In what direction do air masses move in winter? Atmospheric circulation. Ocean-atmosphere interactions
Since childhood, I have been fascinated by the invisible movements around us: a gentle breeze swirling autumn leaves in a cramped courtyard or a powerful winter cyclone. It turns out that these processes have completely understandable physical laws.
What forces cause air masses to move?
Warm air is lighter than cold air - this simple principle can explain the movement of air on the planet. It all starts at the equator. Here, the sun's rays fall on the Earth's surface at right angles, and a small particle of equatorial air receives a little more heat than its neighbors. This warm particle becomes lighter than its neighbors, which means it begins to float upward until it loses all the heat and begins to descend again. But the downward movement is already occurring in the thirties latitudes of the Northern or Southern Hemisphere.
If there were no additional forces, the air would move from the equator to the poles. But there is not one, but several forces at once that force air masses to move:
- Buoyancy force. When warm air rises and cold air stays below.
- Coriolis force. I'll tell you about it a little lower.
- Relief of the planet. Combinations of seas and oceans, mountains and plains.
Deflection force of the Earth's rotation
It would be easier for meteorologists if our planet did not rotate. But it rotates! This generates the deflecting force of the Earth's rotation, or Coriolis force. Due to the movement of the planet, that very “light” particle of air is not only displaced, say, to the north, but also shifted to the right. Or it is forced to the south and deviates to the left.
This is how constant winds of western or eastern directions arise. Perhaps you've heard of the West Winds or the Roaring Forties? These constant air movements arose precisely due to the Coriolis force.
Seas and oceans, mountains and plains
The final confusion comes from the relief. The distribution of land and ocean changes the classical circulation. Thus, in the Southern Hemisphere there is much less land than in the Northern Hemisphere, and nothing prevents the air from moving over the water surface in the direction it needs, there are no mountains or large cities, while the Himalayas radically change the air circulation in their area.
Along with geographic latitude, an important climate-forming factor is atmospheric circulation, i.e. movement air masses.
Air masses- significant volumes of troposphere air, which has certain properties (temperature, moisture content), depending on the characteristics of the region of its formation and moving as a single whole.
The length of the air mass can be thousands of kilometers, and upward it can extend to the upper boundary of the troposphere.
Air masses are divided into two groups according to their speed of movement: moving and local. Moving Air masses, depending on the temperature of the underlying surface, are divided into warm and cold. A warm air mass is moving towards a cold underlying surface, a cold air mass is moving towards a warmer surface. Local air masses are air masses that long time don't change theirs geographical position. They can be stable and unstable depending on the season, as well as dry and wet.
There are four main types of air masses: equatorial, tropical, temperate, arctic (Antarctic). In addition, each type is divided into subtypes: marine and continental, differing in humidity. For example, the Arctic marine mass is formed over the northern seas - the Barents and White Seas, and is characterized like a continental air mass, but with slightly increased humidity (see Fig. 1).
Rice. 1. Area of formation of Arctic air masses
The climate of Russia shapes, to one degree or another, all air masses, with the exception of the equatorial one.
Let us consider the properties of various masses circulating in our country. Arctic air mass is formed predominantly over the Arctic in polar latitudes, characterized low temperatures winter and summer. It is characterized by low absolute humidity and high relative humidity. This air mass dominates all year round in the Arctic zone, and in winter it moves to the subarctic. Moderate The air mass is formed in temperate latitudes, where the temperature varies depending on the time of year: relatively high in summer, relatively low in winter. According to the seasons of the year, humidity also depends on the place of formation. This air mass dominates the temperate zone. Partly, on the territory of Russia they predominate tropical air masses. They form in tropical latitudes and have high temperatures. Absolute humidity depends on the place of formation, and the relative humidity is usually low (see Fig. 2).
Rice. 2. Characteristics of air masses
The passage of various air masses on the territory of Russia causes differences in weather. For example, all the “cold waves” on the territory of our country coming from the north are Arctic air masses, and tropical air masses from Asia Minor or, sometimes, from the north of Africa come to the south of the European part (they bring hot, dry weather).
Let's consider how air masses circulate throughout our country.
Atmospheric circulation is a system of movement of air masses. A distinction is made between general atmospheric circulation on a global scale and local atmospheric circulation over individual territories and water areas.
The process of circulation of air masses provides the area with moisture and also affects the temperature. Air masses move under the influence of centers atmospheric pressure, and the centers change depending on the time of year. That is why the directions of the prevailing winds, which bring air masses to the territory of our country, change. For example, European Russia and the western regions of Siberia are under the influence of constant western winds. They are supplied with marine temperate air masses of temperate latitudes. They form over the Atlantic (see Fig. 3).
Rice. 3. Movement of marine temperate air masses
When the westerly transport weakens, with north winds Arctic air mass arrives. It brings a sharp cooling, early autumn and late spring frosts (see Fig. 4).
Rice. 4. Movement of the Arctic air mass
Continental tropical air enters the Asian part of our country from Central Asia or Northern China, and in European part countries comes from the Asia Minor peninsula or even from North Africa, but more often such air is formed in the territory of North Asia, Kazakhstan, and the Caspian lowland. These territories lie in the temperate climate zone. However, the air above them warms up very much in summer and acquires the properties of a tropical air mass. Continental moderate air mass prevails all year round in the western regions of Siberia, so winters here are clear and frosty, and summers are quite warm. Even over the Arctic Ocean, Greenland experiences warmer winters.
Due to strong cooling over the Asian part of our country, an area of strong cooling is formed in Eastern Siberia (high pressure area - ). Its center is located in the regions of Transbaikalia, the Republic of Tyva and Northern Mongolia. Very cold continental air spreads from it in different directions. It extends its influence over vast territories. One of its directions is the northeast up to the Chukotka coast, the second is to the west through Northern Kazakhstan and the south of the Russian (East European) Plain to approximately 50ºN. The weather is clear and frosty with some snow. In summer, due to warming, the Asian maximum (Siberian anticyclone) disappears and low pressure sets in (see Fig. 5).
Rice. 5. Siberian anticyclone
Seasonal alternation of areas of high and low pressure forms on Far East monsoon circulation of the atmosphere. It is important to realize that, passing through certain territories, air masses can change depending on the properties of the underlying surface. This process is called transformation of air masses. For example, the Arctic air mass, being dry and cold, passing through the territory of the East European (Russian) Plain heats up and in the region of the Caspian lowland becomes very dry and hot, which is the cause of hot winds.
Asian high, or, as it is called, the Siberian anticyclone, is an area of high pressure that forms over Central Asia and Eastern Siberia. It appears in winter and is formed as a result of cooling of the territory in conditions of enormous size and basin relief. In the central part of the maximum over Mongolia and Southern Siberia, the pressure in January sometimes reaches 800 mm Hg. Art. This is the highest pressure recorded on earth. In winter, the great Siberian anticyclone extends here, especially stable from November to March. The winter here is so windless that with little snowfall, tree branches turn white for a long time from the “unshaken” snow. Frosts already from October reach -20... -30ºС, and in January they often reach -60ºC. The average temperature per month drops to -43º, it is especially cold in the lowlands, where cold, heavy air stagnates. When there is no wind very coldy They are not so difficult to bear, but at -50º it is already difficult to breathe, and low-lying fogs are observed. Such frosts make it difficult for planes to land.
Bibliography
- Geography of Russia. Nature. Population. 1 part 8th grade / V.P. Dronov, I.I. Barinova, V.Ya Rom, A.A. Lobzhanidze.
- V.B. Pyatunin, E.A. Customs. Geography of Russia. Nature. Population. 8th grade.
- Atlas. Geography of Russia. Population and economy. - M.: Bustard, 2012.
- V.P. Dronov, L.E. Savelyeva. UMK (educational and methodological set) “SPHERES”. Textbook “Russia: nature, population, economy. 8th grade". Atlas.
- Climate-forming factors and atmospheric circulation ().
- Properties of air masses that shape the climate of Russia ().
- Western transfer of air masses ().
- Air masses ().
- Atmospheric circulation ().
Homework
- What type of air mass transfer prevails in our country?
- What properties do air masses have, and what does this depend on?
Condensation is a change in the state of a substance from gaseous to liquid or solid. But what is condensation in the mastaba of the planet?
At any given time, the atmosphere of planet Earth contains over 13 billion tons of moisture. This figure is practically constant, since losses due to precipitation are ultimately continuously replenished by evaporation.
The rate of moisture circulation in the atmosphere
The rate of moisture circulation in the atmosphere is estimated at a colossal figure - about 16 million tons per second or 505 billion tons per year. If all the water vapor in the atmosphere suddenly condensed and fell as precipitation, this water could cover the entire surface of the globe with a layer of about 2.5 centimeters, in other words, the atmosphere contains an amount of moisture equivalent to only 2.5 centimeters of rain.
How long does a vapor molecule stay in the atmosphere?
Since the average annual precipitation on Earth is 92 centimeters, it follows that moisture in the atmosphere is renewed 36 times, that is, 36 times the atmosphere is saturated with moisture and freed from it. This means that a molecule of water vapor stays in the atmosphere for an average of 10 days.
Path of the water molecule
Once evaporated, a molecule of water vapor usually drifts for hundreds and thousands of kilometers until it condenses and falls with precipitation on the Earth. Water, snow or hail at higher elevations Western Europe, travels approximately 3000 km from the North Atlantic. Several physical processes occur between liquid water turning into vapor and precipitation falling on Earth.
From the warm surface of the Atlantic, water molecules enter the warm wet air, which subsequently rises above the surrounding colder (more dense) and drier air.
If strong turbulent mixing of air masses is observed, then a layer of mixing and clouds will appear in the atmosphere at the boundary of two air masses. About 5% of their volume is moisture. Air saturated with steam is always lighter, firstly, because it is heated and comes from a warm surface, and secondly, because 1 cubic meter of pure steam is about 2/5 lighter than 1 cubic meter of clean dry air at the same temperature and pressure. It follows that moist air is lighter than dry air, and warm and humid air even more so. As we will see later, this is very important fact for weather change processes.
Movement of air masses
Air can rise for two reasons: either because it becomes lighter as a result of heating and humidification, or because forces act on it, causing it to rise above some obstacles, such as over masses of colder and denser air or over hills and mountains.
Cooling
The rising air, having entered layers with lower atmospheric pressure, is forced to expand and cool at the same time. Expansion requires the expenditure of kinetic energy, which is taken from the thermal and potential energy of atmospheric air, and this process inevitably leads to a decrease in temperature. The cooling rate of a rising portion of air often changes if this portion is mixed with surrounding air.
Dry adiabatic gradient
Dry air, in which there is no condensation or evaporation, and no mixing, and does not receive energy in any other form, cools or warms by a constant amount (1 ° C every 100 meters) as it rises or falls. This quantity is called the dry adiabatic gradient. But if the rising air mass is moist and condensation occurs in it, then latent heat of condensation is released and the temperature of the steam-saturated air drops much more slowly.
Moist adiabatic gradient
This amount of temperature change is called the moist-adiabatic gradient. It is not constant, but changes with changes in the amount of latent heat released, in other words, it depends on the amount of condensed steam. The amount of steam depends on how much the air temperature drops. In the lower layers of the atmosphere, where the air is warm and humidity is high, the moist-adiabatic gradient is slightly more than half the dry-adiabatic gradient. But the wet-adiabatic gradient gradually increases with height and at very high altitudes in the troposphere is almost equal to the dry-adiabatic gradient.
The buoyancy of moving air is determined by the relationship between its temperature and the temperature of the surrounding air. Typically, in the real atmosphere, air temperature falls unevenly with height (this change is simply called a gradient).
If the air mass is warmer and therefore less dense than the surrounding air (and the moisture content is constant), then it rises upward in the same way as a child's ball immersed in a tank. Conversely, when the moving air is colder than the surrounding air, its density is higher and it sinks. If the air has the same temperature as neighboring masses, then their density is equal and the mass remains motionless or moves only with the surrounding air.
Thus, there are two processes in the atmosphere, one of which promotes the development of vertical air movement, and the other slows it down.
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Atmospheric circulation diagram
Air in the atmosphere is in constant motion. It moves in both horizontal and vertical directions.
The primary reason for the movement of air in the atmosphere is the uneven distribution of solar radiation and the heterogeneity of the underlying surface. They cause uneven air temperature and, accordingly, atmospheric pressure over the earth's surface.
The pressure difference creates air movement, which moves from areas of high to low pressure. As they move, air masses are deflected by the force of the Earth's rotation.
(Remember how bodies moving in the Northern and Southern Hemispheres are deflected.)
You, of course, have noticed how on a hot summer day a light haze forms over the asphalt. This heated, light air rises. A similar, but much larger scale picture can be observed at the equator. Very hot air constantly rises, forming updrafts.
Therefore, a constant low pressure belt is formed here near the surface.
The air rising above the equator in the upper layers of the troposphere (10-12 km) spreads towards the poles. It gradually cools and begins to fall above approximately 30 t° north and south latitudes.
This creates an excess of air, which contributes to the formation of a tropical high-pressure zone in the surface layer of the atmosphere.
In the polar regions, the air is cold, heavy and sinks, causing downward movements. As a result, high pressure is formed in the surface layers of the polar belt.
Active atmospheric fronts form between the tropical and polar high-pressure belts in temperate latitudes. Massively colder air displaces warmer air upward, causing updrafts.
As a result, a surface low pressure belt is formed in temperate latitudes.
Map climatic zones Earth
If the earth's surface were homogeneous, the atmospheric pressure belts would spread in continuous stripes. However, the planet's surface is an alternation of water and land, which have different properties. Sushi heats up and cools down quickly.
The ocean, on the contrary, heats up and releases its heat slowly. This is why the atmospheric pressure belts are torn into separate sections - areas of high and low pressure. Some of them exist throughout the year, others - in a certain season.
On Earth, belts of high and low pressure regularly alternate. High pressure is at the poles and near the tropics, low pressure is at the equator and in temperate latitudes.
Types of atmospheric circulation
In the Earth's atmosphere there are several powerful links in the circulation of air masses. All of them are active and inherent in certain latitudinal zones. Therefore, they are called zonal types of atmospheric circulation.
At the Earth's surface, air currents move from the tropical high pressure belt to the equator. Under the influence of the force arising from the rotation of the Earth, they are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
This is how constant powerful winds are formed - trade winds. In the Northern Hemisphere, trade winds blow from the northeast, and in the Southern Hemisphere, from the southeast. So, the first zonal type of atmospheric circulation is trade wind.
From the tropics, air moves to temperate latitudes. Deflected by the force of the Earth's rotation, they begin to gradually move from west to east. It is precisely this flow from the Atlantic that covers the temperate latitudes of all of Europe, including Ukraine. Western air transport in temperate latitudes is the second zonal type of planetary atmospheric circulation.
It is also natural for air to move from the circumpolar high pressure zones to the temperate latitudes, where the pressure is low.
Under the influence of the deflecting force of the Earth's rotation, this air moves from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. The eastern subpolar flow of air masses forms the third zonal type of atmospheric circulation.
On the atlas map, find the latitudinal zones where Various types zonal air circulation.
Due to uneven heating of land and ocean, the zonal pattern of movement of air masses is disrupted. For example, in the east of Eurasia in temperate latitudes, westerly air transport operates only for six months - in winter. In summer, when the continent warms up, air masses with the coolness of the ocean move to land.
This is how monsoon air transfer occurs. Changing the direction of air movement twice a year - characteristic feature monsoon circulation. The winter monsoon is a flow of relatively cold and dry air from the mainland to the ocean.
Summer monsoon- movement of moist and warm air in the opposite direction.
Zonal types of atmospheric circulation
There are three main zonal type of atmospheric circulation: trade wind, westerly air transport and eastern subpolar flow of air masses. Monsoon air transport disrupts the general atmospheric circulation pattern and is an azonal type of circulation.
General atmospheric circulation (page 1 of 2)
Ministry of Science and Education of the Republic of Kazakhstan
Academy of Economics and Law named after U.A. Dzholdasbekova
Faculty of Humanities and Economics Academy
Discipline: Ecology
On the topic: “General circulation of the atmosphere”
Completed by: Tsarskaya Margarita
Group 102 A
Checked by: Omarov B.B.
Taldykorgan 2011
Introduction
1. General information about atmospheric circulation
2. Factors determining the general circulation of the atmosphere
3. Cyclones and anticyclones.
4. Winds affecting the general circulation of the atmosphere
5. Hair dryer effect
6. Scheme general circulation"Planet Machine"
Conclusion
List of used literature
Introduction
On the pages scientific literature V Lately The concept of general atmospheric circulation is often encountered, the meaning of which is understood by each specialist in his own way. This term is systematically used by specialists involved in geography, ecology, and the upper part of the atmosphere.
Meteorologists and climatologists, biologists and doctors, hydrologists and oceanologists, botanists and zoologists, and of course ecologists are showing increasing interest in the general circulation of the atmosphere.
There is no consensus whether this scientific direction has emerged recently or whether research here has been going on for centuries.
Below we propose definitions of the general circulation of the atmosphere as a set of sciences and list the factors influencing it.
A certain list of achievements is given: hypotheses, developments and discoveries that mark well-known milestones in the history of this body of science and give a certain idea of the range of problems and tasks it considers.
Described distinctive features general circulation of the atmosphere, and also presents the simplest scheme of general circulation called the “planet machine”.
1. General information about atmospheric circulation
The general circulation of the atmosphere (Latin Circulatio - rotation, Greek atmos - steam and sphaira - ball) is a set of large-scale air currents in the troposphere and stratosphere. As a result, air masses are exchanged in space, which contributes to the redistribution of heat and moisture.
The general circulation of the atmosphere is the circulation of air on the globe, leading to its transfer from low latitudes to high latitudes and back.
The general circulation of the atmosphere is determined by zones of high atmospheric pressure in the polar regions and tropical latitudes and zones of low pressure in temperate and equatorial latitudes.
The movement of air masses occurs in both latitudinal and meridional directions. In the troposphere, atmospheric circulation includes trade winds, westerly air currents of temperate latitudes, monsoons, cyclones and anticyclones.
The reason for the movement of air masses is the unequal distribution of atmospheric pressure and the heating by the Sun of the surface of land, oceans, ice at different latitudes, as well as the deflecting effect on the air flow of the Earth's rotation.
The main patterns of atmospheric circulation are constant.
In the lower stratosphere, jet air currents in temperate and subtropical latitudes are predominantly western, and in tropical latitudes - eastern, and they travel at speeds of up to 150 m/s (540 km/h) relative to the earth's surface.
In the lower troposphere, the prevailing directions of air transport differ across geographic zones.
In polar latitudes there are easterly winds; in temperate regions - western ones with frequent disruption by cyclones and anticyclones; trade winds and monsoons are the most stable in tropical latitudes.
Due to the diversity of the underlying surface, regional deviations - local winds - arise in the shape of the general circulation of the atmosphere.
2. Factors determining the general circulation of the atmosphere
– Uneven distribution of solar energy over the earth’s surface and, as a consequence, uneven distribution of temperature and atmospheric pressure.
– Coriolis forces and friction, under the influence of which air flows acquire a latitudinal direction.
– Influence of the underlying surface: the presence of continents and oceans, heterogeneity of relief, etc.
The distribution of air currents on the earth's surface is zonal. In equatorial latitudes there is a calm or weak variable winds are observed. Trade winds dominate in the tropical zone.
Trade winds – constant winds, blowing from 30 latitudes to the equator, having a northeastern direction in the northern hemisphere, and a southeastern direction in the southern hemisphere. At 30-35? With. and S. – calm zone, so-called. "horse latitudes".
In temperate latitudes, westerly winds predominate (southwest in the northern hemisphere, northwest in the southern hemisphere). In polar latitudes, easterly winds blow (in the northern hemisphere, northeasterly, in the southern hemisphere, southeasterly winds).
In reality, the wind system above the earth's surface is much more complex. In the subtropical zone, in many areas, trade wind transport is disrupted by the summer monsoons.
In temperate and subpolar latitudes, cyclones and anticyclones have a huge influence on the nature of air currents, and in eastern and northern coasts- monsoons.
In addition, in many areas local winds arise due to the characteristics of the territory.
3. Cyclones and anticyclones.
The atmosphere is characterized by vortex movements, the largest of which are cyclones and anticyclones.
A cyclone is an ascending atmospheric vortex with low pressure in the center and a system of winds from the periphery to the center, directed counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. Cyclones are divided into tropical and extratropical. Consider extratropical cyclones.
The diameter of extratropical cyclones is on average about 1000 km, but there are also more than 3000 km. Depth (pressure in the center) – 1000-970 hPa or less. Strong winds blow in a cyclone, usually up to 10-15 m/sec, but can reach 30 m/sec or more.
The average speed of the cyclone is 30-50 km/h. Most often, cyclones move from west to east, but sometimes they come from the north, south and even east. The zone of greatest frequency of cyclones is the 80th latitude of the northern hemisphere.
Cyclones bring cloudy, rainy, windy weather, cooling in summer, warming in winter.
Tropical cyclones (hurricanes, typhoons) form in tropical latitudes; this is one of the most dangerous and hazardous phenomena nature. Their diameter is several hundred kilometers (300-800 km, rarely more than 1000 km), but they are characterized by a large difference in pressure between the center and the periphery, which causes strong hurricane winds, tropical downpours, and severe thunderstorms.
An anticyclone is a downward atmospheric vortex with increased pressure in the center and a system of winds from the center to the periphery, directed clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. The sizes of anticyclones are the same as those of cyclones, but in the late stage of development they can reach up to 4000 km in diameter.
Atmospheric pressure in the center of anticyclones is usually 1020-1030 hPa, but can reach more than 1070 hPa. The greatest frequency of anticyclones is over the subtropical zones of the oceans. Anticyclones are characterized by partly cloudy weather without precipitation, with weak winds in the center, severe frosts in winter, and heat in summer.
4. Winds affecting the general circulation of the atmosphere
Monsoons. Monsoons are seasonal winds that change direction twice a year. In summer they blow from ocean to land, in winter - from land to ocean. The reason for its formation is unequal heating of land and water according to the seasons of the year. Depending on the zone of formation, monsoons are divided into tropical and extratropical.
Extratropical monsoons are especially pronounced on the eastern edge of Eurasia. The summer monsoon brings moisture and coolness from the ocean, while the winter monsoon blows from the mainland, lowering the temperature and humidity.
Tropical monsoons are most pronounced in the Indian Ocean basin. The summer monsoon blows from the equator, it is opposite to the trade wind and brings clouds, precipitation, softens the summer heat, the winter monsoon coincides with the trade wind, strengthens it, bringing dryness.
Local winds. Local winds have a local distribution, their formation is associated with the characteristics of a given territory - the proximity of water bodies, the nature of the relief. The most common are breezes, bora, foehn, mountain-valley and katabatic winds.
Breezes (light wind - fr) - winds along the shores of seas, large lakes and rivers, changing direction to the opposite twice a day: the daytime breeze blows from the reservoir to the shore, the night breeze - from the shore to the reservoir. Breezes are caused by the daily variation of temperature and, accordingly, pressure over land and water. They capture a layer of air 1-2 km.
Their speed is low - 3-5 m/s. A very strong daytime sea breeze is observed on the western desert coasts of continents in tropical latitudes, washed by cold currents and cold water, rising off the coast in the upwelling zone.
There it invades tens of kilometers inland and produces a strong climatic effect: it reduces the temperature, especially in summer by 5-70 C, and in western Africa up to 100 C, increases relative air humidity to 85%, promotes the formation of fog and dew.
Phenomena similar to daytime sea breezes can be observed on the outskirts of large cities, where there is a circulation of colder air from the suburbs to the center, since “heat spots” exist over the cities throughout the year.
Mountain-valley winds have a daily periodicity: during the day the wind blows up the valley and along the mountain slopes, at night, on the contrary, the cooled air descends. The daytime rise of air leads to the formation of cumulus clouds over the slopes of the mountains; at night, as the air descends and adiabatically warms, the cloudiness disappears.
Glacial winds are cold winds that constantly blow from mountain glaciers down slopes and valleys. They are caused by cooling of the air above the ice. Their speed is 5-7 m/s, their thickness is several tens of meters. They are more intense at night, as they are amplified by slope winds.
General atmospheric circulation
1) Due to the tilt of the earth's axis and the sphericity of the earth, equatorial regions receive more solar energy than polar regions.
2) At the equator, the air heats up → expands → rises → a low pressure area is formed. 3) At the poles, the air cools → becomes denser → falls down → a high pressure area is formed.
4) Due to the difference in atmospheric pressure, air masses begin to move from the poles to the equator.
The direction and speed of winds are also influenced by:
- properties of air masses (humidity, temperature...)
- underlying surface (oceans, mountain ranges, etc.)
- rotation of the globe around its axis (Coriolis force)1) general (global) system of air currents over the earth's surface, the horizontal dimensions of which are comparable to the continents and oceans, and the thickness from several km to tens of km.
Trade winds - These are constant winds blowing from the tropics to the equator.
Reason: At the equator there is always low pressure (updrafts), and in the tropics there is always high pressure (downdrafts).
Due to the action of the Coriolis force: the trade winds of the Northern Hemisphere have a northeast direction (deviate to the right)
Southern Hemisphere trade winds - southeast (deviate to the left)
Northeast winds(in the Northern Hemisphere) and southeast winds(in the Southern Hemisphere).
Reason: air currents move from the poles to moderate latitudes and, under the influence of the Coriolis force, are deflected to the west. Western winds are winds blowing from the tropics to temperate latitudes mainly from west to east.
Reason: in the tropical region there is high pressure, and in temperate latitudes it is low, so part of the air from the E.D. region moves to area N,D,. When moving under the influence of the Coriolis force, air currents are deflected to the east.
Western winds bring warm and humid air to Estonia, because air masses form over the waters of the warm North Atlantic Current.
The air in the cyclone moves from the periphery to the center;
In the central part of the cyclone, the air rises and
It cools, so clouds and precipitation form;
During cyclones, cloudy weather with strong winds prevails:
in summer– rainy and cool,
in winter– with thaws and snowfalls.
Anticyclone- This is an area of high atmospheric pressure with a maximum in the center.
the air in the anticyclone moves from the center to the periphery; in the central part of the anticyclone, the air descends and heats up, its humidity drops, the clouds dissipate; During anticyclones, clear, windless weather sets in:
in summer it’s hot,
in winter it is frosty.
Atmospheric circulation
Definition 1
Circulation is a system of movement of air masses.
The circulation can be general on a global scale and local circulation, which occurs over individual territories and water areas. Local circulation includes day and night breezes that occur on the coasts of the seas, mountain-valley winds, glacial winds, etc.
Local circulation at certain times and in certain places can be superimposed on general circulation currents. With the general circulation of the atmosphere, huge waves and vortices arise in it, which develop and move in different ways.
Such atmospheric disturbances are cyclones and anticyclones, which are characteristic features general atmospheric circulation.
As a result of the movement of air masses, which occurs under the influence of atmospheric pressure centers, areas are provided with moisture. As a result of the fact that air movements of different scales simultaneously exist in the atmosphere, overlapping each other, atmospheric circulation is a very complex process.
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The movement of air masses on a planetary scale is influenced by 3 main factors:
These factors complicate the general circulation of the atmosphere.
If the Earth were homogeneous and did not rotate around its axis - then the temperature and pressure at the surface of the earth would correspond to thermal conditions and be of a latitudinal nature. This means that the temperature decrease would occur from the equator to the poles.
With this distribution, warm air at the equator rises, and cold air at the poles sinks. As a result, it would accumulate at the equator in the upper part of the troposphere, and the pressure would be high, and at the poles it would be low.
At altitude, the air would flow out in the same direction and lead to a decrease in pressure over the equator and its increase over the poles. The outflow of air near the earth's surface would occur from the poles, where the pressure is high, towards the equator in the meridional direction.
It turns out that the thermal reason is the first reason for the circulation of the atmosphere - different temperatures lead to different pressures at different latitudes. In reality, the pressure is low above the equator and high at the poles.
On a uniform rotating On Earth in the upper troposphere and lower stratosphere, the winds, when they flow out to the poles, in the northern hemisphere should deviate to the right, in the southern hemisphere - to the left and at the same time become westerly.
In the lower troposphere, the winds, flowing from the poles towards the equator and deflecting, would become easterly in the northern hemisphere, and southeasterly in the southern hemisphere. The second reason for atmospheric circulation is clearly visible - dynamic. The zonal component of the general circulation of the atmosphere is determined by the rotation of the Earth.
The underlying surface with an uneven distribution of land and water has a significant influence on the general circulation of the atmosphere.
Cyclones
The lower layer of the troposphere is characterized by vortices that appear, develop and disappear. Some vortices are very small and go unnoticed, while others have a big impact on the planet's climate. First of all, this applies to cyclones and anticyclones.
Definition 2
Cyclone is a huge atmospheric vortex with low pressure in the center.
In the Northern Hemisphere, the air in a cyclone moves counterclockwise, in the Southern Hemisphere - clockwise. Cyclonic activity in mid-latitudes is a feature of atmospheric circulation.
Cyclones arise due to the rotation of the Earth and the deflecting force of Coriolis, and in their development they go through stages from inception to filling. As a rule, cyclones occur on atmospheric fronts.
Two air masses of opposite temperatures, separated by a front, are drawn into a cyclone. Warm air at the interface is injected into a region of cold air and deflected into high latitudes.
The balance is disrupted, and cold air in the rear part is forced to penetrate into low latitudes. A cyclonic frontal bend occurs, which is a huge wave moving from west to east.
The wave stage is first stage cyclone development.
Warm air rises and slides along the frontal surface at the front of the wave. The resulting waves with a length of $1000$ km or more are unstable in space and continue to develop.
At the same time, the cyclone moves east at a speed of $100$ km per day, the pressure continues to fall, and the wind becomes stronger, the amplitude of the wave increases. This second stage– stage of a young cyclone.
On special maps, a young cyclone is outlined by several isobars.
As warm air moves into high latitudes, a warm front forms, and as cold air moves into tropical latitudes, it forms a cold front. Both fronts are parts of a single whole. A warm front moves slower than a cold front.
If a cold front catches up with a warm front and merges with it, a occlusion front. Warm air rises and twists in a spiral. This third stage cyclone development – occlusion stage.
Fourth stage– filling it out is final. The warm air is finally pushed upward and cooled, temperature contrasts disappear, the cyclone becomes cold over its entire area, slows down and finally fills. From inception to filling, the life of a cyclone lasts from $5$ to $7$ days.
Note 1
Cyclones bring cloudy, cool and rainy weather in summer and thaw in winter. Summer cyclones move at a speed of $400$-$800$ km per day, winter ones - up to $1000$ km per day.
Anticyclones
Cyclonic activity is associated with the emergence and development of frontal anticyclones.
Definition 3
Anticyclone is a huge atmospheric vortex with high pressure in the center.
Anticyclones form in the rear of the cold front of a young cyclone in cold air and have their own stages of development.
There are only three stages in the development of an anticyclone:
General atmospheric circulation
The objects of study of the general circulation of the atmosphere are moving cyclones and anticyclones of temperate latitudes with their rapidly changing meteorological conditions: trade winds, monsoons, tropical cyclones, etc. Typical features of the general circulation of the atmosphere, stable over time or repeating more often than others, are revealed by averaging meteorological elements over long periods of time. long-term observation periods,
In Fig. 8, 9 shows the average long-term distribution of wind at the earth's surface in January and July. In January, i.e.
In winter, in the Northern Hemisphere, giant anticyclonic vortices are clearly visible over North America and a particularly intense vortex over Central Asia.
In summer, anticyclonic eddies over land are destroyed due to the warming of the continent, and over the oceans such eddies intensify significantly and spread to the north.
Pressure at the Earth's surface in millibars and prevailing air currents
Due to the fact that in the troposphere the air in equatorial and tropical latitudes is heated much more intensely than in the polar regions, air temperature and pressure gradually decrease in the direction from the equator to the poles. As meteorologists say, the planetary gradient of temperature and pressure is directed in the middle troposphere from the equator to the poles.
(In meteorology, the gradient of temperature and pressure is taken in the opposite direction compared to physics.) Air is a highly mobile medium. If the Earth did not rotate around its axis, then in the lower layers of the atmosphere the air would flow from the equator to the poles, and in the upper layers it would return back to the equator.
But the Earth rotates at an angular speed of 2n/86400 radians per second. Air particles, moving from low to high latitudes, retain high linear velocities relative to the earth's surface, acquired at low latitudes, and therefore are deflected as they move east. A west-east air transfer is formed in the troposphere, which is reflected in Fig. 10.
However, such correct mode currents are observed only on maps of average values. “Snapshots” of air currents give very diverse, each time new, non-repeating positions of cyclones, anticyclones, air currents, zones of meeting warm and cold air, i.e., atmospheric fronts.
Atmospheric fronts play a large role in the general circulation of the atmosphere, since significant transformations of the energy of air masses from one type to another occur in them.
In Fig. Figure 10 schematically shows the position of the main frontal sections in the middle troposphere and near the earth's surface. Numerous weather phenomena are associated with atmospheric fronts and frontal zones.
Here cyclonic and anticyclonic vortices originate, thick clouds and precipitation zones form, and wind increases.
When an atmospheric front passes through a given point, a noticeable cooling or warming is usually clearly observed, and the entire character of the weather changes sharply. Interesting features are found in the structure of the stratosphere.
Planetary frontal zone in the middle troposphere
If heat is located in the troposphere near the equator; air masses are cold at the poles, then in the stratosphere, especially in the warm half of the year, the situation is just the opposite; at the poles here the air is relatively warmer, and at the equator it is cold.
The temperature and pressure gradients are directed in the opposite direction relative to the troposphere.
The influence of the deflecting force of the Earth's rotation, which led to the formation of west-east transfer in the troposphere, creates a zone of east-west winds in the stratosphere.
Average location of jet stream axes in the Northern Hemisphere in winter
The highest wind speeds, and therefore the highest kinetic energy of air, are observed in jet streams.
Figuratively speaking, jet streams are air rivers in the atmosphere, rivers flowing at the upper boundary of the troposphere, in the layers separating the troposphere from the stratosphere, i.e. in layers close to the tropopause (Fig. 11 and 12).
Wind speed in jet streams reaches 250 - 300 km/h - in winter; and 100 - 140 km/h - in summer. Thus, a low-speed aircraft, falling into such a jet stream, can fly “backwards”.
Average location of jet stream axes in the Northern Hemisphere in summer
The length of jet streams reaches several thousand kilometers. Below the jet streams in the troposphere, wider and less fast air “rivers” are observed - planetary high-altitude frontal zones, which also play a large role in the general circulation of the atmosphere.
The occurrence of high wind speeds in jet streams and in planetary high-altitude frontal zones occurs due to the presence here of a large difference in air temperatures between neighboring air masses.
The presence of a difference in air temperature, or as they say, "temperature contrast", leads to an increase in wind with height. Theory shows that such an increase is proportional to the horizontal temperature gradient of the air layer in question.
In the stratosphere, due to the reversal of the meridional air temperature gradient, the intensity of jet streams decreases and they disappear.
Despite the large extent of planetary high-altitude frontal zones and jet streams, they, as a rule, do not encircle the entire globe, but end where horizontal temperature contrasts between air masses weaken. The most frequent and dramatic temperature contrasts occur in the polar front, which separates the air of temperate latitudes from the tropical air.
Position of the axis of the altitudinal frontal zone with insignificant meridional exchange of air masses
Planetary high-altitude frontal zones and jet streams often occur in the polar front system. Although on average the planetary altitude frontal zones have a direction from west to east, in specific cases the direction of their axes is very diverse. Most often in temperate latitudes they have a wave-like character. In Fig.
13, 14 show the positions of the axes of high-altitude frontal zones in cases of stable west-east transport and in cases of developed meridional exchange of air masses.
A significant feature of air currents in the stratosphere and mesosphere over the equatorial and tropical regions is the existence there of several layers of air with almost opposite directions of strong winds.
The emergence and development of this multi-layer structure of the wind field here changes at certain, but not entirely coincident, intervals of time, which can also serve as some kind of prognostic sign.
If we add to this that the phenomenon of sharp warming in the polar stratosphere, which regularly occurs in winter, is in some way connected with processes in the stratosphere occurring in tropical latitudes, and with tropospheric processes in moderate and high latitudes, then it will become clear how complex and whimsical those atmospheric conditions develop processes that directly affect the weather regime in temperate latitudes.
Position of the axis of the altitudinal frontal zone with significant meridional exchange of air masses
The state of the underlying surface, especially the state of the upper active layer of water in the World Ocean, is of great importance for the formation of large-scale atmospheric processes. The surface of the World Ocean makes up almost 3/4 of the entire surface of the Earth (Fig. 15).
Sea currents
Due to their high heat capacity and ability to mix easily, ocean waters store heat for a long time during encounters with warm air in temperate latitudes and throughout the year in southern latitudes. The stored heat is carried far to the north by sea currents and warms nearby areas.
The heat capacity of water is several times greater than the heat capacity of the soil and rocks that make up the land. The heated water mass serves as a heat accumulator, with which it supplies the atmosphere. It should be noted that land reflects the sun's rays much better than the surface of the ocean.
The surface of snow and ice reflects the sun's rays especially well; 80-85% of all solar radiation falling on snow is reflected from it. The surface of the sea, on the contrary, absorbs almost all the radiation that falls on it (55-97%). As a result of all these processes, the atmosphere directly from the Sun receives only 1/3 of all incoming energy.
It receives the remaining 2/3 of its energy from the underlying surface heated by the Sun, primarily from the water surface. Heat transfer from the underlying surface to the atmosphere occurs in several ways. Firstly, a large number of Solar heat is spent on the evaporation of moisture from the surface of the ocean into the atmosphere.
When this moisture condenses, heat is released, which warms the surrounding layers of air. Secondly, the underlying surface gives off heat to the atmosphere through turbulent (i.e., vortex, disordered) heat exchange. Thirdly, heat is transferred by thermal electromagnetic radiation. As a result of the interaction of the ocean with the atmosphere, important changes occur in the latter.
The layer of the atmosphere into which the heat and moisture of the ocean penetrates, in cases of invasion of cold air onto the warm ocean surface, reaches 5 km or more. In cases where warm air invades the cold water surface of the ocean, the height to which the influence of the ocean extends does not exceed 0.5 km.
In cases of cold air invasion, the thickness of its layer, which is influenced by the ocean, depends primarily on the magnitude of the water-air temperature difference. If the water is warmer than the air, then powerful convection develops, i.e., disordered upward movements of air, which lead to the penetration of heat and moisture into the high layers of the atmosphere.
On the contrary, if the air is warmer than the water, then convection does not occur and the air changes its properties only in the lowest layers. Above the warm Gulf Stream in the Atlantic Ocean, during the invasion of very cold air, heat transfer from the ocean can reach up to 2000 cal/cm2 per day and extends to the entire troposphere.
Warm air can lose 20-100 cal/cm2 per day over the cold ocean surface. Changes in the properties of air falling on a warm or cold ocean surface occur quite quickly - such changes can be noticed at a level of 3 or 5 km within a day after the start of the invasion.
What air temperature increments can occur as a result of its transformation (change) above the underlying water surface? It turns out that in the cold half of the year the atmosphere over the Atlantic warms up by 6° on average, and sometimes it can warm up by 20° per day. The atmosphere can cool by 2-10° per day. It is estimated that in the North Atlantic Ocean, i.e.
where the most intense heat transfer from the ocean to the atmosphere occurs, the ocean gives off 10-30 times more heat than it receives from the atmosphere. It is natural that the heat reserves in the ocean are replenished by the influx of warm ocean waters from tropical latitudes. Air currents distribute the heat received from the ocean over thousands of kilometers. The warming influence of the oceans in winter leads to the fact that the difference in air temperature between the northeastern parts of the oceans and continents is 15-20° at latitudes 45-60° near the earth's surface, and 4-5° in the middle troposphere. For example, the warming effect of the ocean on the climate of Northern Europe has been well studied.
In winter, the northwestern part of the Pacific Ocean is under the influence of the cold air of the Asian continent, the so-called winter monsoon, which extends 1-2 thousand km deep into the ocean in the surface layer and 3-4 thousand km in the middle troposphere (Fig. 16) .
Annual amounts of heat transferred by sea currents
In summer, it is colder over the ocean than over the continents, so the air coming from the Atlantic Ocean cools Europe, and the air from the Asian continent warms Pacific Ocean. However, the picture described above is typical for average circulation conditions.
Day-to-day changes in the magnitude and direction of heat flows from the underlying surface to the atmosphere and back are very diverse and have a great influence on changes in the atmospheric processes themselves.
There are hypotheses according to which the peculiarities of the development of heat exchange between different parts of the underlying surface and the atmosphere determine the stable nature of atmospheric processes over long periods of time.
If the air warms up above the anomalously (above normal) warm water surface of one or another part of the World Ocean in the temperate latitudes of the Northern Hemisphere, then an area of high pressure (pressure ridge) forms in the middle troposphere, along the eastern periphery of which the transfer of cold air masses from the Arctic begins, and along its western part - the transfer of warm air from tropical latitudes to the north. This situation can lead to the persistence of a long-term weather anomaly at the earth's surface in certain areas - dry and hot or rainy and cool in the summer, frosty and dry or warm and snowy in the winter. Cloudiness plays a very significant role in the formation of atmospheric processes by regulating the flow of solar heat to the earth's surface. Cloud cover significantly increases the share of reflected radiation and thereby reduces the heating of the earth's surface, which, in turn, affects the nature of synoptic processes. It turns out some semblance of feedback: the nature of atmospheric circulation affects the creation of cloud systems, and cloud systems, in turn, affect changes in circulation. We have listed only the most important of the studied “terrestrial” factors that influence the formation of weather and air circulation. The activity of the Sun plays a special role in the study of the causes of changes in the general CIRCULATION of the atmosphere. Here it is necessary to distinguish between changes in air circulation on Earth in connection with changes in the total flow of heat coming from the Sun to Earth as a result of fluctuations in the value of the so-called solar constant. However, as recent research shows, in reality it is not a strictly constant value. The atmospheric circulation energy is continuously replenished by the energy sent by the Sun. Therefore, if the total energy sent by the Sun fluctuates significantly, this can affect changes in circulation and weather on Earth. This issue has not yet been sufficiently studied. As for changes in solar activity, it is well known that various disturbances appear on the surface of the Sun, sunspots, faculae, floccules, prominences, etc. These disturbances cause temporary changes in the composition of solar radiation, the ultraviolet component and corpuscular (i.e. consisting of charged particles, mainly protons) radiation from the Sun. Some meteorologists believe that changes in solar activity are associated with tropospheric processes in the Earth's atmosphere, i.e., with the weather.
This last statement requires further research, mainly due to the fact that the well-manifested 11-year cycle of solar activity is not clearly visible in weather conditions on Earth.
It is known that there are entire schools of meteorological forecasters who are quite successful in predicting the weather in connection with changes in solar activity.
Wind and general atmospheric circulation
Wind is the movement of air from areas of higher air pressure to areas of lower pressure. Wind speed is determined by the magnitude of the difference in atmospheric pressure.
The influence of wind in navigation must be constantly taken into account, since it causes ship drift, storm waves, etc.
Due to uneven heating various parts On the globe there is a system of atmospheric currents on a planetary scale (general circulation of the atmosphere).
The air flow consists of individual vortices moving randomly in space. Therefore, the wind speed measured at any point changes continuously over time. The greatest fluctuations in wind speed are observed in the near-water layer. In order to be able to compare wind speeds, a height of 10 meters above sea level was taken as the standard height.
Wind speed is expressed in meters per second, wind force in points. The relationship between them is determined by the Beaufort scale.
Beaufort scale
Fluctuations in wind speed are characterized by the gustiness coefficient, which is understood as the ratio of the maximum speed of wind gusts to its average speed obtained over 5 - 10 minutes.
As the average wind speed increases, the gustiness coefficient decreases. At high wind speeds, the gustiness coefficient is approximately 1.2 - 1.4.
Trade winds are winds that blow all year round in one direction in the zone from the equator to 35° N. w. and up to 30° south. w. Stable in direction: in the northern hemisphere - northeast, in the southern hemisphere - southeast. Speed – up to 6 m/s.
Monsoons are winds of temperate latitudes, blowing from the ocean to the mainland in summer and from the mainland to the ocean in winter. Reach speeds of 20 m/s. Monsoons bring dry, clear and cold weather to the coast in winter, and cloudy weather with rain and fog in summer.
Breezes arise due to uneven heating of water and land during the day. During the daytime, wind arises from sea to land (sea breeze). At night from the chilled coast - to the sea (shore breeze). Wind speed 5 – 10 m/s.
Local winds arise in certain areas due to the characteristics of the relief and differ sharply from the general air flow: they arise as a result of uneven heating (cooling) of the underlying surface. Detailed information about local winds is given in sailing directions and hydrometeorological descriptions.
Bora is a strong and gusty wind directed down a mountain slope. Brings significant cooling.
Observed in areas where low mountain range borders the sea, during periods when atmospheric pressure increases over land and temperature decreases compared to the pressure and temperature over the sea.
In the area of the Novorossiysk Bay, the bora operates in November - March with average wind speeds of about 20 m/s (individual gusts can be 50 - 60 m/s). Duration of action is from one to three days.
Similar winds are observed on Novaya Zemlya, on the Mediterranean coast of France (mistral) and off the northern shores of the Adriatic Sea.
Sirocco - hot and humid wind of the central part Mediterranean Sea accompanied by clouds and precipitation.
Tornadoes are whirlwinds over the sea with a diameter of up to several tens of meters, consisting of water spray. They last up to a quarter of a day and move at speeds of up to 30 knots. The wind speed inside a tornado can reach up to 100 m/s.
Storm winds occur predominantly in areas with low atmospheric pressure. Especially great strength tropical cyclones reach, with wind speeds often exceeding 60 m/s.
Strong storms are also observed in temperate latitudes. When moving, warm and cold air masses inevitably come into contact with each other.
The transition zone between these masses is called the atmospheric front. The passage of the front is accompanied by a sharp change in the weather.
An atmospheric front can be stationary or in motion. There are warm, cold and occlusion fronts. The main atmospheric fronts are: arctic, polar and tropical. On synoptic maps, fronts are depicted as lines (front line).
A warm front is formed when warm air masses attack cold ones. On weather maps, a warm front is marked by a solid line with semicircles along the front indicating the direction of colder air and the direction of movement.
As the warm front approaches, pressure begins to drop, clouds thicken, and heavy precipitation begins to fall. In winter, low stratus clouds usually appear when a front passes. The temperature and humidity are slowly increasing.
As a front passes, temperatures and humidity typically rise quickly and winds pick up. After the front passes, the wind direction changes (the wind turns clockwise), the pressure drop stops and its slight increase begins, the clouds dissipate, and precipitation stops.
A cold front is formed when cold air masses attack warmer ones (Fig. 18.2). On weather maps, a cold front is depicted as a solid line with triangles along the front pointing towards the colder warm temperatures and direction of movement. The pressure ahead of the front drops strongly and unevenly, the ship finds itself in a zone of showers, thunderstorms, squalls and strong waves.
An occlusion front is a front formed by the merger of a warm and cold front. It appears as a solid line with alternating triangles and semicircles.
Section of a warm front
Cross section of a cold front
A cyclone is an atmospheric vortex of huge diameter (from hundreds to several thousand kilometers) with low air pressure in the center. The air in a cyclone circulates counterclockwise in the northern hemisphere and clockwise in the southern hemisphere.
There are two main types of cyclones – extratropical and tropical.
The first are formed in temperate or polar latitudes and have a diameter of a thousand kilometers at the beginning of development, and up to several thousand in the case of the so-called central cyclone.
A tropical cyclone is a cyclone formed in tropical latitudes; it is an atmospheric vortex with low atmospheric pressure in the center with storm-like wind speeds. Formed tropical cyclones move along with air masses from east to west, while gradually deviating towards high latitudes.
Such cyclones are also characterized by the so-called The “eye of the storm” is a central area with a diameter of 20–30 km with relatively clear and windless weather. About 80 tropical cyclones are observed annually in the world.
View of a cyclone from space
Paths of tropical cyclones
In the Far East and Southeast Asia, tropical cyclones are called typhoons (from the Chinese tai feng - big wind), and in Northern and South America– hurricanes (Spanish: huracán, named after the Indian god of the wind).
It is generally accepted that a storm becomes a hurricane when the wind speed exceeds 120 km/h; at a speed of 180 km/h, the hurricane is called a strong hurricane.
7. Wind. General atmospheric circulation
Lecture 7. Wind. General atmospheric circulation
Wind – This is the movement of air relative to the earth's surface, in which the horizontal component predominates. When upward or downward wind movement is considered, the vertical component is also taken into account. The wind is characterized direction, speed and impetuosity.
The cause of wind is the difference in atmospheric pressure at different points, determined by the horizontal pressure gradient. The pressure is not the same primarily due to different degrees of heating and cooling of the air and decreases with altitude.
To get an idea of the distribution of pressure on the surface of the globe, pressure measured at the same time at different points and normalized to the same height (for example, sea level) is applied to geographic maps. Points with the same pressure are connected by lines - isobars.
In this way, areas of high (anticyclones) and low (cyclones) pressure and the directions of their movement are identified for weather forecasting. Using isobars, you can determine the amount of pressure change with distance.
In meteorology, the concept is accepted horizontal pressure gradient is the change in pressure per 100 km along a horizontal line perpendicular to the isobars from high pressure to low pressure. This change is usually 1-2 hPa/100 km.
The movement of air occurs in the direction of the gradient, but not in a straight line, but in a more complex manner, which is caused by the interaction of forces that deflect the air due to the rotation of the earth and friction. Under the influence of the Earth's rotation, air movement deviates from the pressure gradient to the right in the northern hemisphere, and to the left in the southern hemisphere.
The largest deviation is observed at the poles, and at the equator it is close to zero. The friction force reduces both the wind speed and the deviation from the gradient as a result of contact with the surface, as well as inside the air mass due to different speeds in the layers of the atmosphere. The combined influence of these forces deflects the wind from the gradient over land by 45-55o, over the sea - by 70-80o.
With increasing altitude, the wind speed and its deviation increase up to 90° at a level of about 1 km.
Wind speed is usually measured in m/sec, less often in km/hour and points. The direction is taken to be where the wind is blowing, determined in bearings (there are 16 of them) or angular degrees.
Used for wind observations vane, which is installed at a height of 10-12 m. A hand-held anemometer is used for short-term observations of speed in field experiments.
Anemorumbometer allows you to remotely measure wind direction and speed , anemormbograph continuously records these indicators.
The diurnal variation of wind speed over the oceans is almost not observed and is well expressed over land: at the end of the night - a minimum, in the afternoon - a maximum. The annual cycle is determined by the patterns of general circulation of the atmosphere and differs among regions of the globe. For example, in Europe in summer there is a minimum wind speed, in winter it is maximum. In Eastern Siberia it’s the other way around.
The direction of the wind in a particular place changes often, but if you take into account the frequency of winds of different directions, you can determine that some occur more often. To study directions in this way, a graph called a wind rose is used. On each straight line of all points of reference, the observed number of wind events for the required period is plotted and the obtained values on the points of reference are connected by lines.
The wind helps maintain the constancy of the gas composition of the atmosphere, mixing air masses, transporting moist sea air inland, providing them with moisture.
Unfavorable effect of wind on Agriculture may manifest itself in increased evaporation from the soil surface, causing drought; wind erosion of soils is possible at high wind speeds.
Wind speed and direction must be taken into account when pollinating fields with pesticides and when irrigating with sprinklers. The direction of the prevailing winds must be known when laying forest strips and snow retention.
Local winds.
Local winds are called winds that are characteristic only of certain geographical areas. They are of particular importance in their influence on weather conditions; their origin is different.
Breezes – winds near the coastline of seas and large lakes, which have a sharp diurnal change in direction. During the day sea breeze blows onto the shore from the sea, and at night - onshore breeze blows from land to sea (Fig. 2).
They are pronounced in clear weather in the warm season, when the overall air transport is weak. In other cases, for example during the passage of cyclones, breezes can be masked by stronger currents.
Wind movement during breezes is observed at a distance of several hundred meters (up to 1-2 km), with an average speed of 3 - 5 m/sec, and in the tropics - even more, penetrating tens of kilometers deep into land or sea.
The development of breezes is associated with the daily variation of land surface temperature. During the day, the land heats up more than the surface of the water, the pressure above it becomes lower and air transfer from the sea to the land is formed. At night, the land cools faster and more strongly, and air is transferred from land to sea.
The daytime breeze lowers the temperature and increases the relative humidity, which is especially pronounced in the tropics. For example, in West Africa, when sea air moves onto land, the temperature can drop by 10°C or more, and the relative humidity can increase by 40%.
Breezes are also observed on the coasts of large lakes: Ladoga, Onega, Baikal, Sevan, etc., as well as on big rivers. However, in these areas the breezes are smaller in their horizontal and vertical development.
Mountain-valley winds are observed in mountain systems mainly in summer and are similar to breezes in their daily frequency. During the day, they blow up the valley and along the mountain slopes as a result of heating by the sun, and at night, when cooled, the air flows down the slopes. Night air movement can cause frost, which is especially dangerous in the spring when gardens are blooming.
Föhn –a warm and dry wind blowing from the mountains to the valleys. At the same time, the air temperature rises significantly and its humidity drops, sometimes very quickly. They are observed in the Alps, the Western Caucasus, South Coast Crimea, in the mountains of Central Asia, Yakutia, on the eastern slopes of the Rocky Mountains and in other mountain systems.
A foehn is formed when an air current crosses a ridge. Since a vacuum is created on the leeward side, air is sucked down in the form of a downward wind. The descending air is heated according to the dry adiabatic law: by 1°C for every 100 m of descent.
For example, if at an altitude of 3000 m the air had a temperature of -8o and a relative humidity of 100%, then, having descended into the valley, it will heat up to 22o, and the humidity will drop to 17%. If the air rises along the windward slope, then water vapor condenses and clouds form, precipitation falls, and the descending air will be even drier.
The duration of hair dryers ranges from several hours to several days. A hairdryer can cause intense snow melting and flooding, drying out soils and vegetation until they die.
Bora–it is a strong, cold, gusty wind that blows from low mountain ranges towards a warmer sea.
The most famous bora is in the Novorossiysk Bay of the Black Sea and on the Adriatic coast near the city of Trieste. Similar to bora in origin and manifestation north in the area of
Baku, mistral on the Mediterranean coast of France, Northser in the Gulf of Mexico.
Bora is created when cold air masses pass through the coastal ridge. The air flows down under gravity, developing a speed of more than 20 m/sec, while the temperature drops significantly, sometimes by more than 25°C. Bora fades away a few kilometers from the coast, but sometimes can cover a significant part of the sea.
In Novorossiysk, bora is observed about 45 days a year, most often from November to March, with a duration of up to 3 days, rarely up to a week.
General atmospheric circulation
General atmospheric circulation– this is a complex system of large air currents that transport very large masses of air over Globe .
In the atmosphere near the earth's surface in polar and tropical latitudes, eastern transport is observed, and in temperate latitudes - western transport.
The movement of air masses is complicated by the rotation of the Earth, as well as by topography and the influence of areas of high and low pressure. The deviation of winds from the prevailing directions is up to 70°.
In the process of heating and cooling huge masses of air above the globe, areas of high and low pressure are formed, which determine the direction of planetary air currents. Based on long-term average pressure values at sea level, the following patterns have been identified.
On both sides of the equator there is a low pressure zone (in January - between 15° north latitude and 25° south latitude, in July - from 35° north latitude to 5° south latitude). This zone, called equatorial depression, extends more to the hemisphere where it is summer in a given month.
In the direction to the north and south of it, the pressure increases and reaches maximum values at subtropical high pressure zones(in January - at 30 - 32o northern and southern latitudes, in July - at 33-37o N and 26-30o S). From the subtropics to the temperate zones, the pressure drops, especially significantly in the southern hemisphere.
The minimum pressure is at two subpolar low pressure zones(75-65o N and 60-65o S). Further towards the poles the pressure increases again.
The meridional baric gradient is also located in accordance with pressure changes. It is directed from the subtropics on the one hand - to the equator, on the other - to the subpolar latitudes, from the poles to the subpolar latitudes. The zonal direction of winds is consistent with this.
Northeast and southeast winds often blow over the Atlantic, Pacific and Indian oceans - trade winds. Western winds in the southern hemisphere, at latitudes 40-60°, bend around the entire ocean.
In the northern hemisphere at temperate latitudes, westerly winds are constantly expressed only over the oceans, and over continents the directions are more complex, although westerlies also predominate.
Eastern winds of polar latitudes are clearly observed only along the outskirts of Antarctica.
In the south, east and north of Asia there is a sharp change in the direction of winds from January to July - these are areas monsoon. The causes of monsoons are similar to the causes of breezes. In summer, the Asian mainland heats up greatly and an area of low pressure spreads over it, where air masses from the ocean rush.
The resulting summer monsoon causes large amounts of precipitation, often of a torrential nature. In winter, high pressure sets over Asia due to more intense cooling of the land compared to the ocean and cold air moves towards the ocean, forming the winter monsoon with clear, dry weather. Monsoons penetrate more than 1000 km in a layer above land up to 3-5 km.
Air masses and their classification.
Air mass- this is a very large amount of air, which occupies an area of millions of square kilometers.
In the process of general circulation of the atmosphere, the air is divided into separate air masses, which remain for a long time over a vast territory, acquire certain properties and cause various types of weather.
Moving to other areas of the Earth, these masses bring with them their own weather patterns. The predominance of air masses of a certain type(s) in a particular area creates the characteristic climate regime of the area.
The main differences in air masses are: temperature, humidity, cloudiness, dust content. For example, in summer the air over the oceans is wetter, colder, and cleaner than over land at the same latitude.
The longer the air stays over one area, the more it undergoes changes, so air masses are classified according to geographical areas where they were formed.
There are main types: 1) Arctic (Antarctic), which move from the poles, from high pressure zones; 2) temperate latitudes“polar” – in the north and southern hemispheres; 3) tropical– move from subtropics and tropics to temperate latitudes; 4) equatorial– are formed above the equator. Within each type, marine and continental subtypes are distinguished, differing primarily in temperature and humidity within the type. The air, being in constant motion, moves from the area of formation to neighboring ones and gradually changes properties under the influence of the underlying surface, gradually turning into a mass of a different type. This process is called transformation.
Cold Air masses are those that move to a warmer surface. They cause cooling in the areas where they come.
As they move, they are heated by the earth's surface, so large vertical temperature gradients arise within the masses and convection develops with the formation of cumulus and cumulonimbus clouds and rainfall.
Air masses moving towards a colder surface are called warm by the masses. They bring warming, but they themselves cool from below. Convection does not develop in them and stratus clouds predominate.
Neighboring air masses are separated from each other by transition zones that are strongly inclined to the Earth's surface. These zones are called fronts.
Due to the following factors:
Baric gradient force (pressure gradient);
Coriolis force;
Geostrophic wind;
Gradient wind;
Friction force.
Pressure gradient results in wind arising due to the movement of air in the direction of the pressure gradient from an area of higher pressure to an area of lower pressure. Atmospheric pressure is 1.033 kg/cm², measured in mmHg, mb and hPa.
Pressure changes occur when air moves due to its heating and cooling. main reason transfer of air masses - convective flows - the rise of warm air and its replacement with cold air from below (vertical convection flow). When they encounter a layer of high-density air, they spread, forming horizontal convection currents.
Coriolis force- repulsive force. Occurs when the Earth rotates. Under its influence, the wind is deflected to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere, i.e. in Northern it deviates to the east. Closer to the poles the deflecting force increases.
Geostrophic wind.
In temperate latitudes, the pressure gradient force and the Coriolis force are balanced, and air does not move from an area of high pressure to an area of low pressure, but flows between them parallel to the isobars.
Gradient wind- this is the circular movement of air parallel to isobars under the influence of centrifugal and centripetal forces.
Impact of friction force.
The friction of air on the earth's surface upsets the balance between the force of the horizontal pressure gradient and the Coriolis force, slows down the movement of air masses, changes their direction so that the air flow does not move along isobars, but crosses them at an angle.
With height, the effect of friction weakens, and the deviation of the wind from the gradient increases. The change in wind speed and direction with height is called Ekman spiral.
The average long-term wind spiral near the Earth is 9.4 m/s, it is maximum near Antarctica (up to 22 m/s), sometimes gusts reach 100 m/s.
With height, wind speed increases and reaches hundreds of m/s. The direction of the wind depends on the pressure distribution and the deflecting effect of the Earth's rotation. In winter, winds are directed from the mainland to the ocean, in summer - from the ocean to the mainland. Local winds are called breeze, fen, bora.