Atmospheric precipitation and its formation

Precipitation does not fall from every cloud. A prerequisite for the formation of precipitation is the simultaneous presence of water in the air in solid, liquid and gaseous states, sometimes in mixed clouds. This only happens when the cloud rises and cools. Therefore, convective, frontal and orographic precipitation are distinguished by origin.

Convective precipitation is characteristic of the hot zone, where during the year there is intense heating, evaporation of water, the ascending movement of warm and humid air. In summer, such processes often occur in the temperate zone.

Frontal precipitation is formed when two air masses with different temperatures and other physical properties. Typical frontal precipitation is observed in the temperate and cold zones.

Orographic precipitation falls on the windward slopes of mountains, especially high ones, since they also cause the air to rise upwards. Having lost moisture and descending, bypassing the mountain range, it again descends and warms up, and the relative humidity decreases, moving away from the saturation state.

By the nature of the fallout, they distinguish: heavy rainfall (intense, short, falling over a small area); heavy rainfall (medium intensity, uniform, long - can last the whole day, often fall over a large area); precipitation, drizzling (characterized as if small droplets suspended in the air).

Precipitation measurement

Precipitation is measured in terms of the thickness of the water layer in millimeters that would result from precipitation on a horizontal surface in the absence of evaporation and infiltration into the soil. To measure the amount of precipitation, a rain gauge is used (a metal cylinder 40 cm high and a cross-sectional area of ​​​​500 cm2 with an inserted diaphragm "to prevent evaporation). A rain gauge differs from a rain gauge in special protection. Solid precipitation (snow, hail, cereals) is pre-melted. into the rain gauge, measured using a glass cylindrical vessel, the bottom area of ​​which is 10 times less than the area of ​​the bottom of the rain gauge.So, when the layer of water drained from the rain gauge at the bottom of the vessel is 20 mm, this means that a layer of water with a height of 2 mm.

All rainfall measurements are summarized for each month and output the monthly and then the annual rainfall. The longer the observation is, the more accurately it is possible to calculate the average monthly and, accordingly, the average annual rainfall for this observation site. Lines on a map connecting points with the same amount of precipitation in millimeters for a certain period of time (for example, a year) are called isohieta.

Distribution of precipitation on the surface of the globe

Geographic distribution of precipitation by earth's surface depends on the combined effect of many factors: temperature, evaporation, air humidity, cloudiness, atmospheric pressure, prevailing winds, distribution of land and sea, ocean currents. The most important among them is the air temperature, which determines the intensity of evaporation and the amount of air evaporation (the amount of moisture in millimeters of a layer of water that can evaporate in a certain place in a year).

In "cold latitudes, evaporation is negligible, it had evaporation, since cold air can contain a small amount of water vapor. And although the relative humidity of the air can be quite high, when a small amount of vapor condenses, a meager amount of precipitation falls. In the hot zone, the opposite phenomenon is observed: a large evaporation and high evaporation, and, consequently, absolute humidity air cause a significant amount of precipitation. Hence, precipitation distributed zonally.

Falls in the equatorial belt the largest number precipitation - 1000-2000 mm or more, because there all year round there are high temperatures, high evaporation, and ascending air currents predominate.

In tropical latitudes, the amount of precipitation decreases to 300-500 mm, and in the interior desert regions of the continents - less than 100 mm. The reason for this is the dominance here of high pressure and downward air currents, while heating up and moving away from the state of saturation. Here only on the eastern coasts of the continents, which

flowing with warm currents, heavy rainfall occurs, especially in summer.

IN temperate latitudes the amount of precipitation again increases to 500-1000 m. Most of them fall on the western coasts of the continents, since westerly winds from the oceans. Warm currents and mountainous terrain also contribute to more precipitation here.

In the polar regions, the amount of precipitation is only 100-200 mm, due to the low moisture content in the air, despite the high cloudiness.

However, the amount of precipitation does not yet determine the conditions of moisture. The nature of moisture is expressed by the coefficient of moisture - the ratio of precipitation to evaporation over the same period. That is, K \u003d O / B, where K is the moisture coefficient, O is the amount of precipitation, B is the amount of evaporation. If K = 1, then humidification is adequate, K> 1 - excessive, K<1 - недостаточное, а К <0,3 - бедное. Коэффициент увлажнения определяет тип природно-растительных зон: при избыточном и достаточном увлажнении и достаточный, количества тепла произрастают леса; недостаточное, близкий к единице, увлажнение характерно для лесостепи, саванн; несколько больше 0,3 - луговых и сухих степей; бедное - для полупустынь и пустынь.

The average annual rainfall is an important part of climate data - those recorded using various methods.

Precipitation (most often includes snow, hail, sleet, and other forms of water falling on the ground) is measured in units over a specified period of time.

In the United States, precipitation is usually presented in inches per 24 hour period. This means that if one inch of rain falls in a 24 hour period and the water does not soak into the ground and flow down after the storm, there would be a layer of one inch of water covering the ground.

For Low Tech rainfall measurements, a container with a flat bottom and straight sides (eg a coffee cylinder) is used. While a cylinder can help you determine if a rainfall is one or two inches of rain, it is difficult for them to measure small amounts of precipitation.

Weather watchers use more sophisticated instruments known as rain gauges and tip buckets to measure precipitation more accurately. Rain gauges have wide openings at the top for rainfall. The rain is directed into a narrow tube, one-tenth the diameter of the top of the neck. Because the pipe is thinner than the top of the funnel, the units are farther apart than they would be on a ruler, and accurate one-hundredth (1/100 or 0.01) of an inch measurements are possible. When the rate is less than 0.01 inches of rain, this amount is called the "footprint" of rain.

A bucket fitted with a sensor records precipitation readings on a rotating drum or electronically. It has a funnel like a simple rain gauge, but the funnels lead to two tiny "buckets". The two buckets are balanced and each has 0.01 inch of water. When a bucket is full, its bottom is emptied while the other bucket is filled with rainwater. Each tip of the bucket triggers a device to record an increase of 0.01 inches of rain.

Snowfall is measured in two ways. First, it is a simple measurement of the layer of snow on the ground with a stick marked with units of measure. The second measurement determines the equivalent amount of water per unit of snow. To obtain this coefficient, the snow must be collected and melted into water. Typically, 10 inches of snow produces one inch of water. However, this may apply to loose, fluffy snow, although as little as 2-4 inches of wet, compacted snow can produce an inch of water.

Wind, buildings, trees, terrain and other factors can change the amount of precipitation, and such snowfall is usually measured from obstacles. The thirty-year mean annual rainfall is used to determine the average annual rainfall for a particular location.

The amount of precipitation is of constant interest to those who follow the weather. It would seem that the forecast is 10-15 mm, and on the streets - knee-deep snow or huge puddles. To make it easier for you to navigate the forecasts, we have prepared information on measuring the amount of precipitation.

Meteorologists distinguish between two concepts: the height of the snow cover and the amount of precipitation. What we see on the street after a snowfall is the height of the snow cover, which sometimes reaches 50 cm, although the amount of precipitation can be no more than 20 mm. One millimeter of fallen snow is equivalent to 1-1.5 cm of snow cover height, depending on the snow structure.

According to meteorological guidance, a millimeter of precipitation is one liter of water per square meter. At all weather stations there are rain gauge buckets, from which, at 9 and 21 hours GMT, precipitation is poured into a special vessel, by which their amount is measured. Solid precipitation - snow, hail - is melted, and then experts measure the resulting water.

In St. Petersburg, everything portends an abnormally warm winter (oh, I wouldn’t jinx it!), And I, rather tired of the two previous winters-reconstructions of the events of the film “The Day After Tomorrow,” are incredibly happy about this. Moreover, a year ago, at about this time, it was already -20 ° outside the window. Snowboarders and skiers will be covered with artificial snow on the slopes, so they will not be offended, but I live well without it.

But while the weather is shaking around zero, every morning turns into a dilemma for me: what to wear so as not to freeze and not to sweat. And that's where two great sites come to my rescue with very accurate weather forecasts. At one time, my friend helped me find them, but he does not write in LiveJournal, so I will carry the light to the people. Who knows about them, do not rush to throw eggs at the button accordion, because many still go to obtuse and lying Gismeteo and Yandex for the weather.

Below is a small overview of two great sites: WP5 And YR.no, as well as answers to a few questions that may arise after getting to know them. If it seems that there are too many letters, just take note of my recommendations and believe that these two resources have never failed or deceived.

This site, a guest from Norway, unlike WP5, in addition to very accurate forecasts, has a very beautiful design. The Russian language, however, is not. But there is English (switches in the upper right corner).
The peculiarity of the site is a bunch of different ways of providing information, ranging from simple forecast tables familiar from Yandex for 9 days in advance (it is worth noting that the decoding is still very detailed), and ending with graphs and meteorological maps that change over time.
For me personally, the best and most understandable is a moderately "busy" graph, which can also acquire a line for pressure and a cloud chart if you click on the Detailed button on the left, but this information seems redundant to me. The blue bars on the time axis are again the level of precipitation in millimeters.

Now I will answer a couple of questions that may arise after reading these sites:

Q: How do the British and Norwegians get information about our weather? Our hydrometeorological center certainly knows better!
A: Not at all. Both the Hydrometeorological Center and everyone else know exactly the same thing about the actual weather. All information is collected by ground-based weather stations and made available to the public in the system of free international exchange of meteorological data. Now anyone with a supercomputer with a thousand or two processors can take this data, process it and try to predict what the weather will be like in a particular place in the near future. It's only up to those who manage to do it more accurately.

Q: I don't understand when precipitation is referred to as 2 mm/6 hours. What to expect really?
A: It is very easy to understand. Here is how RP5 explains it:
"The ratio is direct: 1 mm corresponds to 1 liter per 1 square meter. That is, 12 mm is a large 12-liter bucket; 10 mm is a 10-liter bucket; 0.5 mm is a half-liter bottle; 0.2 mm is a glass of water per 1 square meter meter Perhaps such an explanation is not very solid, but understandable.
This opens up new horizons compared to those weather forecasts, where rain, regardless of the predicted intensity, is indicated by a drop, or an umbrella. It is possible to understand whether this umbrella is needed at all precisely by these millimeters: 0.2-1 mm is very small, and most likely means heavy rains in places (that is, all 10 millimeters will fall on 10% of the city, and the sun will shine over the remaining 90%) . And 4-10 mm is already an impressive amount, spread over a huge area, and most likely it will rain for a long time and everywhere.

Q: What a rain, we have winter, frost -30! How to measure snow in millimeters?
A: Simply multiply by 10. 1 millimeter of precipitation equals 1 centimeter snowdrift.

Q: It would be great if we could average forecasts from 10 different sources.
Yeah, someone already before

Not every cloud carries precipitation, after all, for the formation of clouds, a prerequisite is the presence of water in three states: gaseous, liquid and solid, characteristic of mixed clouds. Precipitation occurs only when the cloud begins to rise higher and cool. By origin, precipitation is divided into the following types: convective, frontal and orographic.

Convective type of precipitation It is typical for hot climatic zones, in which intense heating takes place throughout the year, as a result of which water evaporates. At this time, the ascending movement of moist and warm air prevails. Such processes can be observed in summer in temperate zones.

Frontal precipitation are formed in the event of a meeting of two air masses of different temperatures and other factors. Frontal precipitation is observed in the temperate and cold zones.

Orographic precipitation characteristic of windward mountain slopes, forcing the air to rise higher. With the loss of moisture, the air descends, bypassing the mountain range, but after it warms up, and the relative humidity is removed from the saturation state.

According to the nature of precipitation, precipitation is divided into showers (short, but intense precipitation over a small area), overcast (long-term and uniform precipitation of medium intensity, covering a fairly large area) and drizzling (they are characterized by small and small rainfall).

Measuring the amount of precipitation.

Precipitation determined by measuring the thickness of a millimeter layer of water formed as a result of their falling onto a horizontal surface and further infiltration into the soil. In order to measure the amount of precipitation, a metal cylinder with a diaphragm installed is used - a rain gauge, as well as a rain gauge with special protection. Precipitation of the solid type is preliminarily melted, and the resulting amount of water is measured by a cylindrical vessel, the bottom area of ​​which is ten times smaller than the bottom of the rain gauge. When the layer of water in the vessel reaches a figure of 20 mm, this will mean that the layer that has fallen to the Earth is 2 m 2 mm in height.

• 1 - Rain gauge installed on the meteorological platform for measuring liquid precipitation;
• 2 - Soil rain gauge, dug in flush with the ground, there is also a bucket installed inside to collect precipitation;
• 3 - Field rain gauge - a glass tall glass with divisions, for assessing precipitation in agricultural fields;
• 4 - Rain gauge - for collecting liquid and solid precipitation (snow, grains ...);
• 5 - Pluviograph - recorder of the amount of liquid precipitation;
• 6 - Total precipitation gauge - for collecting precipitation over a long period (a week, 10 days, ...) in hard-to-reach places;
• 7 - Radio rain gauge.

All measurements are taken into account for a specific month to derive monthly indicators, and subsequently annual ones. The longer the observation, the more accurate the calculation will be. rainfall for different time intervals for a particular observation site. Those lines on the map, the points of which are connected with the same amount of precipitation in millimeters, are called isohyets and indicate the amount of precipitation for a certain period of time (for example, for a year).

Distribution of precipitation on the Earth's surface.

The geographical position of precipitation on the earth's surface is influenced by many factors: temperature, evaporation, humidity, cloudiness, atmospheric pressure, ocean currents, wind, and the location of land and sea. Temperature is the dominant factor, as it affects the rate of evaporation and the amount of moisture.

In cold latitudes, the level of evaporation is negligible, since the air in these latitudes contains very little water vapor. Even though the relative humidity can be quite high, there will be little precipitation in any case when the steam condenses. In warm regions, the opposite situation is observed, in which, with a large level of evaporation, there is a huge rainfall. That is why precipitation is usually distributed zonal.

The greatest amount of precipitation (1000-2000 mm and more) is observed in the equatorial zone, where there are high temperatures all year round, high evaporation and the predominance of ascending air currents.

In tropical latitudes rainfall less - from 300 to 500 mm, and in desert continental areas less than 100 mm. The reason for this was the dominance of high pressure in combination with downdrafts. The eastern coasts, which are washed by warm currents, are characterized by a large amount of precipitation, especially in summer.

In temperate latitudes, the amount of precipitation increases to 500-1000 mm and the greatest amount of precipitation falls on the western coasts, with prevailing western winds from the oceans. Huge rainfall also caused by warm currents and the presence of mountainous terrain.

In the polar zones, the amount of precipitation is quite low - from 100 to 200 mm. This is due to low humidity in the air, but with a lot of cloudiness.

The amount of precipitation does not always determine the conditions of moisture. The nature of moisture is expressed using the moisture coefficient - the ratio of precipitation to evaporation for the same period - K \u003d O / B, where is the moisture coefficient, O is the annual amount of precipitation, and B is the evaporation value. If K=1, then moisture is sufficient, if more - excessive, and if less - insufficient. Humidification implies one or another type of natural zones: with excessive and sufficient moisture, forests can grow, insufficient and close to unity moisture is typical for forest-steppes and savannahs, low and closer to zero indicators imply steppes, deserts and semi-deserts.