Formation of atmospheric fronts. Atmospheric fronts - what is it? What are they like? Zones of atmospheric fronts of a young cyclone

Air masses that differ in their physical properties are separated from each other by a layer of air called the frontal surface. In the layer of the frontal zone, temperature, humidity, density, and wind change sharply. The frontal zone is always inclined towards cold air. Above it is warm air, as less dense and light, and above it in the form of a wedge - cold. The main reason for the formation of fronts is the convergence of dissimilar air masses. The front is considered to be dynamically expressed if the temperature difference between warm and cold air is 8-10C for 1000 km distance. The speed of the front depends on the angle of intersection of the front with the isobars.

The fronts that separate the main geographic types of air masses are called main fronts.

Distinguish:

the arctic front separating the arctic air from the air temperate latitudes;

polar front separating temperate and tropical air;

a tropical front lying between tropical and equatorial air.

In terms of speed, these fronts can be stationary (the average speed of their movement is 5-10 km / h. They are located on the periphery of a cyclone or anticyclone), slow moving, fast moving. By temperature, warm, cold and occlusion fronts. According to the height of development - surface, tropospheric, high-altitude.

warm A front is a section of the main front moving towards cold air; warm air moves behind this front, which, being less dense, flows into cold air.

cold A front is a section of the main front moving towards warm air. Behind these fronts, cold air moves, which is denser and wedged under the warm air.

The front formed as a result of the merging of warm and cold air is called the front. occlusion.

3.3 Warm front in winter and summer. flight conditions.

At the warm front, warm air flows into the cold, located in the form of a wedge at the bottom. Ahead of the surface line, there is an area of pressure drop, which is due to the replacement of cold air with warm air. As the pressure drops, the wind increases, reaches its maximum speed before the passage of the front, then weakens. Winds of the southeast direction predominate ahead of the front, passing behind the front to the south and southwest.

The slow upward movement of warm air along the frontal surface leads to its adiabatic cooling and the formation of a cloud system and a large precipitation zone, the width of the cloud zone extends up to 600-700 km.

The slope of the frontal surface is observed within 1/100 to 1/200.

The main cloud system of the front is nimbostratus and highly stratified Ns-As clouds located in the lower and middle tiers (5-6 km). Their upper border is almost horizontal, and the lower one decreases from the front edge to the front line, where it reaches a height of about 100 m (in cold weather it can be lower). Above As-Ns are cirrostratus and cirrus clouds. Sometimes they merge with the underlying cloud system. But often the clouds of the upper tier are separated from the Ns-As system by a cloud layer. A zone of extensive precipitation is observed under the main cloud system. It lies in front of the surface front line and has a length along the normal from the front up to 400 km.

In the precipitation zone, low broken-rain clouds with a lower boundary of 50-100 m are formed, sometimes frontal fogs occur, and ice is observed at temperatures from 0 to -3.

In winter, when strong winds the passage of the front is accompanied heavy snowstorms, In summer, separate pockets of cumulonimbus clouds with showers and thunderstorms can appear on the warm front. Most often they occur at night. Their development is explained by the strong nighttime cooling of the upper layer of the main frontal cloud system at a relatively constant temperature in the lower layers of the cloud. This leads to an increase in temperature gradients and to an increase in vertical currents, which lead to the formation of cumulonimbus clouds. They are usually masked by nimbostratus clouds, which makes it difficult to visually identify them. When approaching nimbostratus clouds, inside which cumulonimbus clouds are hidden, turbulence (turbulence) begins, increased electrization, which negatively affects the operation of instrumentation.

In winter, in the zone of negative temperatures of the warm front cloudiness, there is a danger of aircraft icing. The lower limit of icing is the zero isotherm. Heavy icing is observed in flight in the zone of supercooled rain. In the cold season, the warm front becomes aggravated and more often gives difficult weather conditions: low cloud cover, poor visibility in snowstorms, precipitation, fog, icing in precipitation, ice on the ground, electrification in the clouds.

Visibility after the passage of the front remains limited for some time, as the air is saturated with a large amount of moisture, which allows for a long time be kept by fogs, haze and low clouds.

The temperature rises behind the warm front. On weather maps, a warm front is indicated by a red line.

3.4 Cold front of the 1st kind in winter and summer. flight conditions.

A cold front of the 1st kind moves at a speed of no more than 30 km/h.

In this case, there is an ordered slow rise of warm air along an invading wedge of cold air. In the cold half-year in the rising warm air, the process of condensation is not violent. As a result, nimbostratus clouds form over the frontal surface. Precipitation begins at the very front line, the width of the precipitation zone is 100-200 km.

In this season, the cloud system resembles the cloudiness of the warm front system, which is in reverse order. The clouds of the upper layer are located behind the surface front line and can be separated from the main cloud system by a cloudless layer.

The upper boundary of nimbostratus and altostratus clouds (Ns-As) is located at an altitude of 4-5 km.

In the warm season, cumulonimbus clouds of high vertical power form in front of the Ns-As cloud system, from which heavy precipitation occurs, accompanied by thunderstorms, these clouds are located in ridges along the front line with a width of 50-100 km. The upper limit can reach the tropopause and above. Under the clouds, showers, thunderstorms, squalls are observed. In the precipitation zone, low broken-rain clouds almost always form. The wind turns to the right after passing the front and weakens, the pressure in front of the front drops, behind the front it gradually increases, the temperature drops.

3.5 Cold front 2 types in winter and summer. flight conditions.

fast moving cold front 2 kinds is the most dangerous of all types of atmospheric fronts. Due to high speed movement (40-50 km / h), cold air with great energy displaces warm air up to great heights. AT summer time as a result of this strong dynamic convection, cumulonimbus clouds of high vertical power are formed in warm air, sometimes breaking through the tropopause. In the cold season

clouds are less powerful.

Cumulonimbus clouds are displaced forward in the direction of the wind at high altitudes, 100-300 km from the front line. Altocumulus lenticular clouds (Ac), which appear 200 km ahead of the surface front line, are a harbinger of the approach of such a front. Near the front line, cumulonimbus clouds are accompanied by squall eddies with destructive wind speeds and thunderstorms. The width of the cloud system reaches several tens of kilometers, the lower boundary is usually at a height of 300-400m, and in the precipitation zone it can drop to 100-200m.

In clouds, ascending currents with a force of up to 30 m/s or more and descending ones of up to 15 m/s or more present a great danger. In addition, there may be thunderstorms, heavy rainfall in the clouds, and intense icing in the zone of negative temperatures. But the width of this dangerous zone is small, about 50 km.

Near the ground, this front is accompanied by squalls, showers, thunderstorms, the width of the rainfall zone is several tens of kilometers and is usually observed ahead of the surface front line. The pressure in front of the front drops sharply, behind the front it grows rapidly. The wind after passing the front sharply changes direction to the right and increases to 20-30 m/s. The temperature behind the front drops by 10-12°C in 1 hour.

The weather is most pronounced on this front in the summer in the afternoon.

In winter, when the front passes, heavy snowfalls and blizzards are observed, which worsen visibility to several tens of meters. The main clouds are cumulonimbus (Cb) with an upper limit of 4-5 km.

Flights at flight level take place in simple weather conditions, and their main influence is manifested at low flight levels during takeoff, landing and climb.

3.6 Fronts of occlusion. flight conditions.

Warm and cold fronts are the fronts of young cyclones. A cold front, being more active and fast moving, usually catches up with a warm front and closes with it. At the same time, two cold air masses- located in front of the warm front and lying behind the cold front. The warm air trapped between the fronts is cut off from the ground and forced upward. The cloud systems of the warm and cold fronts converge and partially overlap each other and are also forced upward. This process is called the cyclone occlusion process, and the resulting front is called the occlusion front (occlusion - "occlusion" - lock close).

Occlusion results in two types of occlusion fronts:

1. warm front of occlusion (occlusion according to the type of warm front);

2. cold front of occlusion (cold front type occlusion).

Warm front of occlusion.

This front occurs if the cold air in the rear of the cyclone is a warmer air mass than the cold air in its front. When a cyclone is occluded, less cold air flows onto colder air, a multi-tiered cloud system is formed, consisting of a system of warm front clouds - stratus and cold front clouds - cumulonimbus, under which low fractured rain clouds can form.

Heavy precipitation begins ahead of the front line for 300-400 km, gradually turning into showers at the point of occlusion. The wind near the ground has a sharp right hand rotation and is getting stronger. The pressure drops quickly. Occlusions of this type are found mainly in the cold half of the year. At medium and high flight altitudes, aircraft may encounter masked cumulonimbus clouds, which cause severe turbulence and icing. The width of such a zone along the normal to the front is 50 km. When flying at low altitudes, there is always low cloudiness, turning into fog, icing, ice at the airfield..

At first glance, the air in the atmosphere appears to be stationary. In fact, movement occurs continuously in both vertical and horizontal directions. In motion, huge air masses interact with each other. Their dimensions are commensurate with the areas of the continents. This is the basis of such a phenomenon as an atmospheric front.

The air in such an array has uniform properties, obtained when it originates above the surface of the land or the ocean, where it was formed. Air vortices of the Earth move the air of the troposphere from one territory to another, transferring and changing their properties along with them. The behavior and properties of air masses determine the types of climate and weather features territory.

Classification of air masses

Depending on the properties, air masses are divided into types. Main criterion classification - the ratio of heat and moisture:

cold and dry - the air of the Arctic and Antarctic;
change the temperature and humidity according to the seasons of the year - polar (temperate latitudes);
hot and dry - tropical;
hot and humid - equatorial.

When moving, air masses collide, and atmospheric events rapidly develop on their border.

Atmospheric front - definition

Geography is a science that studies various natural phenomena. The concept of an atmospheric front is also considered here. It can be very extensive: several tens of kilometers long, hundreds of meters high and thousands of kilometers long. The transition zone from one property to another is called the frontal surface, and its intersection with the earth's surface is called the front line. It unfolds the main events, accompanied by sudden changes in the weather. The weather conditions will depend on what kind of air the front brought.

Thus, the atmospheric front in geography is the boundary between air masses of different properties.

The difference of atmospheric fronts from each other is due not only to air temperature, but also to how they are generated.

warm front

It is formed when light warm air at a higher speed catches up with a cold mass, which, due to gravity, is not able to move quickly. Upon contact with cold air, warm air begins to creep up the gentle slope formed by the cold massif. Already two air masses together continue to move in the direction where the warm air moved. As the warm air rises, it cools and forms rain clouds.

A warm atmospheric front can always be recognized by the following features:

barometers show a decline atmospheric pressure;
there is an increase in air temperature;
harbingers of rain appear - cirrus clouds, gradually turning into cirrostratus, and then - into altostratus;
the wind intensifies, changing its direction;
the clouds are filled with heaviness;
precipitation falls.

Warming is a constant companion of the warm front. In summer, precipitation is protracted, so rainy weather sets in, although warm weather. In winter, the arrival of a warm front is associated with heavy snowfalls and thaws.

cold front

An atmospheric cold front occurs when cold air in motion catches up with warm air, picks it up and rapidly lifts it up. Due to its lightness, warm air quickly rises to a high altitude, and also cools quickly. Moisture from warm air turns into steam and forms clubs of cumulonimbus clouds. The air continues to move in the direction that the cold air moved. Always accompanied by showers and cooling.

Characteristic features of a cold front:

there are pressure surges both behind the front line and in front of it;
cumulus clouds appear;
a squally wind is blowing, sharply changing direction from left to right;
a downpour begins with a thunderstorm, hail is possible, precipitation can last several hours;
it gets colder, the temperature difference can be up to 10 0 С;
clearings are visible behind the cloud line.

The weather that accompanies a cold front is always a challenge, especially for those who are on the road.

Depending on the intensity of air movement, an atmospheric front of the 1st kind is distinguished, characterized by slow movement, and a front of the 2nd kind, moving rapidly and bringing rain and squally wind in summer, and snowfalls and blizzards in winter. They also differ in the speed of atmospheric processes taking place inside.

Fronts of occlusion

These are areas of connection of several fronts. They are also warm and cold. The mechanism of their formation is complex and depends on the properties of the air encountered. As a rule, two cold massifs and one warm massif participate in their formation, and vice versa.

With occlusion fronts, the following are observed:

overcast and heavy rains;
not an increase, but a change in wind direction;
lack of jumps in atmospheric pressure;
temperature constancy;
the formation of cyclones.

Cyclones and anticyclones

Characterization of weather phenomena during the passage of all types of fronts is impossible without mentioning cyclonic and anticyclonic types of weather.

Air over the surface of the planet is distributed unevenly, so it flows from where there is a lot of it to areas where there is not enough air. As a result, there is a difference in air pressure on the earth's surface. When air masses flow in the atmosphere, vortices are formed.

An air funnel with low pressure in the center is called a cyclone, and with high pressure - an anticyclone. Cyclonic is called cloudy, snowy or rainy weather, anticyclonic - dry and clear, in winter - frosty.

Geographic atmospheric differences

The geographical classification of atmospheric fronts is based on two features:

geographic latitudes in which the formation of frontal zones occurs;
front-forming (atmospheric) underlying surface.

On the border climatic zones, differing by dominant air masses, belts of frontal zones are formed. There are three of them on the globe:

In the polar zone of the Northern and Southern Hemispheres, at the border of cold polar and temperate air masses, the Arctic (in the Northern Hemisphere) and Antarctic (in the Southern Hemisphere) frontal zones formed.
An atmospheric polar front has formed between temperate and tropical latitudes. He encircles Earth in the region of the northern and southern tropics.
The tropical frontal zone is located on the border of tropical and equatorial air.

Depending on the season, the zones shift in the meridional direction. Circulation processes in geographic frontal zones form climatic zones.

Underlying surface and frontal zones

Dry continental air masses form over the continent, and wet sea masses form over the ocean. In the process of atmospheric circulation, they also collide, frontal zones are formed at the boundary, in which the properties of the air are transformed. Marine and continental atmospheric fronts are formed. The types of weather associated with them depend on the properties of the air.

So, we have dealt with such a concept as an atmospheric front, the definition of which is as follows - this is the line of contact of air masses of different types. The properties of the atmospheric front depend on the direction in which the air masses move relative to each other. The passage of atmospheric fronts is always accompanied by a change weather conditions and atmospheric phenomena characteristic of each front.

The lower part of the Earth's atmosphere, the troposphere, is in constant motion, shifting over the surface of the planet and mixing. Its individual sections have different temperatures. When such atmospheric zones meet, atmospheric fronts arise, which are boundary zones between air masses of different temperatures.

Formation of an atmospheric front

The circulation of tropospheric currents causes warm and cold air currents to meet. At the place of their meeting, due to the temperature difference, active condensation of water vapor occurs, which leads to the formation of powerful clouds, and subsequently to heavy precipitation.

The boundary of atmospheric fronts is rarely even, it is always tortuous and inhomogeneous, due to the fluidity of air masses. Warmer atmospheric currents flow on cold air masses and rise up, colder ones displace warm air, forcing it to rise higher.

Rice. 1. Approach of the atmospheric front.

Warm air is lighter than cold air and always rises, cold air, on the contrary, accumulates near the surface.

Active fronts move with average speed 30-35 km. per hour, but they can temporarily stop their movement. Compared with the volume of air masses, the boundary of their contact, which is called the atmospheric front, is very small. Its width can reach hundreds of kilometers. In length - depending on the magnitude of the colliding air currents, the front can be thousands of kilometers long.

Signs of a weather front

Depending on which atmospheric current moves more actively, warm and cold fronts are distinguished.

Weather and atmospheric front

In areas where atmospheric fronts pass, the weather changes.

Rice. 3. Collision of warm and cold air currents.

Its changes depend on:

temperatures of the air masses encountered . How more difference temperatures - the stronger the winds, the more intense the precipitation, the more powerful the clouds. And vice versa, if the temperature difference of air currents is small, then the atmospheric front will be weakly expressed and its passage over the Earth's surface will not bring any special weather changes;
air current activity . Depending on their pressure, atmospheric flows can have different speeds of movement, on which the rate of weather change will depend;
front shapes . The simpler linear forms of the front surface are more predictable. With the formation of atmospheric waves or the closure of individual outstanding tongues of air masses, vortices are formed - cyclones and anticyclones.

After the passage of a warm front, the weather sets in with more high temperature. After the passage of the cold - there is a cooling.

What have we learned?

atmospheric fronts- These are the boundary regions between air masses with different temperatures. The greater the temperature difference, the more intense the weather change will be during the passage of the front. The approach of a warm or cold front can be distinguished by the shape of the clouds and the type of precipitation.

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We have considered the types of atmospheric fronts. But when forecasting the weather in yachting, it should be remembered that the types of atmospheric fronts considered reflect only the main features of the development of a cyclone. In reality, there may be significant deviations from this scheme.
Signs of an atmospheric front of any type can in some cases be pronounced, or exacerbated, in other cases - weakly expressed, or blurry.

If the type of atmospheric front is sharpened, then when passing through its line, the air temperature and other meteorological elements change sharply, if it is blurred, the temperature and other meteorological elements change gradually.

The processes of formation and sharpening of atmospheric fronts are called frontogenesis, and the processes of erosion are called frontolysis. These processes are observed continuously, just as air masses are continuously formed and transformed. This must be remembered when forecasting the weather in yachting.

For the formation of an atmospheric front, it is necessary to have at least a small horizontal temperature gradient and such a wind field, under the influence of which this gradient would increase significantly in a certain narrow band.

A special role in the formation and erosion different types atmospheric fronts are played by baric saddles and the deformation fields of the wind associated with them. If the isotherms in the transition zone between adjacent air masses are parallel to the extension axis or at an angle of less than 45° to it, then they converge in the deformation field and the horizontal temperature gradient increases. On the contrary, if the isotherms are located parallel to the compression axis or at an angle of less than 45° to it, the distance between them increases, and if an already formed atmospheric front falls under such a field, it will be washed out.

Surface profile of the atmospheric front.

The slope angle of the surface profile of the atmospheric front depends on the difference in temperature and wind speed of warm and cold air masses. At the equator, atmospheric fronts do not intersect with the earth's surface, but turn into horizontal layers of inversion. It should be noted that the slope of the surface of a warm and cold atmospheric front is somewhat influenced by air friction on the earth's surface. Within the friction layer, the velocity of the frontal surface increases with height, and above the friction level it almost does not change. This has a different effect on the surface profile of a warm and cold atmospheric front.

When the atmospheric front began to move as a warm front, in the layer where the speed of movement increases with height, the frontal surface becomes more sloping. A similar construction for a cold atmospheric front shows that, under the influence of friction, the lower part of its surface becomes steeper than the upper one, and can even get a reverse slope below, so that warm air earth's surface can be located in the form of a wedge under the cold. This complicates the prediction of future events in yachting.

Movement of atmospheric fronts.

An important factor in yachting is the movement of atmospheric fronts. The lines of atmospheric fronts on weather maps run along the axes of baric troughs. As is known, in a trough, the streamlines converge to the axis of the trough, and, consequently, to the line of the atmospheric front. Therefore, when passing it, the wind changes its direction rather sharply.

The wind vector at each point in front of and behind the atmospheric front line can be decomposed into two components: tangential and normal. For the movement of the atmospheric front, only the normal component of the wind speed matters, the value of which depends on the angle between the isobars and the front line. The speed of movement of atmospheric fronts can fluctuate over a very wide range, since it depends not only on the speed of the wind, but also on the nature of the pressure and thermal fields of the troposphere in its zone, as well as on the influence of surface friction. Determining the speed of movement of atmospheric fronts is extremely important in yachting when performing the necessary actions to avoid a cyclone.

It should be noted that the convergence of winds to the atmospheric front line in the surface layer stimulates upward air movements. Therefore, near these lines there are the most favorable conditions for the formation of clouds and precipitation, and the least favorable for yachting.

In the case of a sharp type of atmospheric front, a jet stream is observed above it and parallel to it in the upper troposphere and lower stratosphere, which is understood as narrow air flows with high speeds and large horizontal extent. Max speed observed along the slightly inclined horizontal axis of the jet stream. The length of the latter is measured in thousands, width - hundreds, thickness - several kilometers. The maximum wind speed along the axis of the jet stream is 30 m/sec or more.

The emergence of jet streams is associated with the formation of large horizontal temperature gradients in high-altitude frontal zones, which, as is known, determine the thermal wind.

The stage of a young cyclone continues until warm air remains in the center of the cyclone near the earth's surface. The duration of this stage is on average 12-24 hours.

Zones of atmospheric fronts of a young cyclone.

Let us once again note that, as in the initial stage of the development of a young cyclone, warm and cold fronts are two sections of the wave-like curved surface of the main atmospheric front, on which the cyclone develops. In a young cyclone, three zones can be distinguished, which differ sharply in terms of weather conditions, and, accordingly, in terms of conditions for yachting.

Zone I - the front and central parts of the cold sector of the cyclone ahead of the warm atmospheric front. Here, the nature of the weather is determined by the properties of the warm front. The closer to its line and to the center of the cyclone, the more powerful the cloud system and the more probable precipitation is, the pressure drop is observed.

Zone II - the rear part of the cold sector of the cyclone behind the cold atmospheric front. Here the weather is determined by the properties of a cold atmospheric front and a cold unstable air mass. With sufficient humidity and significant instability of the air mass, showers fall. Atmospheric pressure behind its line increases.

Zone III - warm sector. Since a warm air mass is predominantly moist and stable, the weather conditions in it usually correspond to those in a stable air mass.

The figure above and below shows two vertical sections through the cyclone area. The upper one is made to the north of the center of the cyclone, the lower one is to the south and crosses all three considered zones. The lower one shows the rise of warm air in the front of the cyclone above the surface of the warm atmospheric front and the formation of a characteristic cloud system, as well as the distribution of currents and clouds near the cold atmospheric front in the rear of the cyclone. The upper section crosses the surface of the main front only in the free atmosphere; only cold air near the earth's surface, warm air flows over it. The section passes through the northern edge of the area of frontal sediments.

The change in wind direction during the movement of the atmospheric front can be seen from the figure, which shows the streamlines of cold and warm air.

Warm air in a young cyclone moves faster than the disturbance itself moves. Therefore, more and more warm air flows through the compensation, descending along the cold wedge in the rear of the cyclone and ascending in its front part.

As the disturbance amplitude increases, the warm sector of the cyclone narrows: the cold atmospheric front gradually overtakes the slowly moving warm one, and there comes a moment when the warm and cold atmospheric fronts of the cyclone merge.

The central region of the cyclone near the earth's surface is completely filled with cold air, and warm air is pushed back into higher layers.

Atmospheric front, tropospheric fronts - a transitional zone in the troposphere between adjacent air masses with different physical properties.

An atmospheric front occurs when masses of cold and warm air approach and meet in the lower layers of the atmosphere or in the entire troposphere, covering a layer up to several kilometers thick, with the formation of an inclined interface between them.

Types :

warm front - an atmospheric front moving towards colder air (heat advection is observed). A warm air mass moves into the region behind a warm front.

On the weather map, a warm front is marked in red or as black semicircles pointing in the direction of the front movement. As the warm front line approaches, pressure begins to drop, clouds thicken, and heavy precipitation falls. In winter, when the front passes, low stratus clouds usually appear. The temperature and humidity of the air are slowly rising. When a front passes, temperature and humidity usually increase rapidly, and the wind increases. After the passage of the front, the direction of the wind changes (the wind turns clockwise), the pressure drop stops and its weak growth begins, the clouds dissipate, and precipitation stops. The baric tendencies field is represented as follows: a closed area of pressure drop is located in front of the warm front, and behind the front there is either an increase in pressure or a relative increase (a drop, but less than in front of the front).

In the case of a warm front, warm air, moving towards a cold front, flows into a wedge of cold air and performs an upward sliding along this wedge and is dynamically cooled. At a certain altitude, determined by the initial state of the rising air, saturation is reached - this is the level of condensation. Above this level, cloud formation occurs in the rising air. The adiabatic cooling of warm air sliding along the cold wedge is enhanced by the development of ascending motions from nonstationarity with a dynamic pressure drop and from wind convergence in the lower layer of the atmosphere. Cooling of warm air during an upward slip over the surface of the front leads to the formation of a characteristic system of stratus clouds (upward slip clouds): cirrus-stratus - high-stratus - nimbostratus (Cs-As-Ns).

When approaching a point of a warm front with well-developed cloudiness, cirrus clouds first appear in the form of parallel bands with claw-like formations in the front (harbingers of a warm front), elongated in the direction of air currents at their level (Ci uncinus). The first cirrus clouds are observed at a distance of many hundreds of kilometers from the front line near the Earth's surface (about 800-900 km). Cirrus clouds then pass into cirrostratus clouds (Cirrostratus). These clouds are characterized by halo phenomena. Clouds of the upper tier - cirrostratus and cirrus (Ci and Cs) consist of ice crystals, and precipitation does not fall out of them. Most often, Ci-Cs clouds are an independent layer, the upper boundary of which coincides with the axis of the jet stream, that is, close to the tropopause.

Then the clouds become denser: altostratus clouds (Altostratus) gradually turn into nimbostratus clouds (Nimbostratus), heavy precipitation begins to fall, which weaken or completely stop after passing the front line. As we approach the front line, the base height Ns decreases. Its minimum value is determined by the height of the level of condensation in the rising warm air. Highly stratified (As) are colloidal and consist of a mixture of tiny droplets and snowflakes. Their vertical power is quite significant: starting at a height of 3-5 km, these clouds extend to heights of the order of 4-6 km, that is, they are 1-3 km thick. The precipitation falling from these clouds in the summer, passing through the warm part of the atmosphere, evaporates and does not always reach the Earth's surface. In winter, precipitation from As in the form of snow almost always reaches the Earth's surface, and also stimulates precipitation from the underlying St-Sc. In this case, the wide precipitation zone can reach a width of 400 km or more. Closest to the Earth's surface (at a height of several hundred meters, and sometimes 100-150 m or even lower) is the lower boundary of nimbostratus clouds (Ns), from which heavy precipitation falls in the form of rain or snow; nimbus clouds often develop under nimbus clouds (St fr).

Clouds Ns extend to heights of 3...7 km, that is, they have a very significant vertical power. The clouds also consist of ice elements and drops, and the drops and crystals, especially in the lower part of the clouds, are larger than in As. The lower base of the As-Ns cloud system in in general terms coincides with the surface of the front. Since the upper boundary of the As-Ns clouds is approximately horizontal, their greatest thickness is observed near the front line. Near the center of the cyclone, where the warm front cloud system has greatest development, the width of the cloudy zone Ns and the zone of extensive precipitation is, on average, about 300 km. In general, As-Ns clouds have a width of 500-600 km, the width of the Ci-Cs cloud zone is about 200-300 km. If projected this system on a surface map, then all of it will be in front of the warm front line at a distance of 700-900 km. In some cases, the zone of cloudiness and precipitation can be much wider or narrower, depending on the angle of inclination of the frontal surface, the height of the condensation level, and the thermal conditions of the lower troposphere.

At night, radiative cooling of the upper boundary of the As-Ns cloud system and a decrease in temperature in the clouds, as well as increased vertical mixing when the cooled air descends into the cloud, contribute to the formation of an ice phase in the clouds, the growth of cloud elements and the formation of precipitation. As you move away from the center of the cyclone, the ascending air movements weaken, and precipitation stops. Frontal clouds can form not only above the inclined surface of the front, but in some cases - on both sides of the front. This is especially typical for the initial stage of the cyclone, when ascending movements capture the region behind the front - then precipitation can also fall from both sides of the front. But behind the front line, the frontal cloudiness is usually highly stratified, and behind the frontal precipitation is more often in the form of drizzle or snow grains.

In the case of a very flat front, the cloud system can be shifted forward from the front line. In the warm season, ascending movements near the front line become convective, and cumulonimbus clouds often develop on warm fronts and showers and thunderstorms are observed (both during the day and at night).

In summer, in the daytime, in the surface layer behind the warm front line, with significant cloud cover, the air temperature over land can be lower than ahead of the front. This phenomenon is called warm front masking.

The cloudiness of old warm fronts can also be stratified along the entire length of the front. Gradually, these layers dissipate and precipitation stops. Sometimes a warm front is not accompanied by precipitation (especially in summer). This happens when the moisture content of warm air is low, when the level of condensation lies at a considerable height. When the air is dry, and especially in the case of its noticeable stable stratification, the upward sliding of warm air does not lead to the development of more or less powerful clouds - that is, there are no clouds at all, or a band of clouds of the upper and middle tiers is observed.

cold front - an atmospheric front (a surface separating warm and cold air masses) moving towards warm air. Cold air advances and pushes warm air: cold advection is observed, a cold air mass comes to the region behind the cold front.

On the weather map, a cold front is marked in blue or as black triangles pointing in the direction of the front movement. When crossing the line of a cold front, the wind, as in the case of a warm front, turns to the right, but the turn is more significant and sharp - from the southwest, south (in front of the front) to the west, northwest (behind the front). This increases the wind speed. Atmospheric pressure ahead of the front changes slowly. It can fall, but it can also grow. With the passage of a cold front, a rapid increase in pressure begins. Behind the cold front, the pressure increase can reach 3–5 hPa/3 h, and sometimes 6–8 hPa/3 h or even more. A change in the pressure trend (from falling to rising, from slow to stronger growth) indicates the passage of a surface front line.

Before the front, precipitation is often observed, and often thunderstorms and squalls (especially in the warm half of the year). The air temperature after the passage of the front drops (cold advection), and sometimes quickly and sharply - by 5 ... 10 ° C or more in 1-2 hours. The dew point decreases along with the air temperature. Visibility tends to improve as the cold front is invaded by a cleaner and less wet air from northern latitudes.

The nature of the weather on a cold front differs markedly depending on the speed of the front displacement, the properties of warm air in front of the front, and the nature of the ascending motions of warm air above the cold wedge.

There are two types of cold fronts:

cold front of the first kind, when cold air advances slowly,

cold front of the second kind, accompanied by a rapid onset of cold air.

Front of occlusion - an atmospheric front associated with a heat ridge in the lower and middle troposphere, which causes large-scale ascending air movements and the formation of an extended zone of clouds and precipitation. Often, the occlusion front occurs due to closure - the process of displacing warm air upwards in the cyclone due to the fact that the cold front “catches up” with the warm front moving ahead and merges with it (the process of cyclone occlusion). Occlusion fronts are associated with intense precipitation, in summer - heavy showers and thunderstorms.

Due to downward movements in the cold air behind the cyclone, the cold front moves faster than the warm front and overtakes it over time. At the stage of cyclone filling, complex fronts arise - occlusion fronts, which are formed when cold and warm atmospheric fronts meet. In the occlusion front system, three air masses interact, of which the warm one no longer comes into contact with the Earth's surface. Warm air in the form of a funnel gradually rises up, and its place is taken by cold air coming from the sides. The interface that occurs when the cold and warm fronts meet is called the occlusion front surface. Occlusion fronts are associated with intense precipitation, and strong thunderstorms in summer.

Air masses closing during occlusion usually have different temperature- one may be colder than the other. In accordance with this, two types of occlusion fronts are distinguished - occlusion fronts of the warm front type and occlusion fronts of the cold front type.

AT middle lane In Russia and the CIS, warm fronts of occlusion predominate in winter, since temperate sea air enters the rear of the cyclone, which is much warmer than continental temperate air in front of the cyclone. In summer, cold fronts of occlusion are mainly observed here.

The baric field of the occlusion front is represented by a well-defined trough with V-shaped isobars. In front of the front on the synoptic map there is an area of pressure drop associated with the surface of the warm front, behind the front of occlusion there is an area of pressure increase associated with the surface of the cold front. The point on the synoptic map from which the remaining open sections of the warm and cold fronts in the occluding cyclone diverge is the point of occlusion. As the cyclone occludes, the occlusion point shifts to its periphery.

In the anterior part of the occlusion front, cirrus (Ci), cirrostratus (Cs), altostratus (As) clouds are observed, and in the case of active occlusion fronts, nimbostratus (Ns). If a cold front of the first kind is involved in the occlusion, then a part of the cold front cloud system may remain above the upper warm front. If a cold front of the second kind is involved, then a clearing occurs behind the upper warm front, but a shaft of cumulonimbus clouds (Cb) can develop near the lower cold front already in the front cold air, displaced by a colder rear wedge. Thus, precipitation from Altostratus and Doge Stratoclouds (As-Ns), if it occurs, may begin before the occurrence of showers, either simultaneously with or after the passage of a lower cold front; Precipitation can fall on both sides of the lower front, and the transition from heavy precipitation to showers, if it occurs, occurs not ahead of the lower front, but in close proximity to it.

The approaching cloud systems of warm and cold fronts mainly consist of As-Ns. As a result of the approach, a powerful Cs-As-Ns cloud system arises with the greatest thickness at the upper cold front. In the case of a young occlusion front, the cloud system starts with Ci and Cs, which change to As, then to Ns. Sometimes Ns can be followed by Cb, followed again by Ns. A weak upward sliding of the rear air along the occlusion surface can lead to the formation of stratus and stratocumulus (St-Sc) clouds along it, which do not reach the level of ice cores. Of these, drizzling precipitation will fall in front of the lower warm front. In the case of an old warm front of occlusion, the cloud system consists of cirrostratus (Cs) and altocumulus (Ac) clouds, sometimes joined by altostratus (As); rainfall may be absent.

Stationary front

1. A front that does not change its position in space.

2. A front along which air masses move horizontally; front without slips.

32) cyclones and anticyclones. Stages of their development, systems of winds and clouds in them.

Anticyclone- an area of high atmospheric pressure with closed concentric isobars at sea level and with a corresponding wind distribution. In a low anticyclone - cold, the isobars remain closed only in the lowest layers of the troposphere (up to 1.5 km), and in the middle troposphere, increased pressure is not detected at all; the presence of a high-altitude cyclone above such an anticyclone is also possible.