Air fronts. Atmospheric fronts. Reasons for education. Types of atmospheric fronts. Atmospheric front - definition

ATMOSPHERE FRONT (tropospheric front), an intermediate, transition zone between air masses in the lower part of the atmosphere - the troposphere. The zone of the atmospheric front is very narrow compared to the air masses it separates, therefore it is approximately considered as the interface (break) of two air masses of different densities or temperatures and is called the frontal surface. For the same reason, on synoptic maps the atmospheric front is depicted as a line (front line). If the air masses were stationary, the surface of the atmospheric front would be horizontal, with cold air below and warm air above it, but since both masses are moving, it is located obliquely to the earth's surface, with the cold air lying in the form of a very gentle wedge under the warm one. The tangent of the angle of inclination of the frontal surface (front inclination) is about 0.01. Atmospheric fronts can sometimes extend all the way to the tropopause, but they can also be limited to the lower kilometers of the troposphere. At the intersection with the earth's surface, the zone of the atmospheric front has a width of the order of tens of kilometers, while the horizontal dimensions of the air masses themselves are of the order of thousands of kilometers. At the beginning of the formation of atmospheric fronts and when they are washed out, the width of the frontal zone will be greater. Vertically, atmospheric fronts represent a transition layer hundreds of meters thick, in which the temperature with height decreases less than usual or increases, that is, a temperature inversion is observed.

At the earth's surface, atmospheric fronts are characterized by increased horizontal gradients of air temperature - in a narrow zone of the front, the temperature sharply changes from values ​​characteristic of one air mass to values ​​characteristic of another, and the change sometimes exceeds 10 ° C. Air humidity and transparency also change in the frontal zone. In a pressure field, atmospheric fronts are associated with troughs of low pressure (see Pressure systems). Extensive cloud systems form above the frontal surfaces, producing precipitation. Atmospheric front moves at a speed equal to the normal component to the wind speed front, therefore the passage of the atmospheric front through the observation location leads to a rapid (within hours) and sometimes sharp change in important meteorological elements and the entire weather regime.

Atmospheric fronts are characteristic of temperate latitudes, where the main air masses of the troposphere border each other. In the tropics, atmospheric fronts are rare, and the intertropical convergence zone, which is constantly present there, differs significantly from them, not being a temperature division. The main reason for the emergence of an atmospheric front (frontogenesis) is the presence of such systems of movement in the troposphere that lead to the convergence (convergence) of air masses with different temperatures. The initially wide transition zone between air masses becomes a sharp front. IN special cases the formation of an atmospheric front is possible when air flows along a sharp temperature boundary on the underlying surface, for example, over the edge of ice in the ocean (the so-called topographic frontogenesis). In the process of general circulation of the atmosphere between air masses of different latitudinal zones with sufficiently large temperature contrasts, long (thousands of km) main fronts, predominantly elongated in latitude, arise - Arctic, Antarctic, polar, on which cyclones and anticyclones form. In this case, the dynamic stability of the main atmospheric front is disrupted, it is deformed and moves in some areas to high latitudes, in others - to low latitudes. On both sides of the surface of the atmospheric front, vertical components of wind speed of the order of cm/s appear. Particularly important is the upward movement of air above the surface of the atmospheric front, which leads to the formation of cloud systems and precipitation.

In the front part of the cyclone, the main atmospheric front takes on the character of a warm front (Figure a), as it moves toward high latitudes, warm air takes the place of retreating cold air. In the rear part of the cyclone, the atmospheric front takes on the character of a cold front (Figure b) with the cold wedge moving forward and displacing warm air in front of it into high layers. When a cyclone occludes, a warm and cold atmospheric front combines, forming a complex occlusion front with corresponding changes in cloud systems. As a result of the evolution of frontal disturbances, the atmospheric fronts themselves are blurred (the so-called frontolysis). However, changes in the field of atmospheric pressure and wind created by cyclonic activity lead to the emergence of conditions for the formation of new atmospheric fronts and, consequently, to the constant resumption of the process of cyclonic activity on the fronts.

In the upper part of the troposphere, in connection with the atmospheric front, so-called jet streams arise. Secondary atmospheric fronts that arise within air masses of one or another are distinguished from the main fronts. natural area with some heterogeneity; they do not play a significant role in the general circulation of the atmosphere. There are cases when the atmospheric front is well developed in the free atmosphere (upper atmospheric front), but is little expressed or does not appear at all near the earth's surface.

Lit.: Petersen S. Weather analysis and forecasts. L., 1961; Palmen E., Newton Ch. Circulation systems of the atmosphere. L., 1973; Ocean - atmosphere: Encyclopedia. L., 1983.

Watching the weather changes is very exciting. The sun gives way to rain, rain to snow, and gusty winds blow over all this diversity. In childhood, this causes admiration and surprise; in older people, it causes a desire to understand the mechanism of the process. Let's try to understand what shapes the weather and how atmospheric fronts are related to it.

Air mass boundary

In the usual perception, “front” is a military term. This is the edge on which the clash of enemy forces occurs. And the concept of atmospheric fronts is the boundaries of contact between two air masses that form over vast areas of the Earth's surface.

By the will of nature, man got the opportunity to live, evolve and populate everything large areas. The troposphere - the lower part of the Earth's atmosphere - provides us with oxygen and is in continuous motion. It all consists of individual air masses, united by a common occurrence and similar indicators. Among the main indicators of these masses are volume, temperature, pressure and humidity. While driving various masses can come close and collide. However, they never lose their boundaries and do not mix with each other. - these are areas where sharp weather changes come into contact and occur.

A little history

The concepts of “atmospheric front” and “frontal surface” did not arise on their own. They were introduced into meteorology by the Norwegian scientist J. Bjerknes. This happened in 1918. Bjerknes proved that atmospheric fronts are the main links in the high and middle layers. However, before the Norwegian’s research, back in 1863, Admiral Fitzroy suggested that violent atmospheric processes begin at meeting points of air masses coming from different directions of the world. But at that moment science community did not pay attention to these observations.

The Bergen School, of which Bjerknes was a representative, not only made its own observations, but also brought together all the knowledge and assumptions expressed by earlier observers and scientists, and presented them in the form of a coherent scientific system.

A-priory, inclined surface, which is the transition area between different air flows, is called the frontal surface. But atmospheric fronts are the display of frontal surfaces on a meteorological map. Typically, the transition region of the atmospheric front begins at the Earth's surface and rises up to those heights at which the differences between air masses are blurred. Most often, the threshold of this altitude ranges from 9 to 12 km.

Warm front

Atmospheric fronts are different. They depend on the direction of movement of warm and cold masses. There are three types of fronts: cold, warm and occlusion, formed at the junction of different fronts. Let's take a closer look at what warm and cold atmospheric fronts are.

A warm front is a movement of air masses in which cold air gives way to warm air. That is, the air is more high temperature, moving forward, is located in an area where cold air masses dominated. In addition, it rises upward along the transition zone. At the same time, the air temperature gradually decreases, which causes condensation of the water vapor in it. This is how clouds are formed.

The main signs by which a warm atmospheric front can be identified:

• atmospheric pressure drops sharply;
• increases ;
• air temperature rises;
• cirrus clouds appear, then cirrostratus clouds, and then altostratus clouds;
• the wind turns slightly to the left and becomes stronger;
• clouds become nimbostratus;
• Precipitation of varying intensity falls.

Usually, after the precipitation stops, it gets warmer, but this does not last long, since the cold front moves very quickly and catches up with the warm atmospheric front.

Cold front

The following feature is observed: a warm front is always inclined in the direction of movement, and a cold front is always inclined in the opposite direction. When fronts move, cold air wedges into warm air, pushing it upward. Cold weather fronts lead to lower temperatures and cooling over a large area. As rising warm air masses cool, moisture condenses into clouds.

The main signs by which a cold front can be identified:

• before the front the pressure drops, behind the atmospheric front it rises sharply;
• cumulus clouds form;
• a gusty wind appears, with a sharp change in direction clockwise;
• heavy rain begins with thunderstorms or hail, the duration of precipitation is about two hours;
• the temperature drops sharply, sometimes by 10°C immediately;
• Numerous clearings are observed behind the atmospheric front.

For travelers, navigating through a cold front is no easy task. Sometimes you have to overcome whirlwinds and squalls in poor visibility conditions.

Front of occlusions

It has already been said that there are different atmospheric fronts; if everything is more or less clear with warm and cold ones, then the front of occlusions raises a lot of questions. The formation of such effects occurs in places where cold and warm fronts meet. Warmer air is forced upward. The main action occurs in cyclones at the moment when a faster cold front overtakes a warm one. As a result, atmospheric fronts move and three air masses collide, two cold and one warm.

The main signs by which the front of occlusions can be determined:

• clouds and precipitation of the blanket type;
• sudden changes without a strong change in speed;
• smooth pressure change;
• no sudden temperature changes;
• cyclones.

The front of occlusions depends on the temperature of cold air masses in front of it and behind its line. There are cold and warm fronts of occlusions. The most difficult conditions observed at the moment of direct closure of the fronts. As warm air is forced out, the front erodes and improves.

Cyclone and anticyclone

Since the concept of “cyclone” was used in the description of the occlusion front, it is necessary to tell what kind of phenomenon this is.

Due to the uneven distribution of air in the surface layers, zones of high and low pressure. Zones high pressure characterized by an excess amount of air, low - by an insufficient amount. As a result of the flow of air between zones (from excess to insufficient), wind is formed. A cyclone is an area of ​​low pressure that draws in, as if into a funnel, the missing air and clouds from areas where they are in abundance.

Anticyclone - area with high blood pressure, which displaces excess air into low pressure areas. The main characteristic is clear weather, since clouds are also displaced from this zone.

Geographical separation of atmospheric fronts

Depending on the climatic zones, over which atmospheric fronts form, they are divided geographically into:

1. Arctic, separating cold Arctic air masses from temperate ones.
2. Polar, located between the temperate and tropical masses.
3. Tropical (trade wind), delimiting the tropical and equatorial zones.

Influence of the underlying surface

On physical properties air masses are influenced by radiation and the appearance of the Earth. Since the nature of such a surface can be different, friction against it occurs unevenly. Complex geographic terrain can deform the line of an atmospheric front and change its effects. For example, there are known cases of the destruction of atmospheric fronts when crossing mountain ranges.

Air masses and atmospheric fronts bring many surprises to weather forecasters. By comparing and studying the directions of movement of masses and the vagaries of cyclones (anticyclones), they create graphs and forecasts that people use every day, without even thinking about how much work is behind it.

Atmospheric fronts have several various characteristics. According to them, this natural phenomenon is divided into different types.

Atmospheric fronts can reach a width of 500-700 km and a length of 3000-5000 km.

Characteristics of atmospheric fronts

Based on their movement, atmospheric fronts can be divided into cold, warm and occlusion fronts.
A warm atmosphere is formed when warm air masses, usually moist, move over drier and colder ones. An approaching warm front brings a gradual decrease in atmospheric pressure, a slight increase in air temperature and light but prolonged precipitation.

A cold front is formed by the influence of northerly winds, which push cold air into areas previously occupied by a warm front. A cold front affects the weather over a small area and is often accompanied by thunderstorms and a decrease in atmospheric pressure. After the front passes, the air temperature drops sharply and the pressure increases.

Considered the most powerful and destructive cyclone in history, it struck the Ganges Delta in eastern Pakistan in November 1970. The wind speed reached more than 230 km/h, and the height of the tidal wave was about 15 meters.

Occlusion fronts arise when one atmospheric front superimposes on another, formed earlier. Between them there is a significant mass of air, the temperature of which is much higher than that of the air that surrounds it. Occlusion occurs when a warm air mass is displaced and separated from the surface of the earth. As a result, the front will mix at the surface of the earth under the influence of two cold air masses. On the occlusion fronts there are often deep wave cyclones formed in the form of very chaotic wave disturbances. At the same time, the wind increases significantly, and the wave becomes clearly defined. As a result, the occlusion front turns into a large blurred frontal zone and, after some time, completely disappears.

Based on geographic characteristics, fronts are divided into arctic, polar and tropical. Depending on the latitudes in which they are formed. In addition, depending on the underlying surface, fronts are divided into continental and sea.

We looked at 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 pattern.
Signs of an atmospheric front of any type can in some cases be pronounced or aggravated, in other cases - weakly expressed or blurred.

If the type of atmospheric front is aggravated, 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 aggravation 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, the existence of at least a small horizontal temperature gradient and such a wind field must exist, under the influence of which this gradient would significantly increase in a certain narrow band.

Special role in formation and erosion different types atmospheric fronts are played by pressure saddles and associated wind deformation fields. If isotherms in the transition zone between neighboring air masses are located parallel to the stretch axis or at an angle of less than 45° to it, then in the deformation field they come closer and the horizontal temperature gradient increases. On the contrary, when 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.

Profile of the surface of the atmospheric front.

The angle of inclination 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 inversion layers. It should be noted that the amount of inclination of the surface of a warm and cold atmospheric front is somewhat influenced by air friction against earth's surface. Within the friction layer, the speed of movement of the frontal surface increases with height, and above the friction level it remains almost unchanged. This affects the surface profile of warm and cold atmospheric fronts differently.

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, and can even receive a reverse slope below, so that warm air at the earth’s surface can be located in the form of a wedge under the cold. This makes it difficult to predict subsequent 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 pressure troughs. As is known, in a trough, streamlines converge to the axis of the trough, and, consequently, to the line of the atmospheric front. Therefore, when passing through it, the wind changes its direction quite 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 an atmospheric front, only the normal component of wind speed is important, the value of which depends on the angle between the isobars and the front line. The speed of movement of atmospheric fronts can fluctuate within very wide limits, since it depends not only on the wind speed, 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 cloud formation 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. Maximum speed is noted along the slightly inclined horizontal axis of the jet stream. The length of the latter is measured in thousands, width - in hundreds, thickness - in 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, cause thermal wind.

The young cyclone stage continues as long as 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 note once again that, as in the initial stage of development of a young cyclone, the warm and cold fronts represent 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, sharply differing in weather conditions, and, accordingly, in conditions for yachting.

Zone I is the front and central parts of the cold sector of the cyclone before the warm atmospheric front. Here the weather pattern is determined by the properties of the warm front. The closer to its line and to the center of the cyclone, the more powerful system clouds and the more likely precipitation is, a drop in pressure is observed.

Zone II is the rear part of the cold sector of the cyclone behind the cold atmospheric front. Here the weather is determined by the properties of the cold atmospheric front and the cold unstable air mass. With sufficient humidity and significant instability of the air mass, rainfall occurs. Atmosphere pressure behind his line grows.

Zone III - warm sector. Because a warm air mass is predominantly moist and stable, its weather conditions generally correspond to those of a stable air mass.

The figure above and below shows two vertical sections through the cyclone area. The upper one is made north of the center of the cyclone, the lower one is made to the south and crosses all three considered zones. The bottom shows the rise of warm air in the front part 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 part of the cyclone. The upper section intersects the surface of the main front only in the free atmosphere; There is only cold air at the earth's surface, warm air flows above it. The section passes through the northern edge of the region of frontal precipitation.

The change in wind direction as the atmospheric front moves can be seen from the figure, which shows the flow lines of cold and warm air.

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

As the amplitude of the disturbance increases, the warm sector of the cyclone narrows: the cold atmospheric front gradually catches up with the slowly moving warm one, and a moment comes when the warm and cold atmospheric fronts of the cyclone close together.

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

It turned out that warm air is drawn into the cyclone not throughout its entire eastern (right) half, but in a fairly limited sector located in the southern and southeastern parts of the cyclone between two convergence lines. Cloudiness and precipitation are distributed unevenly in the cyclone. Covering rains fall mainly in front of the first (eastern) line of convergence of air flows, as well as in the center of the cyclone. Showers and thunderstorms are concentrated in a narrow band along the second (western) convergence line. These lines were subsequently called atmospheric fronts. Since in temperate latitudes Cyclones usually move from west to east; the eastern front of the cyclone first passes through the observation point, followed by warm air. This atmospheric front was called a warm front. In the vicinity of a warm atmospheric front, warm air actively advances onto the front line, moves almost perpendicular to it, and cold air is transported almost parallel to this line, i.e. slowly retreats from her. Consequently, the warm air mass catches up and overtakes the cold one. Then the western (cold) front of the cyclone approaches the observation point, as it passes the air temperature drops sharply. Near a cold atmospheric front, the dynamics are different: cold air catches up with warm air and rapidly displaces it upward.

The upward sliding covers thick layers of warm air over the entire frontal surface and an extensive system of high-stratus clouds with overlying precipitation arises. The warm front has an anticyclonic curvature and moves towards the cold air. On a weather map, a warm front is marked in red or with blackened semicircles directed in the direction of the front’s movement (Fig. 1). As the warm front line 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 direction of the wind changes (the wind turns clockwise), its speed decreases, the pressure drop stops and its slight increase begins, the clouds dissipate, and precipitation stops. The field of baric tendencies is presented as follows: before warm front there is a closed area of ​​pressure drop; behind the front there is either an increase in pressure or a relative increase (a drop, but less than in front of the front). The passage of a warm front is usually accompanied by a powerful layer of rain covering the entire sky with heavy rain. The first sign of a warm front is cirrus clouds. Gradually they turn into a continuous white veil of cirrostratus clouds. IN upper layers Warm air is already moving into the atmosphere. The pressure drops. The closer the front line is to us, the denser the clouds become. The sun shines through as a dim spot. Then the clouds drop lower and the sun disappears completely. The wind intensifies and changes its direction clockwise (for example, at first it was easterly, then southeasterly and even southwest). Approximately 300-400 km before the front, the clouds thicken. Light continuous rain or snow begins. But the warm front has passed. The rain or snow has stopped, the clouds are dissipating, warming is coming - a warmer air mass has arrived. The warm front in a vertical section is shown in Fig. 2.

If the warm air retreats and the cold air spreads out after it, it means that a cold front is approaching. His arrival always causes a chill. But when moving, not all layers of air have the same speed. As a result of friction with the earth's surface, the lowest layer is slightly delayed, while the higher ones are pulled forward. Thus, cold air falls on warm air in the form of a shaft. Warm air is quickly forced upward and powerful piles of cumulus and cumulonimbus clouds are created. The clouds of the cold front carry showers, thunderstorms, accompanied by strong gusty winds. They can reach very high altitude, but in the horizontal direction they extend only 20...30 km. And since the cold front usually moves quickly, stormy weather does not last long - from 15...20 minutes. up to 2...3 hours. As a result of the interaction of cold air with the warm underlying surface, cumulus clouds with gaps are formed. Then comes complete clarity.

In the case of a cold front, the upward movement of warm air is limited to a narrower zone and is especially strong in front of the cold wedge, where warm air is displaced by cold air. The clouds here will be largely cumulonimbus with showers and thunderstorms (Fig. 3, Fig. 4). A cold front has a cyclonic curvature (bulges toward warm air) and moves toward warm air. On a weather map, a cold front is marked in blue or with blackened triangles directed in the direction the front is moving (Fig. 1). The flow in cold air has a component directed towards the front line, so cold air, moving forward, occupies the space where warm air was previously located, which increases its instability.

When crossing the line of a warm 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 southwestern, southern (in front of the front) to the western, northwestern (behind the front). At the same time, the wind speed increases. Atmospheric pressure changes slowly ahead of the front. It may fall, but it can also rise. With the passage of a cold front, a rapid increase in pressure begins. Behind the cold front there is a closed isallobaric region of pressure growth, and the increase can reach 3-5 hPa/3h. A change in pressure in the direction of its growth (from a decrease to an increase, from a slow increase to a stronger one) indicates the passage of the surface front line.

Thunderstorms and squalls are common ahead of the front. After the front passes, the air temperature drops, often quickly and sharply – by 10 °C or more in 1-2 hours. The mass fraction of water vapor decreases simultaneously with the air temperature. Visibility typically improves as polar or arctic air moves in behind the cold front. In addition, the instability of the air mass prevents condensation near the Earth's surface.

The nature of the weather on a cold front varies markedly depending on the speed of the front's movement, the properties of warm air ahead of the front, and the nature of the upward movements of warm air above the cold wedge. Cold fronts of the 1st type are dominated by an orderly rise of warm air above a wedge of cold air. A type 1 cold front is a passive upward sliding surface. Slowly moving or slowing down their movement fronts belong to this type, mainly on the periphery of cyclonic regions in deep baric troughs. In this case, the clouds are located mainly behind the front line. There is still a difference from the cloudiness of a warm front. Due to friction, the surface of the cold front in lower layers becomes cool. Therefore, just before the front line, instead of a calm and gentle upward sliding, a steeper (convective) rise of warm air is observed (Fig. 3). Due to this, powerful cumulus and cumulonimbus clouds sometimes appear in the front part of the cloud system, stretching for hundreds of kilometers along the front, with showers in summer, snowfall in winter, thunderstorms, hail and squalls. Over the overlying part of the frontal surface with a normal slope as a result of the upward sliding of warm air, the cloud system represents a uniform cover of stratus clouds. Rainfall ahead of the front after the passage of the front is replaced by more uniform blanket precipitation. Finally, cirrostratus and cirrus clouds appear. The vertical power of the system and the width of the cloud system and precipitation area will be almost 2 times less than in the case of a warm front. The upper boundary of the system is approximately at an altitude of 4-4.5 km. Stratus broken clouds may develop beneath the main cloud system, and frontal fogs may sometimes form. The duration of passage of a cold front of the 1st type through an observation point is 10 hours or more.

Fronts of the 2nd kind in the lower layer of the atmosphere are a passive ascending slip surface, and above them are an active downward sliding surface. Most fast-moving cold fronts in cyclones belong to this type. This is where warm air is displaced lower layers upward moving forward cold shaft. The surface of the cold front in the lower layers is very steep, even forming a bulge in the form of a shaft (Fig. 4). The rapid movement of a wedge of cold air causes forced convection of displaced warm air in a narrow space at the front of the frontal surface. Here a powerful convective flow is created with the formation of cumulonimbus clouds, intensifying as a result of thermal convection. The harbingers of the front are altocumulus lentiform clouds, which spread in front of it at a distance of up to 200 km. The emerging cloud system has a small width (50-100 km) and does not represent individual convective clouds, but a continuous chain, or a cloud bank, which sometimes may not be continuous. In the warm half of the year, the upper boundary of cumulonimbus clouds extends to the height of the tropopause. On cold fronts of the 2nd type, intense thunderstorm activity, showers, sometimes with hail, and squally winds are observed. There is strong turbulence and icing in the clouds. Zone width hazardous phenomena weather distance is several tens of kilometers. In the cold half of the year, the tops of cumulonimbus clouds reach 4 km. The width of the snowfall zone is 50 km. This cloudiness is associated with heavy snowfalls, blizzards with visibility less than 1000 m, a sharp increase in wind speed, and roughness.

When cold fronts of the 2nd type pass through an observation point, cirrus clouds first appear (3-4 hours before the front line passes near the Earth), which are quickly replaced by altostratus, sometimes lenticular, which are quickly replaced by a huge cloud with showers, thunderstorms, hail, and squalls. The duration of movement of a cloud system with showers and thunderstorms usually does not exceed 1-2 hours. After the cold front passes, the rainfall stops. A feature of cold fronts of both the first and second types are prefrontal squalls. Since in the front part of the cold wedge, due to friction, a steep slope of the frontal surface is created, part of the cold air appears above the warm one. Next, the cold air masses “collapse” down in the front part of the advancing cold shaft. The collapse of cold air leads to the displacement of warm air upward and to the appearance of a vortex with a horizontal axis along the front. Squalls on land are especially intense in the summer, when there is a large temperature difference between warm and cold air on both sides of the front and when warm air is unstable. Under these conditions, the passage of a cold front is accompanied by destructive wind speeds. The wind speed often exceeds 20-30 m/s, the duration of the phenomenon is usually several minutes, and gusts are sometimes observed.

Occlusion fronts
Due to downward movements in the cold air at the rear of the cyclone, the cold front moves faster than the warm front and over time catches up with it. At the stage of filling the cyclone, complex fronts arise - occlusion fronts, which are formed when cold and warm atmospheric fronts close.

In the occlusion front system, three air masses interact, of which the warm one no longer comes into contact with the Earth’s surface. The process of displacing warm air into the upper layers is called occlusion. In this case, the rear wedge of cold air of the cyclone closes with the front wedge of cold air. Warm air in the form of a funnel gradually rises upward, and its place is taken by cold air coming from the sides (Fig. 5). The interface that occurs when cold and warm fronts meet is called the occlusion front surface.

In the case of an occluded cold front, precipitation can occur on both sides of the lower front, and the transition from blanket precipitation to showers, if it occurs, occurs not ahead of the lower front, but in close proximity to it. In the case of a warm occlusion front, the warm air funnel is displaced by more warm air, flowing onto the wedge of colder air. The rear wedge of less cold air overtakes the front wedge of colder air, and the cold front, having separated from the Earth's surface, rises along the surface of the warm front.

A weak upward sliding of the rear air along the front along the occlusion surface can lead to the formation of St-Sc clouds along it, not reaching the level of the ice cores. These will produce some drizzle ahead of the lower warm front.