Atmospheric fronts, or simply fronts, are transition zones between two different air masses. The transition zone starts from the surface of the Earth and extends upward to the height where the differences between air masses are erased (usually to the upper border of the troposphere). The width of the transition zone at the Earth's surface does not exceed 100 km.
In the transition zone - the zone of contact of air masses - there are sharp changes in the values of meteorological parameters (temperature, humidity). There is significant cloudiness, the most precipitation falls, the most intense changes in pressure, wind speed and direction occur.
Depending on the direction of movement of warm and cold air masses located on both sides of the transition zone, the fronts are divided into warm and cold. Fronts that change their position little are called sedentary. A special position is occupied by the fronts of occlusion formed when the warm and cold fronts meet. Occlusion fronts can be either cold or warm fronts. On weather maps, fronts are drawn either by colored lines or by symbols (see Fig. 4). Details on each of these fronts will be discussed below.
2.8.1. Warm front
If the front moves in such a way that cold air recedes, giving way to warm air, then such a front is called warm. Warm air, moving forward, not only occupies the space where the cold air used to be, but also rises up along the transition zone. As it rises, it cools down and the water vapor in it condenses. As a result, clouds are formed (Fig. 13).Fig, 13. Warm front in the vertical section and on the weather map.
The figure shows the most typical cloudiness, precipitation and air currents of the warm front. The first sign of a warm front approaching will be the appearance of cirrus clouds (Ci). At the same time, the pressure will begin to drop. After a few hours, the cirrus clouds, getting denser, pass into a veil of cirrostratus clouds (Cs). Following cirrostratus clouds, even denser high-stratus clouds (As) flow, gradually becoming opaque by the moon or sun. At the same time, the pressure drops more, and the wind, turning slightly to the left, increases. Precipitation can fall from highly layered clouds, especially in winter, when they do not have time to evaporate along the way.
After a while, these clouds turn into stratus (Ns), under which there are usually broken-rain (Frob) and broken-stratus (Frst). Precipitation from stratus clouds falls more intensively, visibility deteriorates, pressure drops rapidly, wind intensifies, and often becomes gusty. When crossing the front, the wind turns sharply to the right, the pressure drop stops or slows down. Precipitation may stop, but usually it only weakens and turns into drizzling. The temperature and humidity of the air are gradually increasing.
Difficulties that can be encountered when crossing a warm front are mainly associated with a long stay in a zone of poor visibility, the width of which ranges from 150 to 200 nm. It is necessary to know that sailing conditions in temperate and northern latitudes, when crossing a warm front in the cold half of the year, worsen due to the expansion of the zone of poor visibility and possible icing.
2.8.2. Cold front
A cold front is a front moving towards a warm air mass. There are two main types of cold fronts:1) cold fronts of the first kind - slowly moving or decelerating fronts, which are most often observed on the periphery of cyclones or anticyclones;
2) cold fronts of the second kind - fast moving or moving with acceleration, they arise in the inner parts of cyclones and troughs moving at high speed.
Cold front of the first kind. A cold front of the first kind, as has been said, is a slowly moving front. In this case, warm air slowly rises upward along a wedge of cold air that invades under it (Fig. 14).
As a result, over the separation zone, first stratus clouds (Ns) are formed, transforming at some distance from the front line into high-stratus (As) and cirrostratus (Cs) clouds. Precipitation begins to fall at the very front line and continues after its passage. The width of the zone of the frontal precipitation is 60-110 NM. In the warm season, in the front of such a front, favorable conditions are created for the formation of powerful cumulonimbus clouds (Cb), from which heavy rainfalls accompanied by thunderstorms fall.
The pressure just before the front drops sharply and a characteristic "thunderous nose" is formed on the barogram - a sharp peak facing downward. The wind turns towards it just before passing through the front, i.e. makes a turn to the left. After passing the front, the pressure begins to grow, the wind turns sharply to the right. If the front is located in a well-defined hollow, then the wind turn sometimes reaches 180 °; for example, a southerly wind can be replaced by a northerly one. With the passage of the front, a cold snap sets in.
Rice. 14. Cold front of the first kind in the vertical section and on the weather map.
The sailing conditions when crossing a cold front of the first kind will be affected by the deterioration of visibility in the precipitation zone and squally wind.
Cold front of the second kind. This is a fast moving front. The rapid movement of cold air leads to a very intense displacement of the prefrontal warm air and, as a consequence, to the powerful development of cumulus clouds (Cu) (Fig. 15).
Cumulonimbus clouds at high altitudes usually extend forward 60-70 NM from the front line. This cloud front is seen as cirrostratus (Cs), cirrocumulus (Cc), and lenticular altocumulus (Ac) clouds.
The pressure in front of the approaching front drops, but weakly, the wind turns to the left, and heavy rain falls. After the front has passed, the pressure grows rapidly, the wind turns sharply to the right and significantly increases - it takes on the character of a storm. The air temperature sometimes drops by 10 ° C in 1-2 hours.
Rice. 15. Cold front of the second kind in the vertical section and on the weather map.
The sailing conditions when crossing such a front are unfavorable, since at the very front line, powerful ascending air currents contribute to the formation of a vortex with destructive wind speeds. The width of such a zone can be up to 30 NM.
2.8.3. Sedentary, or stationary, fronts
The front, which does not experience a noticeable displacement either towards the warm or towards the cold air mass, is called stationary. Stationary fronts are usually located in the saddle or in a deep hollow, or at the periphery of the anticyclone. The stationary front cloud system is a cirrostratus, altostratus, and nimbostratus cloud system that looks similar to a warm front. Cumulonimbus clouds often form at the front in summer.The wind direction on such a front hardly changes. The wind speed is lower on the cold air side (Fig. 16). The pressure does not experience significant changes. In a narrow strip (30 NM) heavy rain falls.
On a stationary front, wave disturbances can form (Fig. 17). The waves move rapidly along the stationary front in such a way that the cold air remains to the left - in the direction of the isobars, i.e. in warm air mass. The travel speed reaches 30 knots and more.
Rice. 16. Sedentary front on the weather map.
Rice. 17. Wave disturbances on a slow-moving front.
Rice. 18. Formation of a cyclone on a slow-moving front.
After passing the wave, the front restores its position. An increase in the wave disturbance before the formation of a cyclone is observed, as a rule, if cold air flows from the rear (Fig. 18).
In spring, autumn, and especially summer, the passage of waves on a stationary front causes the development of intense thunderstorm activity, accompanied by squalls.
Navigation conditions when crossing a stationary front are complicated due to deterioration of visibility, and in the summer period - due to increased wind to stormy.
2.8.4. Occlusion fronts
Occlusion fronts are formed as a result of the closing of cold and warm fronts and the displacement of warm air upward. The closing process takes place in cyclones, where a cold front, moving at high speed, overtakes a warm one.Three air masses are involved in the formation of the occlusion front - two cold and one warm. If the cold air mass behind the cold front is warmer than the cold mass ahead of the front, then it, displacing the warm air upwards, will simultaneously flow itself onto the front, colder mass. This front is called warm occlusion (Fig. 19).
Rice. 19. Front of warm occlusion in the vertical section and on the weather map.
If the air mass behind the cold front is colder than the air mass in front of the warm front, then this rear mass will flow under both the warm and the forward cold air mass. This front is called cold occlusion (Fig. 20).
Occlusion fronts go through a number of stages in their development. The most difficult weather conditions at the fronts of occlusion are observed at the initial moment of the closure of the thermal and cold fronts. During this period, the cloud system, as seen in Fig. 20 is a combination of warm and cold front clouds. Overburden precipitation begins to fall out of stratus and cumulonimbus clouds, in the front zone they turn into torrential ones.
The wind in front of the warm front of the occlusion increases, after its passage it weakens and turns to the right.
Before the cold front of the occlusion, the wind intensifies to a stormy one; after it passes, it weakens and turns sharply to the right. As warm air is displaced into higher layers, the occlusion front gradually erodes, the vertical thickness of the cloud system decreases, and cloudless spaces appear. Stratus cloudiness gradually turns into stratus, altostratus - into altocumulus and cirrostratus - into cirrocumulus. Precipitation stops. The passage of old occlusion fronts is manifested in the accumulation of high-cumulus cloudiness of 7-10 points.
Rice. 20. Front of cold occlusion in the vertical section and on the weather map.
The conditions of swimming through the zone of the front of occlusion at the initial stage of development almost do not differ from the conditions of swimming, respectively, when crossing the zone of warm or cold fronts.
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Atmospheric fronts have several different characteristics. According to them, this natural phenomenon is divided into different types.
Atmospheric fronts can be 500-700 km wide and 3000-5000 km long.Atmospheric fronts are classified by movement relative to the location of air masses. Another criterion is spatial extent and circulation significance. Finally, there is a geographic feature.
Characterization of atmospheric fronts
By displacement, atmospheric fronts can be divided into cold, warm, and occlusion fronts.
Warm atmospheric is formed when warm air masses, as a rule, humid ones approach drier and colder ones. The approaching warm front brings a gradual decrease in atmospheric pressure, a slight increase in air temperature and small but prolonged precipitation.
A cold front is formed under the influence of northerly winds, which inject cold air into areas that were previously occupied by a warm front. A cold atmospheric front affects the weather in a small strip and is often accompanied by thunderstorms and a drop in atmospheric pressure. After the front passes, the air temperature drops sharply and the pressure increases.
The cyclone, considered the most powerful and destructive in history, hit the Ganges delta in eastern Pakistan in November 1970. The wind speed reached over 230 km / h, and the tidal wave height was about 15 meters.
Occlusion fronts arise when one atmospheric front is superimposed on another, formed earlier. Between them 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 earth's surface. As a result, the front is mixed at the surface of the earth already under the influence of two cold air masses. Deep wave cyclones formed in the form of very chaotic wave disturbances are often located at the occlusion fronts. At the same time, the wind increases significantly, and the wave becomes clearly pronounced. As a result, the front of the occlusion turns into a large blurred frontal zone and, after some time, completely disappears.
Geographically, the fronts are divided into arctic, polar and tropical. Depending on the latitudes in which they are formed. In addition, depending on the underlying surface, the fronts are divided into continental and sea.
A warm front is a moving section between the advancing warm air and the receding cold air.
As you know, the frontal surface, like the front line itself, under the "pressure" of masses of warm air moves towards the cold air. Warm air, being lighter, flows into the cold air, making a gradual forced rise along the entire frontal surface. As it rises, it cools adiabatically, as a result of which the water vapor in it condenses and forms a cloud system. In the immediate vicinity of the front line, where the rise of warm air occurs along the steeper part of the frontal surface, low stratus rain clouds (N8) form, from which heavy rain falls in the warm season, and snow in the cold season. The width of the precipitation zone in the front part of the warm front varies widely, but on average it is 300-400 km.
Further behind the zone, where the frontal surface becomes higher and flatter, stratus clouds gradually turn into highly stratus (Ab), from which light precipitation falls. In summer, due to the high temperature, the raindrops falling from Ae evaporate and do not reach the ground; in winter, light snow falls out of them.
With further distance from the front line, high-stratus clouds, already at high altitudes, gradually transform into cirrostratus (Ce), and the latter - into cirrus (Ci). These clouds appear at a distance of 80P-1000 km (horizontally) from the warm front line.
Thus, the cloudy systems of the warm front have quite regular alternation. As the warm front approaches this point, the clouds alternate in the following sequence: Ci, Ce, Av and N8.
The speed of movement of the warm front is different. On average, it is 25-30 km / h (maximum 50-60 km / h). From the moment the warm front line appears, it can reach the observation point in 20-30 hours, and the precipitation zone - in 10-15 hours.
Behind the line of the warm front, masses of warm air move, carrying with them the weather characteristic of them: noticeable and sometimes abrupt warming, cessation of precipitation, and the appearance of advective fogs. At the moment of the front passage, the wind changes its direction in the layer - from SE to SE and SW (turning to the right).
However, there are also such warm fronts (poorly expressed), which, due to the dryness of the air masses interacting at the frontal surface of ordinary tenlophroite clouds, do not form and pass without noticeable changes in the weather.The passage of such a warm front is limited only by a slight increase in temperature and a change in the direction of the wind. Low-cloud and dry warm fronts are found most often in the southern continental regions.
Thus, warm fronts in most cases, from the point of view of the navigator, bring with them unfavorable weather: prolonged (overburden) precipitation, poor visibility and a possible increase in wind.
As signs of the approaching warm front can serve the above sequence in the change of cloud cover and a gradual drop in pressure.
On the synoptic map, a warm front is indicated by a red line and a one-color print - a black line with oval teeth facing the direction of the front movement.
Special weather phenomena are associated with atmospheric fronts. On the one hand, the transition from one air mass to another is accompanied by a sharp fluctuation of meteorological elements. On the other hand, ascending air movements in the frontal zones lead to the formation of vast cloud systems, from which precipitation falls over large areas, and huge atmospheric waves arising in the air masses on both sides of the front lead to the formation of atmospheric disturbances - large-scale eddies - cyclones and anticyclones.
The peculiarities of the atmospheric circulation are formed in such a way that atmospheric fronts are constantly eroded and re-emerged. Together with them, air masses on both sides of the front are formed, change their properties (transform).
The approach of atmospheric fronts can be traced rather reliably by some signs.
Warm front
If the front moves in such a way that cold air recedes, giving way to warm air, then such a front is called warm.
The angle of inclination of the warm front to the horizontal surface is about 0.5 ◦. There are two air masses vertically in the troposphere. The cold air remains in a narrow wedge at the ground. Warm air rises up the frontal surface. Since the rise at all heights is slow, layered clouds are formed over huge spaces. Warm air, moving forward, not only occupies the space where the cold air used to be, but also rises up along the transition zone. As it rises, the warm air cools, the water vapor in it condenses. As a result, clouds are formed, which are characterized by special cloudiness, precipitation and air currents of a warm front. The first sign of a warm front approaching will be the appearance of cirrus clouds (Ci). At the same time, the pressure will begin to drop. After a few hours, the cirrus clouds, getting denser, pass into a veil of cirrostratus clouds (Cs). Following on to cirrostratus clouds, even denser high-stratus clouds (As) flow, gradually becoming not translucent by the moon or the sun. At the same time, the pressure drops more, and the wind, turning slightly to the left, increases. Precipitation can fall from high-stratus clouds, especially in winter, when they do not have time to evaporate along the way.
After a while, these clouds turn into stratus-rain (Ns), under which there are usually broken-rain (Fr nb) and broken-stratus (St fr). Precipitation from stratus clouds falls more intensively, visibility deteriorates, pressure drops rapidly, wind intensifies, and often becomes gusty. When crossing the front, the wind turns sharply to the right, the pressure drop stops or slows down. Precipitation may stop, but usually it only weakens and turns into drizzling. The temperature and humidity of the air are gradually increasing.
After the front has passed, the temperature increases, and precipitation stops. In winter, visibility can be poor already due to advective fog in the warm air. Drizzle is possible. In the summertime, visibility behind the front line is improved. Before the warm front, the pressure drops.
Signs that a warm front is approaching are a drop in pressure, an increase in the density and water content of clouds, a decrease in their lower boundary, the appearance of nimbostratus, heavy precipitation, the appearance of scraps of stratus fractus (St, fr) or fractonimbus ().
Difficulties that can be encountered when crossing a warm front are mainly associated with prolonged stay in a zone of poor visibility, the width of which ranges from 150 to 200 miles.
In the cold season, 400 km before the front, precipitation in the form of snow or snow pellets can fall from high-layered clouds. In summer, the precipitation zone narrows to 300 km, since precipitation in the form of light rain or drizzle from As evaporates in warm air, before reaching the underlying surface.
Cold front
When cold air mass replaces warm air, the line along which the frontal surface intersects with the horizontal surface at sea level is called a cold front.
A cold front is a front moving towards a warm air mass. There are two main types of cold fronts:
1) cold fronts of the first kind - slowly moving or decelerating fronts, which are most often observed on the periphery of cyclones or anticyclones;
2) cold fronts of the second kind - fast moving or moving with acceleration, they arise in the inner parts of cyclones and troughs moving at high speed.
On a cold front of the first kind, warm air rises rather slowly up the wedge of cold air. In this case, warm air slowly rises upward along the wedge of cold air invading under it. Above the zone of separation of air masses, first stratus (Ns) clouds are formed, transforming at some distance behind the front into altostratus (As) and cirrostratus (Cs) clouds. Precipitation falls directly on the front line and behind the front. The width of the precipitation zone usually does not exceed 50-120 miles. In summer over the oceans in especially deep cyclones and in winter in the front part of the cold front of the first kind, powerful cumulonimbus (Cb) clouds are formed, from which heavy rainfalls, accompanied by thunderstorms, fall. The atmospheric pressure in front of the front drops sharply, and behind the front it rises. At the same time, the wind turns to the left ahead of the front and its sharp turn to the right behind the front. The wind changes its direction especially sharply (sometimes by 180 °) when the front is located near the axis of a narrow hollow. With the passage of the front, a cold snap sets in. The sailing conditions when crossing a cold front of the first kind will be affected by the deterioration of visibility in the precipitation zone and squally wind.
On a cold front of the second kind, the rapid movement of cold air leads to the development of intense convective movement of prefrontal warm moist air and, consequently, to the powerful development of cumulus (Cu) and cumulonimbus (CL) clouds.
At high altitudes (near the tropopause), cumulonimbus clouds stretch forward 50–80 miles from the front line. The front part of the cloud system of a cold front of the second kind is observed in the form of cirrostratus (Cs), cirrocumulus (Cc), and lenticular altocumulus (Ac) clouds. Useful and reasonably timely information about the approaching cold front can be obtained using ship radars.
The atmospheric pressure in front of a cold front of the second kind decreases slowly, while behind the front line it increases rapidly. The wind turns to the left, and behind the front it turns sharply to the right and often intensifies to a stormy one. Heavy rain falls in front of the front and at the front, thunderstorms are possible. In the warm season, at some distance from the front (in the cold air mass), the formation of a secondary cold front with rainstorms and thunderstorms is possible.
The sailing conditions when crossing such a front are unfavorable, because at the very front line, powerful ascending air currents contribute to the formation of a vortex with destructive wind speeds. The width of such a zone can be up to 30 miles.
Occlusion fronts
A front consisting of two fronts and formed in such a way that a cold front overlaps a warm or stationary front is called an occlusion front. Complex complex fronts - occlusion fronts are formed by the closing of cold and warm fronts during the occlusion of cyclones. A cold front follows a warm front. The cold front usually moves quickly. Over time, it catches up with the warm one and the fronts close up.
This is a common process in the last stage of cyclone development, when a cold front catches up with a warm one. There are three main types of occlusion fronts due to the relative coolness of the air mass following the initial cold front towards the air ahead of the warm front. These are the fronts of cold, warm and neutral occlusion.
A distinction is made between a warm front of occlusion, if the air behind the cold front is warmer than the air before the warm front, and a cold front of occlusion, when the air behind the cold front is colder than the air ahead of the warm front.
Occlusion fronts go through a number of stages in their development. The most difficult weather conditions at the fronts of occlusions are observed at the initial moment of closing of the warm and cold fronts. During this period, the cloud system is a combination of warm and cold front clouds. Overburden precipitation begins to fall out of stratus and cumulonimbus clouds, in the front zone they turn into torrential ones.
The wind in front of the warm front of the occlusion increases, after passing it weakens and turns to the right.
Before the cold front of the occlusion, the wind intensifies to a stormy one, after passing it weakens and turns sharply to the right. As warm air is displaced into higher layers, the occlusion front gradually erodes, the vertical thickness of the cloud system decreases, and cloudless layers appear. Stratus cloudiness gradually turns into stratus, altostratus - into altocumulus and cirrostratus - into cirrocumulus. Precipitation stops. The passage of old occlusion fronts is manifested in the accumulation of high-cumulus cloudiness of 7-10 points.
The conditions for swimming through the fronts of occlusion at the initial stage of development hardly differ from the conditions for swimming when crossing the warm or cold fronts, respectively.
In their development, the occlusion fronts go through three stages. Particularly difficult weather conditions at the fronts are observed at the moment of the closing of the warm and cold fronts. The cloud system is a complex combination of clouds that are characteristic of both warm and cold fronts. Prefrontal overburden precipitation from nimbostratus and cumulonimbus clouds transforms into storm clouds directly in the front zone. The direction and speed of the wind when passing the occlusion fronts change in the same way as on simple fronts. Over time, warm air is forced upward and the occlusion front gradually erodes, the vertical thickness of the cloud system decreases, and gaps appear in the cloud cover. At the same time, stratus cloudiness gradually turns into stratus, altostratus - into altocumulus, and cirrostratus, in turn, into cirrocumulus. This restructuring of cloud systems results in an end to precipitation.
The hydrometeorological conditions of navigation in the zones of the fronts of occlusion differ slightly from the conditions of navigation during the passage of simple fronts: cold or warm.
The cloud system is a complex combination of clouds that are characteristic of both warm and cold fronts. The weather conditions during the passage of such fronts are also unfavorable for yachtsmen - they are accompanied by rains with thunderstorms and hail, strong and gusty winds with a sharp change in direction and, at times, poor visibility.
Prefrontal overburden precipitation from stratus and cumulonimbus clouds transforms into storm clouds directly in the front zone. The direction and speed of the wind when passing the occlusion fronts change in the same way as on simple fronts. Over time, warm air is forced upward and the occlusion front gradually erodes, the vertical thickness of the cloud system decreases, and gaps appear in the cloud cover. At the same time, stratus cloudiness gradually turns into stratus, altostratus - into altocumulus, and cirrostratus, in turn, into cirrocumulus. This restructuring of cloud systems is causing precipitation to end.
Sedentary, or stationary fronts
The front, which does not experience a noticeable displacement either towards the warm or towards the cold air mass, is called stationary.
Stationary fronts are usually located in the saddle, or in a deep hollow or at the periphery of the anticyclone. A stationary front cloud system is a cirrostratus, altostratus, and nimbostratus cloud system that looks something like a warm front. Cumulonimbus clouds often form at the front in summer.
The wind direction on such a front hardly changes. The wind force is less on the cold air side. The pressure does not experience significant changes. Heavy rain falls in a narrow strip (30 miles).
Wave disturbances can form on the stationary front. The waves move rapidly along the stationary front in such a way that the cold air remains to the left, that is, in the direction of isobars in the warm air mass. The travel speed reaches 30 knots or more.
After passing the wave, the front restores its position. An increase in the wave disturbance before the formation of a cyclone is observed, as a rule, if cold air flows from the rear.
In spring and autumn, and especially in summer, the passage of waves on a stationary front causes the development of intense thunderstorm activity, accompanied by squalls.
Navigation conditions when crossing a stationary front are complicated due to the deterioration of visibility, and in the summer period - due to increased wind to stormy.
The warm front is marked with red or blackened semicircles directed towards the front movement. As the line of the warm front approaches, the pressure begins to drop, clouds thicken, and heavy precipitation falls. In winter, when the front passes, low stratus clouds usually appear. Temperature and humidity rise slowly. When the front passes, temperature and humidity usually rise rapidly, and the wind intensifies. After the front has passed, the wind direction changes (the wind turns clockwise), the pressure drop stops and its weak growth begins, the clouds dissipate, and precipitation stops. The field of baric tendencies is presented as follows: a closed region of pressure drop is located in front of the warm front, behind the front there is either an increase in pressure or a relative increase (a drop, but less than before the front).
In the case of a warm front, warm air, moving towards the cold one, flows onto a wedge of cold air and makes an upward slide 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 upward motions from unsteadiness with a dynamic pressure drop and from the convergence of the wind in the lower atmosphere. Cooling of warm air during an ascending glide along the surface of the front leads to the formation of a characteristic system of stratus clouds (clouds of ascending sliding): cirrostratus - high-stratified-nimbus (Cs-As-Ns).
When approaching the point of a warm front with well-developed cloudiness, cirrus clouds first appear in the form of parallel stripes with claw-like formations in the front (precursors 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 at the Earth's surface (about 800-900 km). Cirrus clouds then pass into cirrostratus clouds (Cirrostratus). These clouds are characterized by halo phenomena. The upper tier clouds - cirrostratus and cirrus (Ci and Cs) - are composed of ice crystals, and no precipitation falls 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, it is close to the tropopause.
Then the clouds become denser: altostratus clouds (Altostratus) gradually turn into nimbostratus (Nimbostratus), heavy precipitation begins to fall, which weaken or completely stop after passing the front line. As the front line is approached, the base height Ns decreases. Its minimum value is determined by the height of the condensation level in the rising warm air. Highly layered (As) are colloidal and consist of a mixture of tiny droplets and snowflakes. Their vertical thickness is quite significant: starting at an altitude 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 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 width of the overburden zone can reach a width of 400 km or more. Closest to the Earth's surface (at an altitude of several hundred meters, and sometimes 100-150 m and even lower) is the lower boundary of stratus clouds (Ns), from which heavy precipitation falls in the form of rain or snow; under nimbostratus clouds, torn raindrops (St fr) often develop.
Ns clouds extend to heights of 3 ... 7 km, that is, they have a very significant vertical thickness. Clouds also consist of icy elements and droplets, and the droplets and crystals, especially in the lower part of the clouds, are larger than in As. The bottom base of the As-Ns cloud system roughly coincides with the front surface. Since the upper boundary of As-Ns clouds is approximately horizontal, their greatest thickness is observed near the front line. Near the center of the cyclone, where the cloud system of the warm front has the greatest development, the width of the cloud zone Ns and the zone of overlying 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 you project this system onto 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 cloudiness and precipitation zone 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, radiation cooling of the upper boundary of the As-Ns cloud system and a decrease in temperature in the clouds, as well as an increase in vertical mixing when the cooled air descends into the cloud, contributes to the formation of an ice phase in clouds, the growth of cloud elements and the formation of precipitation. As the distance from the cyclone center increases, the ascending air movements weaken, 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 the ascending movements capture the area behind the front - then precipitation can fall on both sides of the front. But behind the front line, the frontal cloudiness is usually strongly stratified, and the frontal precipitation is more often presented in the form of drizzle or snow grains.
In the case of a very flat front, the cloud system can be displaced forward from the front line. In the warm season, ascending movements near the front line acquire a convective character, and cumulonimbus clouds often develop on warm fronts, and heavy rainfall and thunderstorms are observed (both during the day and at night).
In summer, during daytime hours, in the surface layer behind the warm front, with significant cloudiness, the air temperature over land can be lower than ahead of the front. This phenomenon is called warm front masking.
The clouds 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 is 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 cloudiness - that is, there are no clouds at all, or a band of upper and middle tiers of clouds is observed.
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