The concept of atmospheric front is usually understood as a transition zone in which adjacent air masses with different characteristics. The formation of atmospheric fronts occurs when warm and cold air masses collide. They can extend for tens of kilometers.
Air masses and atmospheric fronts
Atmospheric circulation occurs due to the formation of various air currents. Air masses located in lower layers atmospheres capable of combining with each other. The reason for this is general properties these masses or identical origin.
Changes in weather conditions occur precisely because of the movement of air masses. Warm ones cause warming, and cold ones cause cooling.
There are several types of air masses. They are distinguished by the source of their occurrence. Such masses are: arctic, polar, tropical and equatorial air masses.
Atmospheric fronts occur when different air masses collide. Collision areas are called frontal or transitional. These zones instantly appear and also quickly collapse - it all depends on the temperature of the colliding masses.
The wind generated by such a collision can reach a speed of 200 km/k at an altitude of 10 km from earth's surface. Cyclones and anticyclones are the result of collisions of air masses.
Warm and cold fronts
Warm fronts are considered to be fronts moving towards cold air. The warm air mass moves along with them.
As warm fronts approach, there is a decrease in pressure, thickening of clouds and heavy precipitation. After the front has passed, the direction of the wind changes, its speed decreases, the pressure begins to gradually rise, and precipitation stops.
A warm front is characterized by the flow of warm air masses onto cold ones, which causes them to cool.
It is also quite often accompanied by heavy rainfall and thunderstorms. But when there is not enough moisture in the air, precipitation does not fall.
Cold fronts are air masses that move and displace warm ones. There are cold fronts of the first kind and cold fronts of the second kind.
The first type is characterized by the slow penetration of its air masses under warm air. This process forms clouds both behind the front line and within it.
The upper part of the frontal surface consists of a uniform cover of stratus clouds. The duration of the formation and decay of a cold front is about 10 hours.
The second type is cold fronts moving at high speed. Warm air is instantly replaced by cold air. This leads to the formation of a cumulonimbus region.
The first signals of the approach of such a front are high clouds that visually resemble lentils. Their formation occurs long before his arrival. The cold front is located two hundred kilometers from where these clouds appear.
Cold front of the 2nd kind in summer period accompanied by heavy precipitation in the form of rain, hail and strong winds. Such weather can extend for tens of kilometers.
In winter, a cold front of the 2nd kind causes snowstorm, strong wind, bumpiness.
Atmospheric fronts of Russia
The climate of Russia is mainly influenced by the North Arctic Ocean, Atlantic and Pacific.
In summer, Antarctic air masses pass through Russia, affecting the climate of the Ciscaucasia.
The entire territory of Russia is prone to cyclones. Most often they form over the Kara, Barents and Okhotsk seas.
Most often, there are two fronts in our country - the Arctic and the polar. They move south or north during different climatic periods.
South part Far East influenced by tropical fronts. Heavy rainfall on middle lane Russia are caused by the influence of the polar dandy, which operates in July.
Air masses that differ in their physical properties, are separated from each other by a layer of air called the frontal surface. In the frontal zone layer, temperature, humidity, density, and wind change sharply. The frontal zone is always inclined towards the cold air. Above it there is warm air, as less dense and light, and above it, in the form of a wedge, there is cold air. The main reason for the formation of fronts is the convergence of dissimilar air masses. A front is considered dynamically expressed if, over a 1000 km distance, the temperature difference between warm and cold air is 8-10C. The speed of the front depends on the angle of intersection of the front with the isobars.
The fronts that separate the major geographic types of air masses are called major fronts.
There are:
· arctic front separating arctic air from air temperate latitudes;
· polar front separating temperate and tropical air;
· tropical front, lying between tropical and equatorial air.
In terms of speed of movement, these fronts can be stationary ( average speed their movements are 5-10 km/hour. 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 - ground, tropospheric, high-altitude.
Warm a front is a section of the main front that moves towards cold air; behind this front warm air moves, which, being less dense, flows onto the cold air.
Cold A front is a section of the main front moving towards warm air. Behind this front, cold air moves, which is denser and wedges itself under the warm air.
The front formed as a result of the closure of warm and cold air is called a front occlusion.
3.3 Warm front in winter and summer. Flight conditions.
On a warm front, warm air flows onto cold air, located in a wedge below. Ahead of the ground line there is an area of pressure drop, which is caused by the replacement of cold air with warm air. As the pressure drops, the wind increases, maximum speed reaches before the front passes, then weakens. Before the front, winds of a south-eastern direction predominate, passing behind the front to the south and south-west.
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 to 600-700 km.
The inclination of the frontal surface is observed in the range of 1/100 to 1/200.
The main cloud system of the front is nimbostratus and high-stratus Ns-As clouds located in the lower and middle tiers (5-6 km). Their upper boundary is almost horizontal, and the lower one decreases from the leading edge to the front line, where it reaches a height of about 100 m (in cold weather it may be lower). Above As-Ns there are cirrostratus and cirrus clouds. Sometimes they merge with the underlying cloud system. But often the upper clouds are separated from the Ns-As system by a cloud layer. A zone of heavy precipitation is observed under the main cloud system. It lies ahead of the surface front line and has a normal length from the front to 400 km.
In the precipitation zone, low broken rain clouds with a lower boundary of 50-100 m are formed, frontal fogs sometimes occur, and ice is observed at temperatures from 0 to –3.
In winter when strong winds The passage of the front is accompanied by strong snowstorms. In summer, isolated pockets of cumulonimbus clouds with showers and thunderstorms may appear on the warm front. Most often they occur at night. Their development is explained by 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 increased temperature gradients and increased vertical currents, which lead to the formation of cumulonimbus clouds. They are usually masked by nimbostratus clouds, making them difficult to visually identify. When approaching nimbostratus clouds, within which cumulonimbus clouds are hidden, bumpiness (turbulence) and increased electrification begin, which negatively affects the operation of instrument equipment.
In winter in the zone negative temperatures cloudiness warm front There is a danger of aircraft icing. The lower limit of icing is the zero isotherm. Severe icing occurs during flight in an area of supercooled rain. In the cold season, the warm front intensifies and more often produces difficult weather: low clouds, poor visibility in snowstorms, precipitation, fog, icing in precipitation, ice on the ground, electrification in the clouds.
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Visibility remains limited for some time after the front passes, as the air is saturated big amount moisture, which allows for a long time withstand fogs, haze and low clouds.
Behind a warm front, temperatures rise. 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 type 1 cold front moves at a speed of no more than 30 km/h.
In this case, there is an orderly, slow rise of warm air over an invading wedge of cold air. In the cold half of the year in the ascending warm air the condensation process is not violent. As a result, nimbostratus clouds form above the frontal surface. Precipitation begins at the very front line, the width of the precipitation zone is 100-200 km.
During this season, the cloud system resembles the cloud cover of a warm front system, located in reverse order. Upper level clouds 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 great vertical thickness form in front of the Ns-As cloud system, from which rainfall falls, 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 higher. Rainfall, thunderstorms, and squalls are observed under the clouds. In the precipitation zone, low broken rain clouds almost always form. After the front passes, the wind turns to the right and weakens, the pressure ahead of the front drops, behind the front it gradually increases, and the temperature drops.
3.5 Cold front of the 2nd kind in winter and summer. Flight conditions.
Type 2 fast moving cold front is the most dangerous of all types of atmospheric fronts. Due to the high speed of movement (40-50 km/h), cold air with great energy displaces warm air upward to great heights. IN summer time As a result of this strong dynamic convection in warm air, cumulonimbus clouds of great vertical power are formed, sometimes breaking through the tropopause. During the cold season
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cloud power is less.
Cumulonimbus clouds are forced forward in the direction of the wind at high altitudes, 100-300 km from the front line. A harbinger of the approach of such a front is altocumulus lenticular clouds (Ac), appearing 200 km ahead of the surface front line. At the very front line, cumulonimbus clouds are accompanied by squalls with destructive wind speeds and thunderstorms. The width of the cloud system reaches several tens of kilometers, the lower boundary is usually at an altitude of 300-400m, and in the precipitation zone it can drop to 100-200m.
In the clouds great danger represent upward flows of up to 30 m/s or more and downward flows of up to 15 m/s or more. In addition, there may be thunderstorms, heavy rainfall in the clouds, and intense icing in the subzero temperature zone. But the width of this danger zone small, about 50 km.
Near the ground, this front is accompanied by squalls, showers, and thunderstorms; the width of the rainfall zone is several tens of kilometers and is usually observed ahead of the surface front line. The pressure ahead of the front drops sharply, behind the front it quickly increases. After the front passes, the wind sharply changes direction to the right and intensifies to 20-30 m/s. The temperature behind the front drops by 10-12°C in 1 hour.
The weather on this front is most pronounced in the summer afternoon.
In winter, when the front passes, heavy snowfalls and blizzards are observed, reducing visibility to several tens of meters. The main clouds are cumulonimbus (Cb) with a top of 4-5 km.
Flights at flight levels take place in simple weather conditions, and their main influence is manifested at low flight levels during takeoff, landing and climb.
3.6 Occlusion fronts. 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 merges with it. At the same time, two cold air masses close together - one located in front of the warm front and one lying behind the cold front. Warm air, enclosed between the fronts, is cut off from the ground and forced upward. The cloud systems of warm and cold fronts come closer and partially overlap each other and are also pushed upward. This process is called the cyclone occlusion process, and the resulting front is called the occlusion front (occlusion - “occlusion” - lock to close).
As a result of occlusion, two types of occlusion fronts arise:
1. warm front of occlusion (occlusion like a warm front);
2. cold front occlusion (occlusion like a cold front).
Warm front occlusion.
This front occurs if the cold air at the rear of the cyclone is a warmer air mass, combing the cold air at its front. When a cyclone occludes, 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 broken rain clouds can appear.
Covering precipitation begins ahead of the front line 300-400 km, gradually turning into showers at the point of occlusion. The wind near the ground has a sharp right-hand rotation and intensifies. The pressure drops quickly. Occlusions of this type occur 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 normal to the front is 50 km. When flying at low altitudes, you always encounter low clouds, turning into fog, icing, and ice at the airfield.
Atmospheric fronts have several various characteristics. This is divided according to them natural phenomenon on different types.
Atmospheric fronts can reach a width of 500-700 km and a length of 3000-5000 km.Atmospheric fronts are classified by their movement relative to the location of air masses. Another criterion is spatial extent and circulation significance. And finally, a geographical feature.
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. Approaching warm front brings gradual decline 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.
ATMOSPHERE FRONT (tropospheric front), 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 low blood pressure(see Pressure systems). Extensive cloud systems form above the frontal surfaces, producing precipitation. The 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 site 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 progress general circulation 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 C. Circulation systems atmosphere. L., 1973; Ocean - atmosphere: Encyclopedia. L., 1983.
Atmospheric front, tropospheric fronts - a transition 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 throughout 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). Behind the warm front in this region warm air mass arrives.
On a weather map, a warm front is marked in red or with blackened semicircles directed in the direction the front is moving. 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 wind direction changes (the wind turns clockwise), the pressure drop stops and its slight increase begins, the clouds dissipate, and precipitation stops. The field of pressure trends is presented as follows: in front of the 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 decrease, but less than in front of the front).
In the case of a warm front, warm air, moving towards the cold air, flows onto a wedge of cold air and glides upward along this wedge and is dynamically cooled. At a certain height, determined by the initial state of the rising air, saturation is achieved - this is the level of condensation. Above this level, cloud formation occurs in the rising air. Adiabatic cooling of warm air sliding along a wedge of cold air is enhanced by the development of upward movements from unsteadiness with a dynamic drop in pressure and from the convergence of wind in the lower layer of the atmosphere. Cooling of warm air during upward sliding along the surface of the front leads to the formation characteristic system stratus clouds (ascending clouds): cirrostratus - altostratus - 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 stripes with claw-shaped formations in the front part (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 become cirrostratus clouds. These clouds are characterized by halo phenomena. The upper tier clouds - cirrostratus and cirrus (Ci and Cs) consist of ice crystals and do not produce precipitation. Most often, Ci-Cs clouds represent 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 more and more dense: altostratus clouds (Altostratus) gradually turn into nimbostratus clouds (Nimbostratus), blanket precipitation begins to fall, which weakens or stops completely after passing the front line. As you approach the front line, the height of the base Ns decreases. Its minimum value is determined by the height of the condensation level in the rising warm air. Altolayers (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 altitudes of the order of 4-6 km, that is, they are 1-3 km thick. 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 as snow almost always reaches the Earth's surface and also stimulates precipitation from the underlying St-Sc. In this case, the width of the zone of continuous precipitation 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 nimbostratus clouds (Ns), from which precipitation falls in the form of rain or snow; Nimbostratus clouds often develop under nimbostratus clouds (St fr).
Ns clouds extend to heights of 3...7 km, that is, they have a very significant vertical thickness. Clouds also consist of ice elements and droplets, and the droplets 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 general outline coincides with the front surface. Since the top of As-Ns clouds is approximately horizontal, their greatest thickness is observed near the front line. At the center of the cyclone, where the cloud system of the warm front is most developed, the width of the cloud zone Ns and the zone of heavy 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 on a ground map, then all of it will be in front of the warm front line at a distance of 700-900 km. IN 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, radiation cooling upper limit cloud system As-Ns and a decrease in temperature in the clouds, as well as increased vertical mixing as cooled air descends into the cloud, promotes 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 upward air movements weaken and precipitation stops. Frontal clouds can form not only over 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 a cyclone, when upward movements capture the frontal region - then precipitation can fall on both sides of the front. But behind the front line, frontal clouds are usually highly stratified and post-frontal precipitation is often in the form of drizzle or snow grains.
In the case of a very flat front, the cloud system may be moved forward from the front line. In the warm season, upward movements near the front line acquire a convective character, and cumulonimbus clouds often develop on warm fronts and showers and thunderstorms are observed (both during the day and at night).
In summer, during the daytime hours in the surface layer behind the line of a warm front with significant cloudiness, the air temperature over land may be lower than in front of the front. This phenomenon is called masking of a warm front.
Cloud cover from old warm fronts can also be stratified throughout 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 significant 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 intense cloudiness - that is, there are no clouds at all, or a strip 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 back warm air: cold advection is observed; behind the cold front, a cold air mass enters the region.
On a weather map, a cold front is marked in blue or with blackened triangles pointing in the direction the front is moving. 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 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, the pressure increase can reach 3-5 hPa/3 hours, and sometimes 6-8 hPa/3 hours or even more. A change in pressure trend (from falling to rising, from slow growth to stronger growth) indicates the passage of the surface front line.
Precipitation is often observed ahead of the front, and often thunderstorms and squalls (especially in the warm half of the year). After the front passes, the air temperature drops (cold advection), sometimes quickly and sharply - by 5...10 °C or more in 1-2 hours. The dew point drops along with the air temperature. Visibility typically improves as cleaner, less humid air from northern latitudes moves in behind the cold front.
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.
There are two types of cold fronts:
cold front of the first kind, when cold air moves in slowly,
cold front of the second type, accompanied by a rapid advance of cold air.
Front of occlusion - an atmospheric front associated with a heat ridge in the lower and middle troposphere, which causes large-scale upward air movements and the formation of an extended zone of clouds and precipitation. Often, an occlusion front arises due to closure - the process of displacing warm air upward in a 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 and, in the summer, heavy showers and thunderstorms.
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. Warm air in the form of a funnel gradually rises upward, and its place is taken by cold air coming from the sides. The interface that occurs when cold and warm fronts meet is called the occlusion front surface. Occlusion fronts are associated with intense precipitation and severe thunderstorms in summer.
Air masses that close during occlusion usually have different temperatures- 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.
In central Russia and the CIS in winter, warm fronts of occlusion predominate, since temperate sea air enters the rear of the cyclone, which is much warmer than the continental temperate air in the front part of the cyclone. In summer, occluded cold fronts are mainly observed here.
The pressure field of the occlusion front is represented by a well-defined trough with V-shaped isobars. Before the front on the synoptic map there is an area of pressure drop associated with the surface of the warm front, and behind the occlusion front 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 occlusion point. As the cyclone occludes, the occlusion point shifts to its periphery.
In the front 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 part of the cold front's cloud system may remain above the upper warm front. If a cold front of the second type is involved, then clearing occurs behind the upper warm front, but the lower cold front can develop a wave of cumulonimbus clouds (Cb) already in the front cold air, displaced by a colder rear wedge. Thus, precipitation from altostratus and stratostratus (As-Ns), if it occurs, may begin before the rainfall occurs, or simultaneously with or after the passage of the lower cold front; precipitation can fall 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.
The converging cloud systems of warm and cold fronts are mainly composed of As-Ns. As a result of the convergence, a powerful Cs-As-Ns cloud system appears with its greatest thickness near the upper cold front. In the case of a young occlusion front, the cloud system begins with Ci and Cs, which turn into As, then into Ns. Sometimes Ns can be followed by Cb, followed again by Ns. Weak upward sliding of the rear air along the occluded surface can lead to the formation of clouds such as stratus and stratocumulus (St-Sc) along it, not reaching the level of the ice cores. These will produce some drizzle ahead of the lower warm front. In the case of an old warm occluded front, the cloud system consists of cirrostratus (Cs) and altocumulus (Ac) clouds, sometimes joined by altostratus (As); there may be no precipitation.
Stationary front
1. A front that does not change its position in space.
2. A front along which air masses move horizontally; front without slipping.
32)cyclones and anticyclones. Stages of their development, wind systems and cloudiness 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 high blood pressure not detected at all; It is also possible that there is a high-altitude cyclone above such an anticyclone.
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