Types of atmospheric fronts. What is an atmospheric front
Weather cold VM
Warm weather VM
Warm VM, moving to a cold area, becomes stable (cooling from the cold underlying surface). The air temperature, falling, can reach the level of condensation with the formation of haze, fog, low stratus clouds with precipitation in the form of drizzle or small snowflakes.
Conditions for flying in a warm aircraft in winter:
Weak and moderate icing in clouds at subzero temperatures;
Cloudless sky, good visibility at H = 500-1000 m;
Weak bumpiness at H = 500-1000 m.
In the warm season, conditions for flights are favorable, with the exception of areas with isolated centers of thunderstorms.
When moving to a warmer area, a cold VM heats up from below and becomes unstable. Powerful upward air movements contribute to the formation of cumulonimbus clouds with showers and thunderstorms.
Atmospheric front- this is the separation between two air masses that differ from each other in physical properties (temperature, pressure, density, humidity, cloudiness, precipitation, wind direction and speed). The fronts are located in two directions - horizontally and vertically
The boundary between air masses along the horizon is called front line, vertical boundary between air masses - called. frontal zone. The frontal zone is always inclined towards the cold air. Depending on which VM arrives - warm or cold, they distinguish warm TF and cold HF fronts.
A characteristic feature of fronts is the presence of the most dangerous (difficult) meteorological conditions for flight. Front-end cloud systems have significant vertical and horizontal extent. On fronts in the warm season there are thunderstorms, roughness, and icing; in the cold season there are fogs, snowfall, and low clouds.
Warm front is a front that moves towards cold air, followed by warming.
Associated with the front is a powerful cloud system consisting of cirrostratus, altostratus, and nimbostratus clouds formed as a result of the rise of warm air along a wedge of cold air. SMC on the TF: low clouds (50-200m), fog ahead of the front, poor visibility in the precipitation zone, icing in clouds and precipitation, ice on the ground.
Flight conditions through the TF are determined by the height of the lower and upper boundaries of the clouds, the degree of stability of the VM, the temperature distribution in the cloud layer, moisture content, terrain, time of year, and day.
1. If possible, stay in the zone of negative temperatures as little as possible;
2. Cross the front perpendicular to its location;
3. Select a flight profile in a zone of positive temperatures, i.e. below the 0° isotherm, and if temperatures throughout the entire zone are negative, fly where the temperature is below -10°. When flying from 0° to -10°, the most intense icing is observed.
When encountering dangerous conditions (thunderstorm, hail, severe icing, severe bumps), it is necessary to return to the departure airfield or land at an alternate airfield.
-Cold front – This is a section of the main front, moving towards high temperatures, followed by cooling. There are two types of cold fronts:
-Cold front of the first kind (HF-1r)- this is a front moving at a speed of 20 - 30 km/h. Cold air, flowing like a wedge under the warm air, displaces it upward, forming cumulonimbus clouds, rainfall, and thunderstorms ahead of the front. Part of the TV flows onto the CW wedge, forming stratus clouds and blanket precipitation behind the front. There is strong bumpiness in front of the front, poor visibility behind the front. The conditions for flying through the HF -1r are similar to the conditions for crossing the TF.
When crossing HF -1p, you can encounter weak and moderate bumpiness, where warm air is displaced by cold air. Flight at low altitudes may be difficult due to low clouds and poor visibility in precipitation areas.
Cold front of the second kind (HF – 2р) – This is a front moving quickly at a speed of = 30 – 70 km/h. Cold air quickly flows under the warm air, displacing it vertically upward, forming vertically developed cumulonimbus clouds, showers, thunderstorms, and squalls in front of the front. It is prohibited to cross the HF – type 2 due to strong roughness, a squall of thunderstorm activity, and strong development of clouds along the vertical – 10 – 12 km. The width of the front near the ground ranges from tens to hundreds of kilometers. After the front passes, the pressure increases.
Under the influence of downward flows, clearing occurs in the front zone after its passage. Subsequently, the cold cloud, falling on the warm underlying surface, becomes unstable, forming cumulus, powerful cumulus, cumulonimbus clouds with showers, thunderstorms, squalls, strong bumps, wind shear, and secondary fronts are formed.
Secondary fronts – These are fronts that form within one VM and separate areas with warmer and colder air. The flight conditions there are the same as on the main fronts, but weather conditions are less pronounced than on the main fronts, but even here you can find low clouds and poor visibility due to precipitation (blizzards in winter). Associated with secondary fronts are thunderstorms, rainfall, squalls, and wind shear.
Stationary fronts – These are fronts that remain motionless for some time and are located parallel to the isobars. The cloud system is similar to the TF cloud, but with a small horizontal and vertical extent. Fog, ice, and icing may occur in the front zone.
Upper fronts – This is a condition where the frontal surface does not reach the ground surface. This happens if a strongly cooled layer of air is encountered on the path of the front or the front is washed out in the surface layer, while difficult weather conditions (jet, turbulence) still persist at altitudes.
Occlusion fronts are formed as a result of the closure of cold and warm fronts. When the fronts close, their cloud systems close. The process of closure of the TF and CP begins in the center of the cyclone, where the CP, moving at a higher speed, overtakes the TF, gradually spreading to the periphery of the cyclone. Three VMs participate in the formation of a front: - two cold and one warm. If the air behind the HF is less cold than in front of the TF, then when the fronts close, a complex front is formed, called WARM FRONT OCCLUSION.
If the air mass behind the front is colder than the front, then the rear part of the air will flow under the front, warmer part. Such a complex front is called COLD FRONT OCCLUSION.
Weather conditions on occlusion fronts depend on the same factors as on the main fronts: - the degree of stability of the CM, moisture content, the height of the lower and upper boundaries of clouds, terrain, time of year, day. At the same time, the weather conditions of cold occlusion in the warm season are similar to the weather conditions of HF, and the weather conditions of warm occlusion in cold times are similar to the weather of TF. Under favorable conditions, occlusion fronts can transform into main fronts - warm occlusion in the TF, cold occlusion in a cold front. The fronts move along with the cyclone, turning counterclockwise.
Air masses move around the planet as a single unit. Atmospheric fronts, or simply fronts, are transition zones between two different air masses. Transition zones between neighboring air masses with different properties are called atmospheric fronts. The main characteristic feature of atmospheric fronts are large values of horizontal gradients: pressure, temperature, humidity etc. Significant cloudiness is observed here, the most precipitation falls, and the most intense changes in pressure, strength and wind direction occur.
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.
The main characteristic feature of atmospheric fronts is the large values of horizontal gradients: pressure, temperature, humidity, etc. The zone of the atmospheric front is very narrow compared to the air masses it separates. When there is movement, the transition surface becomes inclined, with denser (cold) air forming a wedge under less dense (warm) air, and warm air sliding upward along this wedge.
The vertical thickness of the frontal surface is very small - several hundred meters, which is much less than the width of the air masses that it separates. Within the troposphere, one air mass overlaps another. The width of the front zone on weather maps is several tens of kilometers, but when analyzing synoptic maps, the front is drawn as a single line. Only in large-scale vertical sections of the atmosphere is it possible to identify the upper and lower boundaries of the transition layer.
For this reason, fronts are depicted on synoptic maps as a line (front line). At the intersection with the earth's surface, the front zone has a width of about ten kilometers, while the horizontal dimensions of the air masses themselves are about thousands of kilometers.
In the horizontal direction, the length of fronts, like air masses, is thousands of kilometers, vertically - about 5 km, the width of the frontal zone to the Earth's surface is about hundreds of kilometers, at altitudes - several hundred kilometers. Frontal zones are characterized by significant changes in air temperature and humidity, wind directions along the horizontal surface, both at Earth level and above.
The fronts between air masses of the above main geographical types are called main atmospheric fronts. The main fronts are: arctic (between arctic and polar air), polar (between polar and tropical air) and tropical (between tropical equatorial air).
According to thermodynamic properties, atmospheric fronts between air masses of the same geographic type are divided into warm, cold and sedentary (stationary), which can be primary, secondary and upper, as well as simple and complex (occluded). A special position is occupied by occlusion fronts, formed when warm and cold fronts close. Occlusion fronts can be either cold or warm fronts. On weather maps, fronts are drawn either as colored lines or as symbols.
Complex complex fronts - occlusion fronts are formed by the closure of cold and warm fronts during the occlusion of cyclones. A distinction is made between a warm front of occlusion, when the air behind a cold front is warmer than the air in front of a warm front, and a cold front of occlusion, when the air behind a cold front is colder than the air in front of a warm front.
A well-defined front has a height of several kilometers, most often 3-5 km. Major fronts are associated with prolonged and heavy precipitation; In the system of secondary fronts, cloud formation processes are less pronounced, precipitation is short-lived and does not always reach the Earth. There are also intramass precipitations not associated with fronts.
In the surface layer, due to the convergence of air flows to the axis of pressure troughs, the greatest contrasts in air temperature are created here - therefore, the fronts near the Earth are located precisely along the axes of pressure troughs. Fronts cannot be located along the axes of pressure ridges, where air flows diverge, but can only intersect the ridge axis at a large angle.
With height, the temperature contrasts on the axis of the pressure trough decrease - the axis of the trough shifts towards lower air temperatures and tends to align with the axis of the thermal trough, where temperature contrasts are minimal. Thus, with height, the front gradually moves away from the axis of the pressure trough to its periphery, where the greatest contrasts are created.
Depending on the direction of movement of warm and cold air masses located on both sides of the transition zone, fronts are divided into warm and cold. Fronts that change their position little are called sedentary. A special position is occupied by occlusion fronts, formed when warm and cold fronts close. Occlusion fronts can be either cold or warm fronts. On weather maps, fronts are drawn either as colored lines or as symbols.
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 ever larger territories. 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. During movement, various masses can approach 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, the scientific 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.
By definition, the inclined surface, which represents 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, air of a higher temperature, moving, is located in the territory 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 are 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 are formed. High pressure zones are characterized by an excess amount of air, while low pressure zones are characterized by an insufficient amount of air. 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.
An anticyclone is an area of high pressure that displaces excess air into areas of low pressure. 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 are formed, they are divided geographically into:
- Arctic, separating cold Arctic air masses from temperate ones.
- Polar, located between the temperate and tropical masses.
- Tropical (trade wind), delimiting the tropical and equatorial zones.
Influence of the underlying surface
The physical properties of air masses are affected 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 cases of 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.
Uneven heating of the earth's surface and air in the troposphere, as we have seen, is the cause of the emergence of horizontal temperature and pressure gradients and the formation of air currents. Due to transport, air masses with different properties can move closer to each other or move away. When air masses with different physical properties approach each other, horizontal gradients of temperature, humidity, pressure and other meteorological elements increase, and wind speeds increase. On the contrary, as they move away from each other, the gradients decrease. Those zones in which dissimilar air masses, for example relatively dry cold and moist warm ones, come together are called transitional or frontal zones. In the frontal zones, there seems to be a struggle between cold and warm air masses. As a result of this struggle, cold air masses break through into the areas where warm masses are located, and warm masses penetrate into areas where cold masses are located. As a result of these processes, both air masses gradually acquire properties inherent in the air of a given geographical area.
The frontal zones of the troposphere can be detected daily in the field of temperature and pressure mainly in extratropical latitudes, where the influx of solar energy is different in the north and south of the temperate zone. The magnitudes of horizontal temperature and pressure gradients here are greater than anywhere else on the globe. Frontal zones continuously arise, become aggravated, and are destroyed. However, they vary in intensity, which depends on the temperature difference between the approaching air masses.
In the lower layers of the atmosphere, when crossing frontal zones in the direction from warm to cold air, in accordance with large horizontal gradients, a rapid decrease in temperature, pressure and humidity occurs and high speeds of air currents are observed. In mid-latitudes at altitudes of 10-12 km in these zones, winds often reach hurricane force, i.e. 200 km/h or more. As we will see below, frontal zones play a leading role in the development of atmospheric processes.
Since cold and warm air masses have different densities, they are located in relation to each other not vertically, but obliquely. Cold air, being denser and heavier, wedges under the warm, lighter one. In this border zone between air masses of different properties, cyclones and anticyclones usually arise, bringing inclement and fair weather.
The dimensions of the transition zones are small compared to air masses. In the frontal zone, interfaces between cold and warm air masses appear, which are called atmospheric fronts. The frontal surfaces are always inclined towards the cold air, which is located under the warm air in the form of a narrow wedge (Fig. 52). The angle of inclination of the frontal surface to the horizon is very small: it is less than 1°, and the tangent of the angle ranges from 0.01-0.02. This means that if you move 200 km away from the front line at the surface of the earth towards the cold air, then the frontal surface will be at an altitude of 1-2 km. When removed in the horizontal direction by 500 km, the frontal surface is at an altitude of 2.5-5.0 km. Since the angles of inclination of the fronts are very small, in order to represent the fronts in the vertical plane more clearly, the horizontal scale is usually taken many times smaller than the vertical one. In the presented diagram of the front, the vertical scale is increased almost 50 times.
The greatest length of fronts in height in middle latitudes is 8-12 km. They often reach the tropopause. According to the studies of E. Palmen, G. D. Zubyan and others, fronts are also observed in the lower layers of the stratosphere.
At tropospheric fronts, multi-layered clouds usually develop, from which precipitation falls. Fronts are most pronounced in cyclones, where upward air movement predominates. In anticyclones, due to downward movements, frontal clouds dissipate.
Atmospheric fronts are divided into cold and warm.
A cold front is a front that moves toward higher temperatures. After the passage of a cold front, a cold snap occurs. A warm front is a front moving towards low temperatures. After the passage of a warm front, warming occurs.
In the field of temperature and wind, fronts are most pronounced at the surface of the earth in the system of developing cyclones and pressure troughs. This is facilitated by the convergence of air currents in the front zone near the surface of the earth, since due to this convergence in the front zone there are air masses with low and high temperatures. In Fig. 53a shows the field of pressure, wind and temperature in the cyclone trough at the surface of the earth. The front is intensifying, since to the north there is a cold air mass with temperatures of 1-2° below zero, and to the south there is a warm air mass with temperatures up to 10-12° above zero.
In anticyclones, fronts near the earth's surface are washed out, since the system of air currents diverges (Fig. 53 6). Here, in the first part of the ridge, the cold section of the front near the surface of the earth is washed away, since the flows are directed not towards the front, but away from the front. In the system of a developing cyclone, the air tends to rise upward and, as a result of dynamic cooling and condensation, clouds appear and precipitation occurs. In the system of a developing anticyclone, on the contrary, there is a downward movement of air and, as a result of dynamic heating, the air moves away from the saturation state, clouds dissipate and precipitation stops.
The speed of the front depends on the magnitude of the normal wind component, which varies widely. In Europe, during the transition seasons of the year, the average speed of movement of fronts reaches approximately 30 km/h, which is about 700 km per day; but often in a cyclone system, fronts travel a distance of more than 1200-1500 km per day. In these cases, the front, located, for example, in Western Europe, within a day turns out to be in the central regions of the European territory of the USSR. If air currents are directed parallel to the front, then the front remains inactive. Since temperature and pressure gradients in winter are much greater than in summer, the activity of fronts in winter is more intense.
We have already said that in the zone of the atmospheric front, especially in the system of a developing cyclone, air rises, adiabatic cooling, and the formation of clouds and precipitation occur. The rise of air occurs not only in the ground layer, but also at heights. But if in the surface layer it is caused by the convergence of the surface wind, then the reason for the rise of air at altitudes is the unsteady movement and the difference in the speed of movement of the transfrontal and prefrontal air.
In the case of a cold front, fast-moving cold air behind the front, flowing under the warm air, displaces it upward. As a result, if dynamic conditions cause a general rise of air, warm air begins to slide upward along the inclined surface of the front and cool adiabatically.
In the case of a warm front, under the same conditions, there is also an upward movement of warm air over a wedge of cold air. The greater the temperature difference between cold and warm air, i.e., the more pronounced the front is not only at the surface of the earth, but also at heights, the more intense under the same conditions the upward movement of warm air, condensation, and the formation of clouds and precipitation occur.
On a well-defined front, clouds of all tiers are represented. The clouds of a warm front can be very powerful; they often extend horizontally perpendicular to the front for 500-700 km, and vertically up to 6-8 km or more. Moreover, the length of such a front can reach 1000-2000 km. The upper part of powerful frontal clouds, even in summer, is located in the zone of negative temperatures, so it usually consists of ice crystals. In Fig. 54 in a vertical section perpendicular to the front shows a cloud system characteristic of a warm front. These clouds are stratified and are located predominantly in warm air above the frontal surface. The uppermost clouds (cirrus and cirrostratus) are located at altitudes of 6-8 km. They are harbingers of a warm front. The appearance of these clouds several hours before the approaching precipitation zone indicates worsening weather. Cirrostratus clouds are replaced by altostratus clouds, through which the sun still shines through, nevertheless they have a greater vertical thickness. This is followed by denser nimbostratus clouds, producing blanket precipitation that reaches the ground. Below are stratus and nimboclouds, the height of the lower boundary of which, depending on the moisture content, can range from zero to several hundred meters. At the same time, as can be seen in Fig. 54, low-level clouds form not only in the warm above-frontal air, but also partially in the cold air in the immediate vicinity of the frontal surface. The arrows in this figure show the direction of air flow in warm and cold air with a general transfer from left to right in the plane of the diagram presented here.
The cloud system of a powerful cold front is shown in Fig. 55. As is easy to see, the profiles of the warm (Fig. 54) and cold (Fig. 55) fronts are noticeably different from each other. This happens because when moving, the warm air in the lower layer, due to friction with the earth's surface, is stretched in the direction opposite to the movement. Meanwhile, the cold front becomes steeper due to friction in the lower 1-2 km layer.
Shown in Fig. 54 and 55 cloud systems of warm and cold fronts refer to those cases when the vertical extent of the fronts is large, the temperature contrasts at the front are significant and there is intense upward air movement. Air masses on both sides of the front are stable. If, under all these conditions, the cold air is stratified unstably, then the cold front is followed not by stratocumulus clouds, but by powerful cumulus and cumulonimbus clouds. If at the same time both cold air and warm air are stratified unstably, then powerful squall clouds form ahead of the front (Fig. 56), giving heavy rainfall, accompanied by thunderstorms and even hail.
The cloud system of a warm front also has variations. When warm air is unstable, convective clouds form and rainfall occurs. It is assumed that the air moisture content is sufficient.
However, the vertical extent of atmospheric fronts is not always significant; often it does not exceed 1-3 km. In accordance with this, frontal cloudiness receives limited development, with the exception of those cases when, due to instability, convective cloudiness is formed, reaching a height of 5-6 km or more. Even with a large vertical extent of the front, frontal clouds do not represent a continuous medium, as shown in Fig. 54 and 55, but consists of a number of layers with cloudless spaces between them (Fig. 57 a). This is due to the fact that in many cases the general rise of warm air is disrupted and layers with ascending and descending air movements alternate in the front zone. In this case, the latter cause the destruction of the cloud system of the front, up to the complete dispersion of clouds. When the air is very dry, cloud formation at the front either does not occur at all, or low-power clouds of the middle and upper tiers appear that do not produce precipitation (Fig. 57 6).
There are other types of fronts that occur when cold and warm fronts meet. The closing of fronts occurs as a result of the fact that they move at different speeds. In a cyclone system, cold fronts typically move at higher speeds than warm fronts. Therefore, the cold front, catching up with the warm one, closes with it, forming a closure front, or, as is usually called, an occlusion front. At first, the cloud systems of both fronts, having closed, persist and give abundant, predominantly blanket precipitation. However, gradually the intensity of the occlusion front weakens due to the already existing process of blurring it. At the same time, powerful cloud systems begin to dissipate and the front is detected in the surface wind field by the remnants of clouds. In Fig. 58 schematically shows the closure of cold and warm fronts as they move from left to right. Cold air, being denser, wedges under the warm air.
All types of fronts, when meeting mountain obstacles, leave a lot of moisture on their windward side. However, as the high mountain obstacle is overcome, the cloud system of fronts is disrupted, and on the leeward side of the mountains the clouds spread out, and precipitation often stops. Only after overcoming the obstacle the cloud frontal system is restored again.
The study of atmospheric fronts is dictated by the need to expand knowledge in this area in connection with the requirements of practice, especially aviation, since powerful clouds, like sudden changes in weather, are associated with fronts. Therefore, their study is one of the most important tasks of meteorologists.
Despite the importance of the task of studying fronts, knowledge about the conditions for their occurrence is still far from sufficient. This primarily applies to the formation and evolution of frontal clouds. The above diagrams only give a general idea of frontal clouds. In reality, clouds in the zone of atmospheric fronts comprise both a continuous medium and thick layers with cloudless spaces between them.
Difficulties in studying the physics of cloud formation at fronts are associated with the lack of methods for a massive and detailed study of all the features of cloud development under certain synoptic conditions, since this requires a long stay at altitudes, which is technically difficult to implement.
Really modern airplanes, flying at high speed, make it possible to make observations and various measurements along the flight path. Balloons are most convenient for studying clouds. But they cannot always enter the cloud of interest to us. In particular, the balloon cannot enter thunderclouds, as it could be ignited by a lightning flash.
It was already mentioned above that the formation of clouds is caused by the condensation of water vapor due to the rise of air and its adiabatic cooling. To imagine the difficulties of studying the evolution of cloudiness, it is enough to say that the vertical air movements that cause the formation and destruction of clouds are not yet amenable to direct measurements. Approximate calculations of vertical movements are currently made mainly from theoretical premises of changes in pressure and wind fields at various heights.
The study of atmospheric fronts and their cloud systems attracts the attention of many scientists both in the USSR and abroad. Often, risking their lives, they fly in thunderclouds and step by step expand their knowledge of frontal activity. The provisions on the structural features of fronts, developed mainly by Norwegian meteorologists (T. Bergeron, S. Petersen, etc.), were revised and clarified by Soviet scientists. Thanks to the works of A. F. Dyubyuk, N. L. Taborovsky, E. G. Zak, E. K. Fedorov, G. D. Zubyan, E. S. Selezneva and others, our knowledge about the emergence and erosion of fronts, the nature of vertical air movements and cloud formation, as well as other issues related to fronts, have been significantly enriched. And yet, many important features of cloud formation and changes in cloud forms during the evolution of fronts remain unknown.
There is no unity of views on the issue of the vertical extent of fronts in the troposphere and on front formation in the stratosphere. However, in recent years, more and more scientists have come to the conclusion that tropospheric fronts in most cases reach the tropopause; higher - in the stratosphere - they also exist (G.D. Zubyan, R. Bergren), but due to the negligible moisture content of the air, clouds do not form at the stratospheric fronts.
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. 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 (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. Different from the main fronts are secondary atmospheric fronts that arise within the air masses of a particular natural zone 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.