Atmospheric front(Greek atmos - steam and Lat. front is - forehead, front side).
When heterogeneous elements come together, transitional or frontal zones arise, which continuously move and blur; the speed and strength of these processes depends on the temperature difference between the meeting masses. In the frontal zone it intensifies, reaching speeds (200 km/h) at an altitude of 9-12 km, large atmospheric vortices, and also identify the interface between cold and warm air masses. These partition surfaces are called atmospheric fronts. Their width is insignificant - several tens of kilometers, vertical thickness - several hundred meters. The inclination of the atmospheric front to the Earth is very slight, less than 1°.
If the front moves towards more high temperatures, this means that cold air is coming. Such a front is called a cold front. When it occurs, heavy cold air masses displace upward lighter warm air masses, which, rising upward, cool, the moisture contained in them is released and forms. The immediate approach of a cold front can be seen by the accumulation of powerful cumulonimbus clouds on. They quickly advance like a wall and soon occupy the entire sky. Their lower edge is so low that it seems to drag along the ground. The dazzling white curly top rises to a height of over 10 km. In nature it becomes quiet and stuffy, nature freezes. Soon the wind begins to blow in gusts and suddenly changes direction. A wall of torrential rain suddenly falls, often accompanied by hail. The darkened sky is crossed by lightning, deafening rumbles of thunder are heard. Bad weather usually does not last long, rarely more than two. Afterwards it becomes colder, as the space is occupied by cold air masses. Afterwards, heavy rains may begin, which will gradually turn into drizzling. What happens next will depend on the speed of the cold front's advance.
When a warm front approaches, warm air moves toward the low temperatures and, flowing onto the cold air mass, slides along it, rising upward and forming clouds. Cirrus clouds form high in the atmosphere. They are harbingers of a warm front. Soon these clouds begin to melt, and a continuous veil of almost imperceptible, thin cirrostratus clouds forms above the earth's surface in the atmosphere. The cloud layer quickly thickens and descends lower. The wind picks up and light rain begins to drizzle (or snowflakes swirl). It gradually intensifies and then pours for several hours. This type of rain is called heavy rain. The advance of a warm front is accompanied by an increase. However, it does not last long, as it soon approaches again cold front, which usually moves faster.
The front line is never straight, it is winding. Line bends to the north are usually caused by tongues of warm air, bends to the south - by tongues of cold air. When the bends of the frontal line close together, powerful atmospheric vortices arise -
), are separated from each other by rather narrow transition zones, which are strongly inclined towards earth's surface(less than 1°). front is the division between those with different physical properties. The intersection of the front with the earth's surface is called the front line. All properties at the front air masses- temperature, wind direction and speed, humidity, precipitation - change dramatically. The passage of the front through the observation location is accompanied by more or less abrupt changes.
There are fronts associated with cyclones and climate fronts.
In cyclones, fronts form when warm and cold air meet, with the top of the frontal system typically located in the center. Cold air, meeting warm air, always ends up at the bottom. It flows under the warm one, trying to push it upward. Warm air, on the contrary, flows onto cold air and if it presses against it, it itself rises along the interface plane. Depending on which air is more active and in which direction the front moves, it is called warm or cold.
A warm front moves toward cold air and signifies the arrival of warm air. It slowly pushes back the cold air. Being lighter, it flows onto the wedge of cold air, gently rising up along the interface. In this case, a vast zone of clouds forms in front of the front, from which heavy precipitation falls. The precipitation strip ahead of the warm front reaches 300, and in cold times even 400 km. Behind the front line, precipitation stops. The gradual replacement of cold air with warm air leads to a decrease in pressure and increased wind. After the front passes, a sharp change in the weather is observed: it rises, changes direction by about 90° and weakens, visibility deteriorates, drizzle is formed, and drizzle can occur.
A cold front moves toward warm air. In this case, cold air - as denser and heavier - moves along the earth's surface in the form of a wedge, moves faster than warm air and, as it were, lifts the warm air in front of it, vigorously pushing it upward. Above the front line and in front of it, large cumulonimbus formations form, from which torrential rains fall, arise, and are observed strong winds. After the front passes, precipitation and cloudiness decrease significantly, the wind changes direction by approximately 90° and weakens somewhat, the temperature drops, air humidity decreases, and its transparency and visibility increases; growing.
The Arctic (Antarctic) front separates the Arctic (Antarctic) air from the air of temperate latitudes, two temperate (polar) fronts separate the air of temperate latitudes and tropical air. A tropical front forms where tropical air and air that differ in temperature, not temperature, meet. All fronts, together with the boundaries of the belts, shift towards the poles in summer, and in winter. They often form separate branches spreading over long distances from. The tropical front is always in the hemisphere where it is summer.
Atmospheric fronts have several different 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.
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. Home characteristic feature 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 the air masses of the main geographic types indicated above are called the 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.
The frontal zone is a transition zone between air masses with different properties, strongly inclined towards the earth's surface towards cold air. It rises up several kilometers, where its horizontal extent can be thousands of kilometers.
The width of the frontal zone at the Earth's surface is tens of kilometers. Since its dimensions are small compared to the dimensions of air masses, it is usually represented as a frontal surface, the line of intersection of which with the surface of the earth is called the front. As the front passes, all weather elements change sharply, extensive cloud systems form, precipitation falls, and the wind increases. Fronts can arise and develop (this process is called frontogenesis), and also erode and disappear (frontolysis).
Depending on the direction of movement of air masses, atmospheric fronts are divided into warm, cold, sedentary and occlusion fronts.
Warm front
A warm front occurs when air masses move when a cold air mass is replaced by a warm one. Warm air, as a lighter one, flows onto the cold wedge, rises, cools, from a certain height the vapors begin to condense, forming a characteristic powerful cloudiness, consisting of cirrus, cirrostratus, altostratus and nimbostratus clouds, forming a huge wedge-shaped massif. A diagram of the change in cloud types characteristic of a warm front is shown in Fig. 12, and the order of changes in meteorological elements during its passage is in table. 1.
Table 1. Changes in meteorological elements during the passage of a warm front.
| Weather elements | Before the front | When the front passes | Behind the front |
| Atmosphere pressure | Falls, usually evenly (cold wedge, more heavy air decreases above the observation point (Fig. 12)). | The fall is slowing down | Changes little or grows little |
| Wind | Intensifies, turns counterclockwise (in the northern hemisphere) | Rotates clockwise (in the northern hemisphere) | Weaken, direction does not change |
| Air temperature | Does not change or grows weakly | Increases (the warm air mass at the observation point replaces the cold one (Fig. 12)) | Little changes |
| Cloudiness | Consistently replace each other: cirrus, cirrostratus, altostratus, nimbostratus clouds. Under the frontal surface it is possible to appear cumulus clouds(Fig. 12) | Nimbostratus | Stratus or stratocumulus |
| Precipitation | 300-400 km before the front line, heavy precipitation begins | Almost stop | Chance of drizzle |
Cold front
A cold front occurs when air masses move when a warm air mass is replaced by a cold one. In this case, the angle of inclination of the frontal surface is, as a rule, greater than that of a warm front. There are cold fronts of the first and second kind.
Cold front of the first kind
This is the name given to a slow-moving cold front. As air masses move, cold air slowly flows under warm air, which leads to the appearance of a cloud system reminiscent of a warm front system located in reverse order as he moves. The horizontal dimensions of the cloud system and precipitation zone for this type of atmospheric front are smaller than for a warm one.
Before the front, cumulonimbus clouds can develop in the warm air mass, the appearance of which is caused by rising air currents. The movement of fronts occurs due to the influence of wind. The direction of the wind in mid-latitudes coincides with the direction of the tangent to the isobar. Therefore, if on a weather map the line of a cold front passes at a slight angle to the isobar, then the wind will blow almost along the front and the speed of movement of the latter will be small. That is, such a front will be a front of the first kind.
Cold front of the second kind
This is the name given to a fast-moving cold front. On the weather map, the line of this front in relation to the isobars is located at an angle close to a straight line (the wind blows almost perpendicular to the front, which leads to fast movement the latter). The rapid flow of cold air under warm air leads to the development of strong convection (updrafts) in a narrow band in front of the front and the appearance of powerful cumulonimbus clouds.
The turbulence of the updrafts causes the presence of squally winds at the surface of the earth. The main type of precipitation is torrential. The precipitation zone is usually so narrow that it is almost not visible on weather maps. The cloud system of altostratus and cirrostratus clouds in the rising flow of warm air is strongly extended forward from the frontal surface and is blurred into separate altocumulus lenticular and small cirrocumulus clouds. The order of changes in meteorological elements during its passage is in table. 2.
Table 2. Changes in meteorological elements during the passage of a cold front.
| Weather elements | Before the front | As the front passes | Behind the front |
| Atmosphere pressure | Falls | Fall gives way to growth | Grows rapidly (the wedge of cold, heavier air above the observer becomes increasingly higher), then growth slows or stops |
| Wind | Intensifies, turns counterclockwise (in the northern hemisphere) | Significantly intensifies, becomes squally, turns sharply clockwise (in the northern hemisphere) | Rotates counterclockwise (in the northern hemisphere). Strong gusty winds persist |
| Air temperature | Stable or slightly decreasing | Decreases sharply | Continues to decrease or changes little |
| Cloudiness | For the front of the 1st kind - powerful Cb. For a front of the 2nd type, individual Cc are possible, and below them – Ac, then the appearance of powerful Cb clouds. | For a cold front of the first kind - Ns. For fronts of the 2nd type – Cb, under which broken-rain clouds are observed. | For a cold front of the first kind, the cloud system is basically the opposite of the warm front (Ns, As, Cs, Ci change sequentially). For a front of the second type, cloudiness quickly disappears. |
| Precipitation | Usually small, starting just before the front | Showers, often strong | Stop quickly or turn into short-term showers |
| Other phenomena | Thunderstorms are common | Thunderstorms, increased wind waves | Strong excitement persists |
Front of occlusion
A cold front always moves faster than a warm front and gradually catches up with it. When fronts close, the warm air mass located between the frontal surfaces is forced upward and breaks away from the earth's surface. This process is called occlusion.
The development of occlusion depends on the thermal regime of air masses. If they have the same temperatures, then the front is eliminated at the surface of the earth. Warm air ends up in a trough formed by the surfaces of previous cold and warm fronts and is called neutral. If the rear cold air is colder than the one in front, then such a front is called a cold front occlusion. In this case, the surface of the warm front slides over the surface of the cold front. If the rear air is warmer than the air in front, then such a front is called a warm front occlusion.
Occlusion fronts are characterized by a wide variety of cloud systems and precipitation. IN general outline the weather during a warm front occlusion is similar to the weather of warm fronts, and during a cold occlusion, it is similar to the weather of cold fronts. Occlusion fronts, as a rule, are associated with well-defined pressure troughs. The order of changes in meteorological elements during the passage of the occlusion front is given in Tables 3 and 4.