What is a cyclone and anticyclone definition briefly. What is a cyclone? Tropical cyclone in the Southern Hemisphere. Cyclones and anticyclones - characteristics and names. Cyclones and anticyclones
Atmospheric phenomena have been an object of study for centuries because of their significance and influence on all spheres of life. Cyclones and anticyclones are no exception. The concept of these weather phenomena is given at school by geography. Cyclones and anticyclones, after such a brief study, remain a mystery to many. and fronts are key concepts that will help capture the essence of these weather events.
air masses
It often happens that for many thousands of kilometers in a horizontal direction, the air has very similar properties. This mass is called air mass.
Air masses are divided into cold, warm and local:
A cold mass is called if its temperature is lower than the temperature of the surface over which it is located;
Warm - this is such an air mass, the temperature of which is higher than the temperature of the surface that is under it;
The local air mass does not differ in temperature from the surface below it.
Air masses form over different parts of the Earth, which leads to peculiarities in their properties. If the mass is formed over the Arctic, then, accordingly, it will be called Arctic. Of course, such air is very cold, it can bring thick fogs or light haze. Polar air considers temperate latitudes to be its deposit. Its properties may vary depending on what time of the year it is. In winter, the polar masses are not much different from the Arctic ones, but in summer such air can bring very poor visibility.
Tropical masses that came from the tropics and subtropics have a high temperature and increased dust content. They are responsible for the haze that covers objects when viewed from a distance. Tropical masses formed on the continental part of the tropical belt lead to dust whirlwinds, storms and tornadoes. Equatorial air is very similar to tropical air, but all these properties are more pronounced.
Fronts
If two air masses with different temperatures meet, a new weather phenomenon is formed - a front, or interface.
According to the nature of the movement, the fronts are divided into stationary and mobile.
Each existing front divides the air masses among themselves. For example, the main polar front is an imaginary mediator between polar and tropical air, the main arctic front is between arctic and polar air, and so on.
When a warm air mass moves over a cold air mass, a warm front occurs. For travelers, the entrance to such a front may herald either heavy rain or snow, which will significantly reduce visibility. When cold air is wedged under warm air, a cold front is formed. Ships entering the cold front suffer from squalls, downpours and thunderstorms.
It happens that air masses do not collide, but catch up with one another. In such cases, an occlusion front is formed. If the role of the catching-up is performed by the cold mass, then this phenomenon is called the front of cold occlusion, if vice versa, then the front of warm occlusion. These fronts bring torrential weather with strong gusts of wind.
Cyclones
To understand what an anticyclone is, you need to understand, This is an area in the atmosphere with a minimum indicator in the center. It is generated by two having different temperatures. Very favorable conditions for their formation are created in the fronts. In a cyclone, air moves from its edges, where the pressure is higher, to the center. In the center, the air seems to be thrown upwards, which makes it possible to form ascending flows.
By the way the air moves in a cyclone, it is easy to determine in which hemisphere it was formed. If its direction coincides with the movement of the hour hand, then this is definitely the Southern Hemisphere, if it is against it, this is
Cyclones provoke such weather phenomena as the accumulation of cloud masses, heavy precipitation, wind and temperature changes.
tropical cyclone
From cyclones formed in temperate latitudes, cyclones are separated, which owe their origin to the tropics. They have many names. These are hurricanes (West Indies), and typhoons (east of Asia), and simply cyclones (Indian Ocean), and arcana (south of the Indian Ocean). The dimensions of such vortices range from 100 to 300 miles, and the diameter of the center is from 20 to 30 miles.
The wind here accelerates to 100 km / h, and this is typical for the entire area of the vortex, which radically distinguishes them from cyclones formed in temperate latitudes.
A sure sign of the approach of such a cyclone is ripples on the water. Moreover, it goes in the opposite direction to the blowing wind or the wind that blew shortly before.
Anticyclone
The area of high pressure in the atmosphere with a maximum in the center is the anticyclone. The pressure at its edges is lower, which allows air to rush from the center to the periphery. The air located in the center constantly descends and diverges towards the edges of the anticyclone. This is how downward flows are formed.
An anticyclone is the opposite of a cyclone also because in the Northern Hemisphere it follows the hour hand, in the Southern Hemisphere it goes against it.
After re-reading all the above information, we can say with confidence what an anticyclone is.
An interesting property of anticyclones of temperate latitudes is that they seem to follow cyclones. In this case, the sedentary state fully characterizes the anticyclone. The weather formed by this vortex is slightly cloudy and dry. There is practically no wind.
The second name of this phenomenon is the Siberian maximum. Its life expectancy is about 5 months, namely the end of autumn (November) - the beginning of spring (March). This is not one anticyclone, but several, which very rarely give way to cyclones. The height of the winds reaches 3 km.
Due to the geographical environment (mountains of Asia) cold air cannot disperse, which leads to even more cooling, the temperature near the surface drops to 60 degrees below zero.
Speaking about what an anticyclone is, we can say with confidence that this is an atmospheric vortex of enormous size, bringing clear weather without precipitation.
Cyclones and anticyclones. Similarities and differences
In order to understand better what an anticyclone and a cyclone are, you need to compare them. We have clarified the definitions and main aspects of these phenomena. The question of how cyclones and anticyclones differ remains open. The table will show this difference more clearly.
| № | Characteristic | Cyclone | Anticyclone |
| 1. | Dimensions | 300-5000 km in diameter | Can reach 4000 km in diameter |
| 2. | Travel speed | 30 to 60 km/h | From 20 to 40 km/h (except for sedentary vehicles) |
| 3. | Places of origin | Anywhere but the equator | Over ice and in the tropics |
| 4. | Causes | Due to the natural rotation of the Earth (Coliolis force), with a deficit of air mass. | Due to the occurrence of a cyclone, with an excess of air mass. |
| 5. | Pressure | Low in the center, high at the edges. | High in the center, low at the edges. |
| 6. | Direction of rotation | In the Southern Hemisphere - clockwise, in the Northern - against it. | In the South - counterclockwise, in the North - clockwise. |
| 7. | Weather | Cloudy, strong wind, lots of rain. | Clear or partly cloudy, no wind or precipitation. |
Thus, we see how cyclones and anticyclones differ. The table shows that these are not just opposites, the nature of their occurrence is completely different.
That this question is in the lead among the questions that are asked to weather forecasters. I've been meaning to write a post about this for a long time.
I remember in the children's story about 38 parrots there was a chapter that someone ruined the weather, but who is not explained there, and four animal friends push the blame on each other. So how do you answer if a child asks who ruined the weather? I answer my children like this: "The cyclone spoiled the weather. And I fixed it - the anticyclone." Probably, for many, knowledge of what these words mean ends there. Yes, I myself quite recently figured out why they affect the weather in this way. And also, why exactly such formations exist in the atmosphere.
Without complicating things too much, a picture that explains a lot might look something like this:
Usually, when describing a cyclone, the emphasis is on the fact that the rotation of air in it occurs counterclockwise (if you look at it from above in the northern hemisphere). In my opinion, it is much more interesting to look at it from the side, as shown in the figure. In the lower layer of the atmosphere, air is drawn into the cyclone, then it rises, and at the top it spreads. In this sense, a thundercloud is a reduced model of a cyclone, since the movement of air in a vertical plane occurs in it in the same way. And even the spreading of air above can be traced along the "anvil". The anticyclone is called so not in vain, because it really is the absolute antipode of the cyclone. In it, at the top, the air moves towards the center, in the central part it descends, and then spreads to the sides near the ground.
So, the fact that the air in a cyclone rises, and in an anticyclone falls down is the main thing that makes the weather. Ascending air movements cause it to cool, its humidity increases, and then clouds form, and precipitation begins to fall from them. And downward movements, on the contrary, lead to the fact that the air warms up, becomes drier, and the clouds disperse. Here is a simple explanation. But after that, a few more questions remain.
1. And what about atmospheric pressure, and why is it lowered in a cyclone, and increased in an anticyclone?
I could not answer this simple question for a long time, but recently I came to the conclusion that pressure is just a side factor, a consequence of vertical movements. Turn on the vacuum cleaner and point it at the wall. Obviously, the air flow will create excess pressure. The same thing happens in an anticyclone. The air moves towards the earth and presses on it. And in a cyclone - on the contrary.
2. What makes air move in a vertical plane?
When a cyclone or anticyclone has existed for a long time, the air moves like this, because other air presses on it from the sides, and you have to go somewhere. But when a cyclone starts, the trigger is that the air below is warmer and therefore lighter than the air above. More precisely, it should be warmer not in absolute terms, but the temperature should drop faster with height than in some equilibrium (adiabatic) distribution. Then there is a force that lifts the air up, as in a balloon. And then air comes from the side in its place, and the process has begun. The best conditions for the occurrence of a cyclone occur on atmospheric fronts: air masses of different temperatures just come into contact there. As soon as one fragment of the front, for some reason, “goes” in one direction, and the neighboring one in the other, a “wave” is formed, which then turns into a young cyclone.
3. What role does the rotation of the Earth play here?
The rotation of the Earth affects the rotation of air in the horizontal plane. If the Earth did not rotate, cyclones and anticyclones would not be able to exist stably, since the resulting pressure drops would quickly level out, and that's all. But, since the Earth rotates, the Coriolis force acts on the air, directed perpendicular to the direction of its movement. It is zero at the equator, so there are no cyclones there. The Coriolis force causes the air in the cyclones to twist, and this also maintains its movement in the vertical plane.
4. Why are there only two such formations? Why can't there be something else besides cyclones and anticyclones?
Because there are only two options: in the vertical plane, either upward movements or downward, and in the horizontal - either movement clockwise or counterclockwise. There is no third.
5. What is more on Earth: cyclones or anticyclones?
Everything is different at every moment, on average there are more cyclones, but on the other hand they are on average smaller in area.
6. Why do cyclones and anticyclones like to form in the same places?
There are places on Earth that are especially favorable for the development of baric formations of one type or another. For example, the North Atlantic is the most characteristic place for the formation of cyclones. There is everything for this: on the one hand - a warm current, and on the other - the glaciers of Greenland. And in more southern latitudes in the Atlantic there is almost always an anticyclone: it is supported by both cyclones in the north and a cold current.
7. Why do cyclones bring warm weather in winter, and anticyclones bring cold weather, and vice versa in summer?
For answering this question, I got 5+/5+ in geography at school :) The main factor here is cloudiness. In winter, the cloud cover itself limits the frost, keeping the ground cool during the long night. And in summer, on the contrary, cloudiness does not allow the sun to heat the earth. In addition to this, specifically, we also have air in cyclones in winter that comes most often from the ocean, and it is warmer.
8. Why is it sometimes the opposite: beautiful weather in a cyclone, and darkness in an anticyclone?
Because nature is much more complicated than the diagram that I drew. For example, in winter there may be an inversion in the anticyclone, when the air below is colder than above, and continuous clouds form, from which drizzling precipitation can even fall. And in some parts of the cyclone, for example, behind a cold front, the air may not rise, but fall. Different cyclones are just as different from each other as different girls :) The weather never repeats itself, and therefore it is so interesting to watch it.
air masses- these are large air masses of the troposphere and lower stratosphere, which are formed over a certain territory of land or ocean and have relatively uniform properties - temperature, transparency. They move as one unit and in the same direction in the atmosphere system.
Air masses occupy an area of thousands of square kilometers, their thickness (thickness) reaches up to 20-25 km. Moving over a surface with different properties, they heat up or cool down, or become drier. Warm or cold air mass is called, which is warmer (colder) than its environment. There are four types of air masses depending on the areas of formation: equatorial, tropical, temperate, arctic (Antarctic) air masses (Fig. 13). They differ primarily in temperature and humidity. All types of air masses, except for equatorial ones, are divided into maritime and continental, depending on the nature of the surface over which they formed.
The equatorial air mass is formed in the belt. It has rather high temperatures and humidity close to the maximum, both over land and over the sea. Continental tropical air mass is formed in the central part of the continents in. It has high temperature, low humidity, high dust content. Marine tropical air mass is formed over the oceans in tropical latitudes, where rather high air temperatures prevail and high humidity is noted.
Continental moderate air mass is formed over the continents in, dominates the Northern Hemisphere. Its properties change with the seasons. In summer, the temperature and humidity are quite high, precipitation is typical. In winter, low and extremely low temperatures and low humidity. Marine temperate air mass forms over oceans with warm currents in temperate latitudes. It is cooler in summer, warmer in winter, and has significant humidity.
The continental Arctic (Antarctic) air mass is formed over the ice of the Arctic and has extremely low temperatures and low humidity, high transparency. Marine Arctic (Antarctic) air mass is formed over periodically freezing seas and oceans, its temperature is slightly higher, humidity is higher.
Air masses are in constant motion; when they meet, transition zones, or fronts, are formed. - the border zone between two having different properties. The width of the atmospheric front reaches tens of kilometers. Atmospheric fronts can be warm or cold, depending on what kind of air is moving into the territory and what is being displaced (Fig. 14). Most often, atmospheric fronts occur in temperate latitudes, where cold air from polar latitudes and warm air from tropical latitudes meet.
The passage of the front is accompanied by changes in . The warm front moves towards the cold air. It is associated with warming, nimbostratus clouds, bringing drizzling precipitation. The cold front moves towards the warm air. It brings abundant short-term heavy rainfall, often with squally and, and cooling.
Cyclones and anticyclones
In the atmosphere, when two air masses meet, large atmospheric vortices arise -. They are flat air vortices covering thousands of square kilometers at a height of only 15-20 km.
Cyclone- an atmospheric vortex of huge (from hundreds to several thousand kilometers) diameter with reduced air pressure in the center, with a system of winds from the periphery to the center against in the Northern Hemisphere. In the center of the cyclone, ascending air currents are observed (Fig. 15). As a result of ascending air currents, powerful clouds form in the center of cyclones and precipitation falls.
In summer, during the passage of cyclones, the air temperature decreases, and in winter it rises, a thaw begins. The approach of a cyclone causes cloudy weather and a change in wind direction.
Tropical cyclones occur in tropical latitudes from 5 to 25° in both hemispheres. Unlike cyclones of temperate latitudes, they occupy a smaller area. Tropical cyclones occur over the warm sea surface in late summer - early autumn and are accompanied by powerful thunderstorms, heavy rainfall and storm-force winds, which have tremendous destructive power.
In tropical cyclones they are called, in the Atlantic -, off the coast of Australia - willy-willy. Tropical cyclones carry a large amount of energy from tropical to temperate latitudes, which makes them an important component of global atmospheric circulation processes. For their unpredictability, tropical ones are given female names (for example, "Catherine", "Juliet", etc.).
Anticyclone- an atmospheric vortex of huge diameter (from hundreds to several thousand kilometers) with an area of high pressure near the earth's surface, with a system of winds from the center to the periphery clockwise in the Northern Hemisphere. Downdrafts of air are observed in the anticyclone.
Both in winter and in summer, the anticyclone is characterized by a cloudless sky and calmness. During the passage the weather is sunny, hot in summer and very cold in winter. Anticyclones form over the ice sheets of Antarctica, over, over the oceans in tropical latitudes.
The properties of air masses are determined by the areas of their formation. When they move from their places of formation to others, they gradually change their properties (temperature and humidity). Due to cyclones and anticyclones, heat and moisture are exchanged between latitudes. The change of cyclones and anticyclones in temperate latitudes leads to sharp changes in the weather.
Short-term processes of wind formation
Short-term processes also lead to the formation of winds, which, unlike the prevailing winds, are not regular, but occur chaotically, often during a certain season. These processes are the formation cyclones, anticyclones and similar phenomena of a smaller scale, in particular thunderstorms.
Cyclone Katharina in the South Atlantic. March 26, 2004
Cyclones And anticyclones are called areas of low or, respectively, high atmospheric pressure, usually those that occur in a space larger than a few kilometers. On Earth, they form over most of the surface and are characterized by their typical circulation structure. Due to the influence of the Coriolis force, in the Northern Hemisphere, the movement of air around the cyclone rotates counterclockwise, and around the anticyclone - clockwise. In the Southern Hemisphere, the direction of movement is reversed. In the presence of friction on the surface, there is a component of movement towards the center or away from the center, as a result, the air moves in a spiral towards the area of low pressure or away from the area of high pressure.
Cyclone
Cyclone (from other Greek κυκλῶν - “rotating”) - an atmospheric vortex of huge (from hundreds to several thousand kilometers) diameter with reduced air pressure in the center.
Air movement (dashed arrows) and isobars (solid lines) in a cyclone in the northern hemisphere
The air in cyclones circulates counterclockwise in the northern hemisphere and clockwise in the southern. In addition, in air layers at a height from the earth's surface up to several hundred meters, the wind has a term directed towards the center of the cyclone along the baric gradient (in the direction of decreasing pressure). The value of the term decreases with height.
Schematic representation of the process of formation of cyclones (black arrows) due to the rotation of the Earth (blue arrows)
A cyclone is not just the opposite of an anticyclone, they have a different mechanism of occurrence. Cyclones constantly and naturally appear due to the rotation of the Earth, thanks to the Coriolis force. A consequence of Brouwer's fixed point theorem is the presence of at least one cyclone or anticyclone in the atmosphere.
There are two main types of cyclones - extratropical And tropical. The first are formed in temperate or polar latitudes and have a diameter of thousands of kilometers at the beginning of development, and up to several thousand in the case of the so-called central cyclone. Among the extratropical cyclones, southern cyclones are distinguished, which form on the southern border of temperate latitudes (Mediterranean, Balkan, Black Sea, South Caspian, etc.) and shift to the north and northeast. Southern cyclones have colossal reserves of energy; It is with the southern cyclones in central Russia and the CIS that the heaviest precipitation, winds, thunderstorms, squalls and other weather phenomena are associated.
Tropical cyclones form in tropical latitudes and are smaller (hundreds, rarely more than a thousand kilometers), but have larger baric gradients and wind speeds reaching storms. Such cyclones are also characterized by the so-called. "eye of the storm" - a central area with a diameter of 20-30 km with relatively clear and calm weather. Tropical cyclones can transform into extratropical cyclones during their development. Below 8-10° north and south latitude, cyclones occur very rarely, and in the immediate vicinity of the equator they do not occur at all.
Cyclones in the atmosphere of Saturn. Photograph of the Cassini probe
Cyclones occur not only in the Earth's atmosphere, but also in the atmospheres of other planets. For example, in the atmosphere of Jupiter for many years there has been a so-called big red spot which is, apparently, a long-lived anticyclone. However, cyclones in the atmospheres of other planets have not been studied enough.
The Great Red Spot in Jupiter's atmosphere (Voyager 1 image)
The Great Red Spot is a giant anticyclone hurricane, 24-40 thousand km long and 12-14 thousand km wide (significantly larger than the Earth). The size of the spot is constantly changing, the general tendency is to decrease; 100 years ago, the BKP was about 2 times larger and much brighter. However, it is the largest atmospheric vortex in the solar system.
Color animation of BKP movement
The Great Dark Spot in Neptune's Atmosphere
The dark, elliptical spot (13,000 km × 6,600 km) was similar in size to the Earth. Around the spot, the wind speed reached 2400 km / h, which was the highest in the entire solar system. The spot is thought to be a hole in Neptune's methane clouds. A large dark spot is constantly changing its shape and size.
Great Dark Spot
extratropical cyclone
Cyclones that form outside the tropics are known as extratropical. Of the two types of large-scale cyclones, they are the larger (classified as synoptic cyclones), the most common, and occur over most of the earth's surface. It is this class of cyclones that is most responsible for day-to-day weather changes, and their prediction is the main goal of modern weather forecasts.
According to the classical (or Norwegian) model of the Bergen School, extratropical cyclones form mainly near the polar front in zones of especially strong high-altitude jet stream and receive energy due to a significant temperature gradient in this region. During the formation of a cyclone, the stationary atmospheric front breaks into sections of warm and cold fronts moving towards each other with the formation of an occlusion front and the swirling of the cyclone. A similar picture also arises in the later Shapiro-Keizer model based on the observation of oceanic cyclones, with the exception of the long movement of the warm front perpendicular to the cold one without the formation of an occlusion front.
Norwegian and Shapiro-Keyser models of extratropical cyclone formation
After the formation, the cyclone usually exists for several days. During this time, it manages to advance over a distance of several hundred to several thousand kilometers, causing sharp changes in winds and precipitation in some areas of its structure.
Although large extratropical cyclones are usually associated with fronts, smaller cyclones can form within a relatively homogeneous air mass. A typical example is cyclones that form in polar air currents at the beginning of the formation of a frontal cyclone. These small cyclones are called polar and often occur over the polar regions of the oceans. Other small cyclones occur on the lee side of mountains under the influence of westerly winds of temperate latitudes.
extratropical cyclone - a cyclone that forms during the year in the extratropical latitudes of each hemisphere. In 12 months there can be many hundreds of them. The sizes of extratropical cyclones are very significant. A well-developed cyclone can be 2-3 thousand km across. This means that it can simultaneously cover several regions of Russia or provinces of Canada and determine the weather regime in this vast territory.
Propagation of an extratropical cyclone
The vertical spread (vertical power) of a cyclone changes as it develops. At first, the cyclone is noticeably pronounced only in the lower part of the troposphere. The temperature distribution in the first stage of a cyclone's life is, as a rule, asymmetric with respect to the center. In front of the cyclone, with air inflow from low latitudes, temperatures are elevated; in the rear, with an influx of air from high latitudes, on the contrary, they are lowered. Therefore, with height, the isobars of the cyclone open up: a ridge of increased pressure is found at heights above the warm front part, and a depression of low pressure is found above the cold rear part. With height, this wave formation, the curvature of the isobars or isohypse, is more and more smoothed out.
Video showing the development of an extratropical cyclone
But with subsequent development, the cyclone becomes high, that is, closed isobars are found in it and in the upper half of the troposphere. At the same time, the air temperature in the cyclone generally decreases, and the temperature contrast between the front and rear parts is more or less smoothed out: a high cyclone is generally a cold region of the troposphere. The penetration of the cyclone into the stratosphere is also possible.
The tropopause above a well-developed cyclone is bent down in the form of a funnel; First, this decrease in the tropopause is observed over the cold rear (western) part of the cyclone, and then, when the cyclone becomes cold in its entire area, a decrease in the tropopause is observed over the entire cyclone. The temperature of the lower stratosphere above the cyclone is increased in this case. Thus, in a well-developed high cyclone, a low-beginning warm stratosphere is observed above the cold troposphere.
Temperature contrasts in the area of the cyclone are explained by the fact that the cyclone arises and develops on the main front (polar and arctic) between air masses of different temperatures. Both these masses are drawn into the cyclonic circulation.
In the further development of the cyclone, warm air is pushed into the upper part of the troposphere, above the cold air, and itself undergoes radiative cooling there. The horizontal temperature distribution in the cyclone becomes more uniform, and the cyclone begins to fade.
The pressure at the center of the cyclone (depth of the cyclone) at the beginning of its development does not differ much from the average: it can be, for example, 1000-1010 mb. Many cyclones do not deepen more than 1000-990 mb. Relatively rarely, the depth of the cyclone reaches 970 mb. However, in especially deep cyclones, the pressure drops to 960–950 mb, and in some cases 930–940 mb (at sea level) was observed with a minimum of 925 mb in the northern hemisphere and 923 mb in the southern hemisphere. The deepest cyclones are observed at high latitudes. Over the Bering Sea, for example, in one third of all cases, the depth of cyclones in winter is from 961 to 980 mb.
As the cyclone deepens, the wind speeds in it increase. Winds sometimes reach storm speeds over large areas. In the cyclones of the southern hemisphere, this happens especially often. Individual gusts of wind in cyclones can reach 60 m/sec, as was the case on December 12, 1957 in the Kuril Islands.
The life of a cyclone lasts several days. In the first half of its existence, the cyclone deepens, in the second it fills up and, finally, disappears altogether (fades out). In some cases, the existence of a cyclone turns out to be long, especially if it combines with other cyclones, forming one common deep, vast and inactive area of low pressure, the so-called central cyclone. They in the northern hemisphere are most often formed in the northern parts of the Atlantic and Pacific oceans. On climatological maps in these regions, well-known centers of action are noted - the Icelandic and Aleutian depressions.
Having already filled in the lower layers, the cyclone can persist for some time in the cold air of the upper layers of the troposphere in the form high-altitude cyclone.
tropical cyclone
Diagram of a tropical cyclone
Cyclones that form in the tropics are somewhat smaller than extratropical cyclones (they are classified as mesocyclones) and have a different mechanism of origin. These cyclones are powered by the upwelling of warm, moist air and can exist exclusively over warm regions of the oceans, which are why they are called warm-core cyclones (as opposed to cold-core extratropical cyclones). Tropical cyclones are characterized by very strong winds and significant rainfall. They develop and gain strength over the surface of the water, but quickly lose it over land, which is why their destructive effect usually manifests itself only on the coast (up to 40 km inland).
For the formation of a tropical cyclone, a section of a very warm water surface is required, the heating of the air above which leads to a decrease in atmospheric pressure by at least 2.5 mm Hg. Art. Humid warm air rises, but due to its adiabatic cooling, a significant amount of retained moisture condenses at high altitudes and falls as rain. The drier and thus denser air that has just been freed from moisture sinks down, forming zones of higher pressure around the core of the cyclone. This process has a positive feedback, so that as long as the cyclone is above a fairly warm water surface, which supports convection, it continues to intensify. Although tropical cyclones most often form in the tropics, sometimes other types of cyclones develop later in their existence, as happens with subtropical cyclones.
tropical cyclone A type of cyclone, or low-pressure weather system, that occurs over a warm sea surface and is accompanied by severe thunderstorms, heavy rainfall, and gale-force winds. Tropical cyclones get their energy from lifting moist air up, condensing water vapor as rain, and sinking the drier air that results from this process down. This mechanism is fundamentally different from that of extratropical and polar cyclones, in contrast to which tropical cyclones are classified as "warm core cyclones".
The term "tropical" means both the geographical area where such cyclones occur in the vast majority of cases, that is, tropical latitudes, and the formation of these cyclones in tropical air masses.
In the Far East and Southeast Asia, tropical cyclones are called typhoons and in North and South America hurricanes(Spanish) huracan, English hurricane), named after the Mayan wind god Huracan. It is generally accepted, according to the Beaufort scale, that storm goes into Hurricane with wind speeds over 117 km/h.
Tropical cyclones can cause not only extreme downpours, but also large waves on the sea surface, storm surges and tornadoes. Tropical cyclones can form and maintain their strength only over the surface of large bodies of water, while over land they quickly lose strength. That is why coastal areas and islands suffer the most from the destruction they cause, while areas inland are relatively safe. However, heavy rains caused by tropical cyclones can cause significant flooding a little further from the coast, at a distance of up to 40 km. Although the effect of tropical cyclones on humans is often very negative, significant amounts of water can end droughts. Tropical cyclones carry a large amount of energy from tropical to temperate latitudes, which makes them an important component of global atmospheric circulation processes. Thanks to them, the difference in temperature in different parts of the Earth's surface is reduced, which allows the existence of a more temperate climate on the entire surface of the planet.
Many tropical cyclones form under favorable conditions from weak atmospheric disturbances, the occurrence of which is influenced by such effects, like the Madden-Julian oscillation, El Niño And North Atlantic oscillation.
Madden-Julian oscillation - fluctuations in the circulation properties of the tropical atmosphere with a period of 30-60 days, which is the main factor in interseasonal variability in the atmosphere on this time scale. These fluctuations have the form of a wave that moves eastward at a speed of 4 to 8 m/s over the warm regions of the Indian and Pacific Oceans.
Long wavelength radiation pattern showing Madden-Julian oscillation
The movement of the wave can be seen in various manifestations, most clearly in changes in the amount of precipitation. First, the changes appear in the western Indian Ocean, gradually shift to the central Pacific, and then fade as you move to the cold eastern regions of this ocean, but sometimes reappear with reduced amplitude over the tropical regions of the Atlantic Ocean. In this case, at first there is a phase of increasing convection and precipitation, followed by a phase of decreasing precipitation.
The phenomenon was discovered by Ronald Madden and Paul Julian in 1994.
El Niño (Spanish) El Nino- baby, boy) or southern oscillation - fluctuations in the temperature of the surface layer of water in the equatorial part of the Pacific Ocean, which has a noticeable effect on the climate. In a narrower sense, El Niño is the phase of the Southern Oscillation, in which the region of heated near-surface waters shifts to the east. At the same time, the trade winds weaken or stop altogether, upwelling slows down in the eastern part of the Pacific Ocean, off the coast of Peru. The opposite phase of the oscillation is called La Niña(Spanish) La Nina- baby girl). The characteristic time of oscillation is from 3 to 8 years, however, the strength and duration of El Niño in reality varies greatly. So, in 1790-1793, 1828, 1876-1878, 1891, 1925-1926, 1982-1983 and 1997-1998 powerful El Niño phases were recorded, while, for example, in 1991-1992, 1993, 1994 this phenomenon , often repeating, was weakly expressed. El Niño 1997-1998 was so strong that it attracted the attention of the world community and the press. At the same time, theories about the connection of the Southern Oscillation with global climate changes spread. Since the early 1980s, El Niño also occurred in 1986-1987 and 2002-2003.
El Niño 1997 (TOPEX)
Normal conditions along the western coast of Peru are determined by the cold Peruvian Current, which carries water from the south. Where the current turns west, along the equator, cold and plankton-rich water rises from deep depressions, which contributes to the active development of life in the ocean. The cold current itself determines the aridity of the climate in this part of Peru, forming deserts. The trade winds drive the heated surface layer of water into the western zone of the tropical Pacific Ocean, where the so-called tropical warm basin (TTB) is formed. In it, the water is heated to depths of 100–200 m. The Walker atmospheric circulation, which manifests itself in the form of trade winds, coupled with low pressure over the Indonesia region, leads to the fact that in this place the level of the Pacific Ocean is 60 cm higher than in its eastern part . And the water temperature here reaches 29-30°C against 22-24°C off the coast of Peru. However, everything changes with the onset of El Niño. The trade winds are weakening, the TTB is spreading, and a huge area of the Pacific Ocean is experiencing a rise in water temperature. In the region of Peru, the cold current is replaced by a warm water mass moving from the west to the coast of Peru, upwelling weakens, fish die without food, and westerly winds bring moist air masses to the desert, showers that even cause floods. The onset of El Niño reduces the activity of Atlantic tropical cyclones.
North Atlantic Oscillation - the variability of the climate in the north of the Atlantic Ocean, which is manifested primarily in changes in the temperature of the sea surface. The phenomenon was first described in 2001 by Goldenberg et al. Although there is historical evidence for this wobble over a long period of time, accurate historical data on its amplitude and relationship to tropical ocean surface temperatures are lacking.
Time dependence of fluctuation in the period 1856-2013
Other cyclones, in particular subtropical cyclones, are able to take on the characteristics of tropical cyclones as they develop. After the moment of formation, tropical cyclones move under the influence of the prevailing winds; if conditions remain favorable, the cyclone gains strength and forms a characteristic vortex structure with eye in the center. If the conditions are unfavorable, or if the cyclone moves to land, it dissipates fairly quickly.
Structure
Tropical cyclones are relatively compact storms of fairly regular shape, typically about 320 km in diameter, with spiraling winds converging around a central area of very low atmospheric pressure. Due to the Coriolis force, the winds deviate from the direction of the baric gradient and twist counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Structure of a tropical cyclone
The structure of a tropical cyclone can be divided into three concentric parts. The outer part has an inner radius of 30-50 km, in this zone the wind speed increases evenly as it approaches the center of the cyclone. The middle part, which has a name eye wall, characterized by high wind speeds. The central part with a diameter of 30-60 km is called eyes, here the wind speed decreases, the air movement is predominantly downward, and the sky often remains clear.
Eye
The central part of the cyclone, in which the air descends, is called eyes. If the cyclone is strong enough, the eye is large and is characterized by calm weather and clear skies, although sea waves can be exceptionally large. The eye of a tropical cyclone is usually a regular round shape, and its size can be from 3 to 370 km in diameter, but most often the diameter is about 30-60 km. The eye of large mature tropical cyclones sometimes expands noticeably at the top, this phenomenon is called the "stadium effect": when viewed from the inside of the eye, its wall resembles the shape of a stadium stand.
Hurricane Isabel 2003 ISS photo - Tropical cyclone eyes, eye wall and surrounding rain bands can be clearly seen
The eye of tropical cyclones is characterized by very low atmospheric pressure, it was here that the lowest value of atmospheric pressure at the level of the earth's surface was recorded (870 hPa in Typhoon Type). In addition, unlike other types of cyclones, the air in the eye of tropical cyclones is very warm, always warmer than at the same altitude outside the cyclone.
The eye of a weak tropical cyclone may be partially or completely covered by clouds, which are called central dense cloud cover. This zone, in contrast to the eye of strong cyclones, is characterized by significant thunderstorm activity.
eye of the storm, abo ofo, Bulls-eye - an area of clearing and relatively calm weather in the center of a tropical cyclone.
A typical storm eye has a diameter of 20 to 30 km, in rare cases - up to 60 km. In this space, the air has a higher temperature and lower humidity than in the surrounding area of wind and rain clouds. The result is a stable temperature stratification.
The wall of wind and rain serves as an insulator for the very dry and warmer air that descends into the center of the cyclone from the upper layers. Along the periphery of the eye of the storm, part of this air mixes with the air from the clouds and cools due to the evaporation of droplets, thereby forming a powerful downward cascade of relatively cold air along the inner side of the clouds.
Eye of Typhoon Odessa (1985)
At the same time, the air in the clouds is rapidly rising.This construction forms the kinematic and thermodynamic basis of a tropical cyclone.
In addition, near the axis of rotation, the horizontal linear wind speed decreases, which for the observer, when it hits the center of the cyclone, gives the impression of a stopped storm, in contrast to the surrounding space.
eye wall
wall of the eye called the ring of dense thunderclouds that surrounds the eye. Here, clouds reach their highest height within the cyclone (up to 15 km above sea level), and precipitation and winds near the surface are the strongest. However, the maximum wind speed is reached at a slightly higher altitude, usually about 300 m. It is during the passage of the eye wall over a certain area that the cyclone causes the greatest damage.
The strongest cyclones (usually category 3 or more) are characterized by several eyewall replacement cycles during their lifetime. At the same time, the old wall of the eye narrows to 10-25 km, and it is replaced by a new one, of a larger diameter, which gradually replaces the old one. During each eyewall replacement cycle, the cyclone weakens (i.e., the winds within the eyewall weaken and the temperature of the eye decreases), but with the formation of a new eyewall, it quickly gains strength back to its previous values.
outer zone
outer part of a tropical cyclone is organized into rain bands - bands of dense thunderclouds that slowly move towards the center of the cyclone and merge with the wall of the eye. At the same time, in the rain bands, as in the wall of the eye, the air rises, and in the space between them, free from low clouds, the air descends. However, the circulation cells formed on the periphery are less deep than the central one and reach a lower height.
When the cyclone reaches land, instead of rain bands, air currents are more concentrated within the wall of the eye, due to increased friction on the surface. At the same time, the amount of precipitation increases significantly, which can reach 250 mm per day.
Tropical cyclones also form cloud cover at very high altitudes (near the tropopause) due to the centrifugal movement of air at that altitude. This cover consists of high cirrus clouds that move from the center of the cyclone and gradually evaporate and disappear. These clouds can be thin enough to show the sun through and can be one of the first signs of a tropical cyclone approaching.
Dimensions
One of the most common definitions of the size of a cyclone, which is used in various databases, is the distance from the center of circulation to the outermost closed isobar, this distance is called radius of the outer closed isobar. If the radius is less than two degrees of latitude, or 222 km, the cyclone is classified as "very small" or "dwarf". A radius from 3 to 6 degrees of latitude, or from 333 to 667 km, characterizes a "medium-sized" cyclone. "Very large" tropical cyclones have a radius in excess of 8 degrees latitude, or 888 km. According to this system, the largest tropical cyclones on Earth occur in the Pacific Northwest, about twice the size of tropical cyclones in the Atlantic Ocean.
Other methods for sizing tropical cyclones are the radius at which tropical storm force winds exist (about 17.2 m/s) and the radius at which the relative wind speed curl is 1×10 −5 s −1 .
Comparative sizes of Typhoon Type, Cyclone Tracy with the territory of the United States
Mechanism
The main source of energy of a tropical cyclone is the energy of evaporation, which is released during the condensation of water vapor. In turn, the evaporation of ocean water proceeds under the influence of solar radiation. Thus, a tropical cyclone can be represented as a large heat engine, which also requires the rotation and gravity of the Earth. In meteorology, a tropical cyclone is described as a type of mesoscale convection system that develops in the presence of a powerful source of heat and moisture.
Directions of convection currents in a tropical cyclone
Warm moist air rises mainly within the wall of the eye of the cyclone, as well as within other rain bands. This air expands and cools as it rises, its relative humidity, already high at the surface, increases even more, as a result of which most of the accumulated moisture condenses and falls as rain. The air continues to cool and lose moisture as it rises to the tropopause, where it loses almost all of its moisture and ceases to cool with altitude. The cooled air sinks down to the ocean surface, where it is rehydrated and rises again. Under favorable conditions, the energy involved exceeds the costs of maintaining this process, excess energy is spent on increasing the volume of updrafts, increasing the speed of winds and accelerating the condensation process, that is, it leads to the formation of a positive feedback. For conditions to remain favorable, a tropical cyclone must be above a warm ocean surface that provides the necessary moisture; when the cyclone passes through a piece of land, it does not have access to this source and its strength drops rapidly. The rotation of the Earth adds twisting to the convection process as a result of the Coriolis effect - the deviation of the wind direction from the baric gradient vector.
Drop in ocean surface temperature in the Gulf of Mexico with the passage of Hurricanes Katrina and Rita
The mechanism of tropical cyclones differs significantly from the mechanism of other atmospheric processes in that it requires deep convection, that is, one that captures a large range of altitudes. At the same time, updrafts capture almost the entire distance from the ocean surface to the tropopause, with horizontal winds limited mainly in the near-surface layer up to 1 km thick, while most of the rest of the 15-km troposphere in tropical areas is used for convection. However, the troposphere is thinner at higher latitudes, and the amount of solar heat is less there, which limits the zone of favorable conditions for tropical cyclones to the tropical belt. Unlike tropical cyclones, extratropical cyclones derive their energy predominantly from the horizontal air temperature gradients that existed before them.
The passage of a tropical cyclone over a section of the ocean leads to a significant cooling of the near-surface layer, both due to heat loss for evaporation, and due to the active mixing of warm near-surface and cold deep layers and the production of cold rainwater. Cooling is also affected by dense cloud cover that covers the ocean surface from sunlight. As a result of these effects, over several days during which the cyclone passes through a certain part of the ocean, the surface temperature on it drops significantly. This effect results in a negative feedback that can result in a loss of tropical cyclone strength, especially if it is moving slowly.
The total amount of energy that is released in a medium-sized tropical cyclone is about 50-200 exajoules (10 18 J) per day, or 1 PW (10 15 W). This is about 70 times more than the consumption of all types of energy by mankind, 200 times more than the world's electricity production and corresponds to the energy that would be released from the explosion of a 10-megaton hydrogen bomb every 20 minutes.
Life cycle
Formation
Map of the path of all tropical cyclones for the period 1985-2005
In all areas of the world where tropical cyclone activity exists, it reaches its maximum at the end of summer, when the temperature difference between the ocean surface and the deep layers of the ocean is greatest. However, seasonal patterns are somewhat different depending on the basin. Globally, May is the least active month, September is the most active, and November is the only month when all pools are active at the same time.
Important Factors
The formation process of tropical cyclones is still not fully understood and is the subject of intense research. Usually, six factors can be identified that are necessary for the formation of tropical cyclones, although in some cases a cyclone may form without some of them.
The formation of trade wind convergence zones, which leads to atmospheric instability and contributes to the formation of tropical cyclones
In most cases, the formation of a tropical cyclone requires a surface ocean water temperature of at least 26.5°C at a depth of at least 50 m; this water temperature is minimally sufficient to cause instability in the atmosphere above it and support the existence of a thunderstorm system.
Another necessary factor is the rapid cooling of the air with height, which makes it possible to release the energy of condensation, the main energy source of a tropical cyclone.
Also, the formation of a tropical cyclone requires high air humidity in the lower and middle layers of the troposphere; under the condition of a large amount of moisture in the air, more favorable conditions are created for the formation of instability.
Another characteristic of favorable conditions is a low vertical wind gradient, since a large wind gradient leads to a break in the cyclone circulation pattern.
Tropical cyclones usually occur at a distance of at least 550 km or 5 degrees of latitude from the equator - only there the Coriolis force is strong enough to deflect the wind and twist the vortex.
Finally, the formation of a tropical cyclone usually requires a pre-existing zone of low pressure or rough weather, albeit without the circulation behavior of a mature tropical cyclone. Such conditions can be created by low-level and low-latitude flares that are associated with the Madden-Julian oscillation.
Formation areas
Most tropical cyclones in the world are formed within the equatorial belt (intertropical front) or its continuation under the influence of monsoons - the monsoon low pressure zone. Areas favorable for tropical cyclone formation also occur within tropical waves, where about 85% of intense Atlantic cyclones and most of the East Pacific tropical cyclones originate.
The vast majority of tropical cyclones form between 10 and 30 degrees latitude in both hemispheres, with 87% of all tropical cyclones occurring within 20 degrees of latitude of the equator. Due to the absence of the Coriolis force in the equatorial zone, tropical cyclones very rarely form closer than 5 degrees from the equator, but it does happen, for example with 2001 Tropical Storm Wamei and Cyclone Agni in 2004.
Tropical Storm Wamei before landfall
Tropical Storm Wamei, sometimes known as Typhoon Wamei, is a tropical cyclone known for forming closer to the equator than any other tropical cyclone on record. Wamei formed on December 26 as the last tropical cyclone of the 2001 Pacific typhoon season at 1.4°N in the South China Sea. It quickly intensified and made landfall in southwestern Malaysia. It practically dissipated over the island of Sumatra on December 28, and its remnants later reorganized over the Indian Ocean. Although officially designated as a tropical storm, the intensity of this tropical cyclone is disputed, with some agencies classifying it as a typhoon based on 39 mph winds and the presence of an eye.This storm caused flooding and landslides in eastern Malaysia, causing $3.6 million in damage (at prices 2001) and five victims.
Motion
Interaction with trade winds
The movement of tropical cyclones along the Earth's surface depends primarily on the prevailing winds resulting from global circulation processes; tropical cyclones are carried along by these winds and move with them. In the zone of occurrence of tropical cyclones, that is, between 20 parallels of both hemispheres, they move westward under the influence of easterly winds - trade winds.
Scheme of the global circulation of the atmosphere
In the tropical regions of the North Atlantic Ocean and the northeast Pacific Ocean, the trade winds form tropical waves that start from the African coast and pass through the Caribbean Sea, North America and attenuate in the central Pacific Ocean. These waves are the origin of most of the tropical cyclones in these regions.
Coriolis effect
Due to the Coriolis effect, the rotation of the Earth not only causes the twisting of tropical cyclones, but also affects the deviation of their movement. Due to this effect, a tropical cyclone that moves westward under the influence of the trade winds in the absence of other strong air currents deviates towards the poles.
An infrared image of Cyclone Monica showing the swirl and rotation of the cyclone
Since easterly winds are applied to the cyclonic movement of air on its polar side, the Coriolis force is stronger there, and as a result, the tropical cyclone is pulled poleward. When a tropical cyclone reaches a subtropical ridge, temperate westerly winds begin to reduce air speed on the polar side, but the difference in distance from the equator between different parts of the cyclone is large enough for the net Coriolis force to be directed poleward. As a result, Northern Hemisphere tropical cyclones deviate north (before turning east) and Southern Hemisphere tropical cyclones deviate south (also before turning east).
Interaction with westerly winds of temperate latitudes
When a tropical cyclone crosses a subtropical ridge, which is a high pressure zone, its path usually deviates into a low pressure zone on the polar side of the ridge. Once in the zone of the westerly winds of the temperate zone, a tropical cyclone tends to move with them to the east, passing the moment of change of course (eng. recurvature). Typhoons moving westward across the Pacific towards the coast of Asia often change course off the coast of Japan to the north and then to the northeast, caught by southwesterly winds from China or Siberia. Many tropical cyclones are also deflected by interactions with extratropical cyclones moving from west to east in these areas. An example of a course change by a tropical cyclone is Typhoon Yoke 2006, which moved along the described trajectory.
The path of Typhoon Yoke that changed course off the Japanese coast in 2006
Landing
Formally, a cyclone is considered to pass over land if this happens to its center of circulation, regardless of the state of the peripheral regions. Storm conditions typically begin over a particular area of land several hours before the center of the cyclone makes landfall. During this period, that is, before the formal landfall of a tropical cyclone, the winds can reach their greatest strength - in this case, one speaks of a “direct impact” of a tropical cyclone on the coast. Thus, the moment of cyclone landfall actually means the middle of the storm period for areas where this happens. Safety measures should be taken before the winds reach a certain speed or until a certain rain intensity is reached, and not be associated with the moment a tropical cyclone makes landfall.
Cyclone interaction
When two cyclones approach each other, their centers of circulation begin to rotate around a common center. In this case, the two cyclones approach each other and eventually merge. If the cyclones are of different sizes, the larger one will dominate this interaction, while the smaller one will rotate around it. This effect is called fujiwara effect, in honor of the Japanese meteorologist Sakuhei Fujiwara.
This image shows Typhoon Melor and Tropical Storm Parma, and their interaction in southeast Asia. This example shows how the strong Melor pulls the weaker Parma towards him.
Satellites capture the dance of twin cyclones over the Indian Ocean
On January 15, 2015, two tropical cyclones formed over the center of the Indian Ocean. None of them threatened settlements due to low intensity and low chances of making landfall. Meteorologists were confident that Diamondra and Eunice would weaken and dissipate in the following days. The close proximity of tropical cyclones made it possible for satellites to take amazing photographs of the dance of eddy systems over the ocean.
January 28, 2015 geostationary satellites owned by EUMETSAT and the Japan Meteorological Agency provided data for the composite image (top). Radiometer (VIRS) on board the satellite Suomi NPP took three pictures of the twin cyclones, combining which resulted in the bottom image.
The two systems were about 1,500 kilometers apart on January 28, 2015. Eunice, the stronger of the two cyclones, was located east of Diamondra. The maximum speed of stable Eunice winds reached almost 160 km/h, while the maximum speed of Diamondra winds did not exceed 100 km/h. Both cyclones were moving in a southeasterly direction.
As a rule, if two tropical cyclones approach each other, they begin to rotate cyclonically around the axis connecting their centers. Meteorologists call this phenomenon the Fujiwara effect. Such double cyclones can even merge into one if their centers converge close enough.
“But in the case of Eunice and Diamondra, the centers of the two vortex systems were too far apart,” explains Brian McNoldy, a meteorologist at the University of Miami. From experience, the centers of cyclones must be at least 1,350 kilometers apart in order to start orbiting each other. According to the latest forecasts from the Joint Typhoon Warning Center, both cyclones are moving southeast at about the same speed, so they probably won't get any closer to each other."
(To be continued)
CYCLONES AND ANTICYCLONES
Cyclones and anticyclones
In the troposphere, eddies of various sizes constantly arise, develop and disappear - from small to giant cyclones and anticyclones.
Cyclone is an area of low pressure in the center. Therefore, the air in the cyclone moves in a spiral from the periphery (from areas of high pressure) to the center (to the area of low pressure) and then rises upwards, forming updrafts. In a cyclone, the air moves along a curved path and is directed counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Cyclones are associated with vast areas of clouds and precipitation, significant temperature changes, and strong winds. However, cyclones are also known that exist throughout the year in constant areas of low pressure: Icelandic cyclone (minimum), located in the North Atlantic in the area of about. Iceland, and Aleutian cyclone (minimum) in the Aleutian Islands in the North Pacific. In addition to temperate latitudes, cyclones are observed in the tropical zone.
Tropical cyclones occur only over the sea, between 10-15 ° N. and y.sh. When moving to land, they quickly fade. These are, as a rule, small cyclones, their diameter is about 250 km, but with very low pressure in the center.
Tropical cyclones move at a speed of 10-20 km / h, mainly from east to west, but their trajectory deviates towards high latitudes (for example, in the Northern Hemisphere they move towards the northwest). These are very powerful whirlwinds with exceptionally strong winds (20-30 m/s, in gusts up to 100 m/s and more), which cause the strongest waves at sea and great destruction on land. On the globe, on average, more than 70 cases of tropical cyclones are recorded per year. They are best known in the Antilles, off the southeast coast of Asia, in the Arabian Sea, the Bay of Bengal, east of about. Madagascar. In different areas they have local names ( cyclone - in the Indian Ocean; Hurricane - in North and Central America; typhoon in East Asia). Cyclones are especially typical for the territory of Europe, where they move from the Atlantic to the east and exist up to 5-7 days, i.e. until the atmospheric pressure equalizes.
Anticyclone is an area with increased pressure in the center. Due to this, the movement of air in the anticyclone is directed from the center (from the region of higher pressure) to the periphery (in the region of lower pressure). In the center of the anticyclone, the air descends, forming descending flows, and spreads in all directions, i.e. from the center to the periphery. At the same time, it also rotates, but the direction of rotation is opposite to the cyclonic one - it occurs clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
Anticyclones in temperate latitudes most often follow cyclones, often they take a sedentary (stationary) state and also exist until the pressure equalizes (6-9 days). Due to downward movements in the anticyclone, the air is not saturated with moisture, cloud formation does not occur, and cloudy and dry weather prevails with light winds and calm. In addition to temperate latitudes, anticyclones are most common in subtropical latitudes - in high pressure zones. Here, these are permanent atmospheric vortices (high-pressure areas) that exist throughout the year: (Azorean) anticyclone (maximum) in the area of the Azores and the South Atlantic anticyclone; North Pacific anticyclone in the Pacific Ocean and South Pacific; Indian anticyclone (maximum) in the Indian Ocean. As you can see, they are all located above the oceans. The only powerful anticyclone over land occurs in winter in Asia with a center over Mongolia - Asian (Siberian) anticyclone.
The sizes of cyclones and anticyclones are comparable: diameter they can reach 3-4 thousand . km, and height - maximum 18-20 km , i.e. they are flat vortices with a strongly inclined axis of rotation. They usually move from west to east at a speed of 20-40 km / h (except for stationary ones).
atmospheric fronts
Air masses, having different physical properties (especially air temperature), are separated from each other by rather narrow transition zones, which are strongly inclined to the earth's surface (less than 1 °).
atmospheric front called the division between air masses with different physical properties. The intersection of the front with the earth's surface is called the front line.
At the front, all the properties of air masses - temperature, wind direction and speed, humidity, cloudiness, precipitation - change dramatically. The passage of the front through the place of observation is accompanied by more or less abrupt changes in the weather. Distinguish fronts associated with cyclones, and climatic fronts. In cyclones, fronts form when warm and cold air meet.
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warm front moves in the direction of cold air and means the onset of warm air. It slowly pushes cold air out. Being lighter, it flows onto the wedge of cold air, gently rising up along the interface. In this case, an extensive zone of clouds forms in front of the front, from which heavy precipitation falls. The precipitation band in front of the warm front reaches 300, and in cold weather 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 an increase in wind. After the passage of the front, a sharp change in the weather is observed: the air temperature rises, the wind changes direction by about 90 ° and weakens, visibility worsens, fogs form, and drizzling precipitation may fall.
cold front moving towards warmer 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 warm air in front of it, vigorously pushing it up. Large cumulonimbus clouds form above the front line and in front of it, from which heavy rains fall, thunderstorms arise, and strong winds are observed. After the passage of the front, precipitation and cloudiness significantly decrease, the wind changes direction by about 90 ° and weakens somewhat, the temperature drops, air humidity decreases, its transparency and visibility increase; the pressure is rising.
Climate fronts - fronts of a global scale, which are sections between the main (zonal) types of air masses.
There are five such fronts: arctic, antarctic, two temperate (polar) and tropical. 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 and equatorial air meet, differing in humidity rather than temperature.
All fronts, together with the boundaries of the belts, shift towards the poles in summer and towards the equator in winter. Often they form separate branches, spreading over long distances from climatic zones. The tropical front is always in the hemisphere where it is summer.