Already from the middle of the 17th century. Mariners knew in general terms the patterns in the zonal structure of the prevailing winds in the World Ocean. Modern science provides an explanation for this pattern in the zonal structure of atmospheric pressure over the Earth's surface (§8).
Rice. 17. Trade winds
Sailors have long known winds that are stable in their constancy or change directions: trade winds, monsoons, breezes, etc.
Trade winds- these are the stable winds of the tropics, northeast in the northern and southeast in the southern hemispheres, blowing on the side of the subtropical center of action of the atmosphere facing the equator (Fig. 17). The speed of the trade winds is low - on average 5–8 m/s earth's surface. The conditions for the distribution of atmospheric pressure change little in the tropics, so the trade winds have great directional stability. However, during the season, the subtropical center of atmospheric action may undergo certain changes. Thus, usually one anticyclone is formed over each ocean of both hemispheres in the subtropics. On daily weather maps there may be more of them - often two, sometimes three over each ocean; over the South Pacific - up to four. As a result, the trade winds of both hemispheres can change the above directions. The migration of the centers of subtropical anticyclones can have a similar effect on the direction of trade winds. Therefore, trade winds in the same place can change the northeast direction to the east and southeast, then again to the northeast, etc.
Monsoons– seasonal winds, most pronounced and stable in tropical latitudes, arising due to ocean-land thermal heterogeneity. Monsoons, like all other air currents on Earth, are associated with cyclonic activity. The tropical monsoon regime consists of a seasonal change in the position of subtropical anticyclones and equatorial depression. The stability of monsoons is associated with a stable distribution of atmospheric pressure during each season, and their seasonal change is associated with fundamental changes in the distribution of pressure from season to season. The total baric gradients change direction sharply from season to season, and along with this the direction of the prevailing winds also changes.
If there is an ocean on both sides of the equator, then the seasonal shifts of the subtropical center of action of the atmosphere are small and the monsoons do not develop much, for example, over the Pacific Ocean.
Another thing with continents. Over Africa, for example, Atmosphere pressure varies greatly from January to June. An area of high pressure dominates over the central regions of Africa in summer, and the crest of the Azores anticyclone in winter; over southern Africa in winter there is also an anticyclone, and in summer there is a deep depression. In this regard, the direction of pressure gradients over the coast tropical Africa from season to season it changes sharply over a wide zone, which is the cause of monsoon winds here.
The monsoon circulation is especially pronounced in the Indian Ocean basin, where seasonal changes in hemispheric temperature are amplified by the huge continent of Eurasia north of the equator, warmed in summer and very cool in winter. The winter monsoon in the Indian Ocean basin is called northeast, and the summer monsoon is called southwest. In eastern China and Korea, the winter monsoon is northern or northwestern, and the summer monsoon is southern or southeastern. This depends on the structure of the pressure field and the direction of the isobars and, consequently, the direction of the pressure gradients (Fig. 18).
Breezes– reversible winds of the coasts of seas and oceans, with a semi-diurnal periodicity of direction changes. Breezes are especially pronounced over the coasts in the zone of subtropical anticyclones, where they are observed in all seasons of the year. In temperate and high latitudes, breezes are observed only in the warm season. For example, on the Black, Azov and Caspian seas, breezes are observed from April to September.
Breeze circulation is explained by the restructuring of the pressure field day and night over land and sea. Thermal heterogeneity of sea and land leads to a change in the direction of baric gradients and, consequently, to a change in wind direction (Fig. 19).
The speed of the sea breeze is slightly higher than the speed of the coastal breeze and is 3–5 m/s (in the tropics up to 8 m/s). Breezes are especially pronounced in clear, windless weather, which is typical for the central regions of anticyclones. Breeze circulation covers a layer of the troposphere up to 1–2 km vertically and extends deep into the sea or land to tens of kilometers from the coastline. Sea breeze decreases by 2–3° average temperature air and increases humidity by 10–20%.
Bora- a strong and gusty wind blowing from low mountains towards the warm sea. Bora belongs to the so-called katabatic winds associated with the flow of cold dense air masses along the slopes of the mountains towards the sea.
Winds similar to the Novorossiysk and Novaya Zemlya Bors are known in many other seas of the World Ocean: the Baku Nord on the Caspian Sea, the Mistral on the Mediterranean coast of France, the North Sea in Gulf of Mexico(Mexico, USA), etc.
The bora is caused by the passage of a cold front through the coastal ranges. Cold air abruptly passes over the low mountains (the wind is especially strong on the passes) and rushes down in a dense stream towards the nearby warm sea.
In straits, narrows, fiords when sailing along the coast, at capes, ends of islands, etc., there may be features of the wind regime associated with the coastal (angular) effect. These features are described in detail in the hydrometeorological sketches of the sailing directions.
When powerful cumulonimbus (thunderstorm) clouds form, which is possible in conditions of particularly strong instability of air masses, vertical vortices of small diameter can arise. Emerging vortices over the sea are called tornadoes, and over land - blood clots (in the USA - tornadoes). A tornado has the appearance of a dark cloud column several tens of meters in diameter (a tornado is up to 100–200 m), descending in the form of a funnel from the lower base of the cloud to the surface of the water or land. Wind speeds in a tornado reach 50–100 m/s and, with a strong vertical component, can cause catastrophic destruction. Proximity atmospheric fronts can stimulate the process of tornado formation, especially in continental tropical air (in the USA in marine tropical air from the Gulf of Mexico).
Scheme general circulation atmosphere.
The Earth's atmosphere is in constant motion. Air currents differ in both speed and direction. Close interaction with the underlying surface, the properties of which quickly change in time and space, leads to the fact that the instantaneous picture of atmospheric movement turns out to be unusually complex.
The construction of average daily, seasonal and long-term synoptic maps allows us to identify general (predominant) patterns of atmospheric movements. System of macroscale air currents over the globe is called the general circulation of the atmosphere.
Identification of the main patterns of general atmospheric circulation is the scientific basis for both long-term and short-term weather forecasts.
It has now been established that the reason for the stable, persistent features of the general circulation of the atmosphere is zonality in the distribution of pressure (§ 8) and the associated cyclonic activity on the planet.
Zonal transports manifesting themselves in the troposphere are characterized by prevailing easterly winds in tropical zone(trade winds), westerly winds in temperate latitudes and again eastern winds in subpolar and polar latitudes (Fig. 20).
Questions for self-control:
1. Causes of wind. What is a pressure gradient? Explain its components.
2. What forces influence the characteristics of the pressure gradient?
3. What are geostrophic and gradient winds?
4. What is the essence of the Beaufort scale?
5. What types of air flows do you know?
6. What are local winds?
7. What is the peculiarity of the wind distribution on the globe?
The air is constantly moving, it constantly falls and rises, and also moves horizontally. We call horizontal air movements wind. Wind is characterized by such quantities as speed, strength, direction. The average wind speed near the earth's surface is 4-9 meters per second. The maximum wind speed of –22 m/s was recorded off the coast of Antarctica, with gusts up to 100 m/s.
The wind arises due to differences in pressure, moving from an area of high pressure to an area of low along the shortest path, deviating, according to the direction of flow, to the left in the Southern Hemisphere, and to the right in the Northern Hemisphere (Coriolis force). At the equator this deviation is absent, but near the poles it is, on the contrary, maximum.
Constant winds
The main directions of winds at different latitudes are determined by the distribution of atmospheric pressure. In each hemisphere, air moves in two directions: from the regions tropical climate, in which high pressure reigns, to moderate latitudes and to the equator. At the same time, it deviates to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere, in the direction of the flow.
In the region between the equator and the tropics, trade winds blow - easterly winds that are constantly directed towards the equator.
In areas of temperate latitudes, on the contrary, westerly winds predominate, which are called westerly transport.
These winds determine the main constant movement of air masses, which interacts with anticyclones and cyclones, and on which regional winds are then superimposed.
Regional winds
At the border of land and ocean water, due to the displacement of high and low pressure zones, monsoons occur, resulting in the appearance of intermediate belts that change the directions of winds according to the seasons. There are no huge land masses in the Southern Hemisphere, so the monsoons dominate in the Northern Hemisphere. IN summer time they blow towards the mainland, and in winter - towards the ocean. Most often, this wind is found on the Pacific coast of Eurasia (northeastern China, Korea, Far East), and in North America (Florida). It is these winds that also blow in Vietnam, which is why there is such a stable wind pattern here.
Tropical monsoons are a cross between trade winds and monsoons. They arose, like trade winds, due to differences in pressure in different climatic zones, but, like monsoons, they change their direction depending on the season. This wind can be encountered on the shores of the Indian Ocean and the Gulf of Guinea.
Regional winds also include sirocco, a wind originating in the Mediterranean. It is a western transport, which, having passed through the tops of the mountains, heats up and becomes dry, since it gave up all its moisture to the windward slopes. Sirocco brings a lot of dust to the regions of Southern Europe from the deserts of North Africa, as well as the Arabian Peninsula.
Local winds
These are winds on the coasts, arising due to the difference in the rate of heating and cooling of the sea and land, and operating in the area of the first tens of kilometers of the coast.
A breeze is a wind that arises at the border of the coast and the water area and changes its direction twice a day: during the day it blows from the water area to the land, and at night - vice versa. Breezes blow along the banks of large lakes and rivers. A change in the direction of this wind occurs due to changes in temperature and, accordingly, in pressure. During the day it is much warmer on land and the pressure is lower than above water, while at night the opposite is true.
Bora (mistral, bizet, nor'east) is a cold wind of hurricane force. It forms on narrow sections of the coast of warm seas during the cold season. Bora is directed from the leeward slopes of the mountains towards the sea. These winds blow, for example, in the mountainous regions of Switzerland and France.
Pampero is a cold gale that blows from the south or southwest in Argentina and Uruguay, sometimes with rain. Its formation is associated with the invasion of cold air masses from Antarctica.
Thermal wind is a general name for winds associated with temperature differences that occur between a hot desert and a relatively cold sea, such as the Red Sea. This is the difference between the conditions of Dahab and Hurghada in Egypt, which is nearby, but the wind does not blow there with such force. The fact is that the city of Dahab is located at the exit of a canyon formed by the Sinai and Arabian Peninsulas. The wind accelerates in the canyon itself, creating a wind tunnel effect, but when entering open space, the wind force gradually decreases. The speed of such winds decreases with distance from the coast. As we move towards the open ocean, global atmospheric winds have a greater influence.
Tramontana is a hurricane north wind of the Mediterranean, generated by the collision of atmospheric currents of the Atlantic with the air of the Gulf of Lyon. After their meeting, a violent squall is formed, which can exceed a speed of 55 m/s and is accompanied by a loud whistle and howl.
Another group of local winds depends on the local topography.
Foehn is a warm dry wind directed from the leeward slopes of the mountains to the plain. The air gives off moisture as it rises along the windward slopes, and this is where precipitation occurs. When the air descends from the mountains, it is already very dry. A type of foehn - the garmsil wind - blows mainly in the summer from the south or southeast in the area of the foothills of the Western Tien Shan.
Mountain-valley winds change their direction twice: during the day they are directed up the valley, and at night, on the contrary, they blow down. This happens because the lower part of the valley warms up more intensely during the day.
There are also winds that occur over large areas of deserts and steppes.
Samum is a hot dry wind tropical deserts, having a stormy, squally character. Gusts accompany dusty and sandstorms. You can meet him in the deserts of the Arabian Peninsula and North Africa.
A dry wind is a warm, dry wind in steppe regions that forms during the warm season under anticyclone conditions and contributes to the occurrence of droughts. These winds are found in the Caspian region and Kazakhstan.
Khamsin is a dry, hot and dusty wind, usually from the south, blowing in northeastern Africa and the eastern Mediterranean. Hasmin blows for about 50 days in the spring, carrying with it a lot of dust and sand. It reaches its greatest strength in the afternoon, waning towards sunset. Often found in Egypt.
Thus, each point on Earth has its own different features that affect wind conditions; for example, we will give some of them.
Anapa is one of the few places in Russia where the climate is subtropical Mediterranean and very pleasant for water sailing. In winter it is humid, but not cold, and in summer the intense heat is softened by a cool sea breeze. The most favorable period for skiing is the season from July to November. The wind force in summer reaches an average of 11-15 knots. After mid-October and in November the wind increases and can reach 24 knots.
The Canary archipelago has a tropical trade wind climate, moderately dry and hot. From the coast of Africa, the “harmattan” comes to the islands of Fuerteventura and Lanzarote, bringing the heat and sand of the Caxapan desert. The main wind prevailing on these islands is the trade wind, which blows for six months and almost constantly in the summer. The wind force is 10-20 knots, in October and November it increases to 25-35.
The Philippines is an island with a tropical monsoon climate. Temperatures on the coast are about 24-28 degrees. The rainy season here begins from November and lasts until April, then the northeast monsoon blows, and from May to October the southwest monsoon blows. Tsunamis and typhoons often occur in the northern regions of the country. The average wind force is 10-15 knots.
So, in a specific area the impact simultaneously appears various types winds: global, depending on areas of high or low blood pressure, and local, blowing only in a given territory, due to its physical and geographical features. This means that for a certain place the wind system can be predictable to some extent. Scientists have long created special maps, with the help of which it became possible to recognize and trace the wind regimes of different regions.
Internet users often find out the characteristics of winds in a particular area with the help of resources and, where you can quite accurately check whether there is wind in a particular point in the world or not.
We invite you to go on an unusual journey. Let us follow the winds of the seasons today. “Mausim” - in Arabic - season, time of year, this is where the word “monsoon” comes from. Winds of the seasons that blow from opposite directions in winter and summer.
Let's first Let's look at the situation in the summer: There is a lot of sun and it heats the land to a greater extent. But why? Everything is not so complicated, firstly, water has the property that it is difficult to heat and difficult to cool. Water, of all substances, is the most difficult to heat, so it is said that its heat capacity is equal to unity. The volumetric heat capacity of air is equal to 0.000307, i.e., to heat the air it is necessary to apply 3257 times less heat than is required for water. Conversely, it is 3257 times easier to cool air than water.
Plus, water, unlike land, is also transparent, which means the sun’s rays penetrate into the water column and warm it, and not just the surface layers.
So, we settled on the fact that in the summer the sun heats the land more than the ocean. Therefore, the air over land is heated and rises, leaving behind an area of low pressure. Above the ocean, the air is colder and, therefore, located closer to the ground, and here an area of high pressure arises. That's almost all!!! A holy place is never empty and cold air is directed from the ocean to land to fill the “empty” space. Or in other words, high pressure forces air into areas of lower pressure.
Why is the air humid in summer? Here, too, everything is simple, it came from the ocean, and there is a lot of water there :) In the summer, under the influence of the sun, it evaporates and saturates the air.
Now let's consider what happens in winter. There is little sun here and it does not play an important role. But again, everything happens thanks to the amazing properties of water discussed earlier. Over the long summer, the water has absorbed a lot of heat, and in winter it begins to slowly release it, while the air above the land cools down almost as soon as the sun goes away. Therefore, now, all the air over the ocean is heated due to the heat accumulated in the water, and the air over land without the sun cools down.
And again, where the air is warm, the pressure is low, and where it is cold, the pressure is high. And the wind blows from an area of high pressure to an area of low pressure. Those. in our case, in winter the monsoon winds blow from land to the ocean, and I think it’s clear why they are dry :-).
For a better understanding, also watch the video: “Why does the wind blow?”
Monsoon climate areas.
Summer monsoons come from the sea and bring with them rain and dampness; in winter, the wind blows from the land and gives dry and clear weather.
India is classic monsoon territory. This has long been a natural phenomenon known to sailors, because the correct change of winds was very important for navigation.
What does spring mean to us? Awakening of nature, rebirth. The onset of the rainy summer monsoon carries the same significance on the Indian mainland. Many poets have sung this season in their works. In addition to India, the South Asian monsoon also covers Indochina, and then China.
Finally, the Australian monsoon covers northern Australia and the Malay Archipelago. These are the areas of the monsoon country.
Modern Russian physical map world with sea currents. Among other things, you can find the monsoon current that goes around India.
And now, we invite you to get acquainted with wet and variable-humid monsoon forests.
Permanently wet forests. Huge humidity and always hot temperature air. Vegetable and animal world very rich. These forests are impenetrable jungles, with several layers of plants that never shed their leaves. The animals are usually small in size, since large individuals would have difficulty making their way through difficult areas. For humans, these forests also pose a challenge. Even today you can find places untouched and unexplored by us.
Variably wet forests. Precipitation does not occur all year round, but only during the rainy season. Plants have to shed their leaves to protect themselves from excess evaporation. Animals also have to adapt, so the diversity of flora and fauna here is inferior to the constantly humid forests.
Unfortunately, these forests are increasingly threatened by our civilization. And restoring their previous appearance takes a lot of time. Therefore, it is worth thinking again about how to preserve this wonderful splendor of nature.
And finally, I suggest watching a video film: BBC: Natural World. Monsoon / Natural World. Monsoon.
Reasons for air movement
Atmospheric air is in constant and continuous movement. The movement of air can be upward, in which it rises up, and downward, in which the air goes down. There is another movement - horizontal.
Definition 1
Horizontal air movement is called wind.
Air movement depends on atmospheric pressure and temperature. In addition to these main reasons, the movement is influenced by friction on the surface of the Earth, meeting with any obstacle, and the deflecting Coriolis force. In the Northern Hemisphere, due to this Coriolis force, air currents are deflected right, in the Southern Hemisphere – left.
Note 1
Air flow it always moves from an area of high pressure to an area of low pressure.
Any wind has its own direction, strength and speed, which depends on pressure. If the pressure difference between two neighboring territories is large, the wind speed will increase. On average, the long-term wind speed at the Earth's surface reaches $4-9$ m/s, sometimes $15$ m/s. Storm winds blow at speeds up to $30$ m/s, with gusts up to $60$ m/s. Tropical hurricanes reach $65$ m/s, and gusts reach $120$ m/s.
In addition to meters per second, kilometers per hour, wind speed is also measured in points on a scale Beaufort from $0-13$. From speed the wind depends on it force which shows dynamic pressure air flow onto any surface. Wind force is measured in kilograms per square meter.
The side of the horizon from which the wind blows determines its direction. To indicate its direction, eight main directions are used, i.e. four main sides of the horizon and four intermediate ones. The direction of the wind will be related to the pressure and deflection force of the Coriolis. Winds are very diverse in their origin, meaning and character.
Temperate latitudes are characterized by westerly winds, because the westerly transport of air masses dominates there - these are northwestern, western and southwestern winds. In the Northern and Southern Hemispheres, this area occupies vast areas. The winds of the polar regions blow from the poles to moderate latitudes, i.e. to areas of low pressure. In the Arctic, northeast winds blow clockwise, while in Antarctica, southeast winds blow counterclockwise. Antarctic winds are faster and more stable. In tropical latitudes, trade winds dominate.
Constant winds
Note 2
Constant winds blow throughout the year in one direction from areas of high to low atmospheric pressure. These include trade winds, westerly winds, Arctic and Antarctic winds.
Definition 2
Trade winds- These are constant winds of tropical latitudes, blowing from 30 parallels towards the equator.
The name of this constant wind was given by the Spaniards, calling it “Viento de pasada”, which means “wind favorable for moving”. Trade winds blow at a speed of $5-6$ m/s, and cover a layer of air with a height of $15-16$ km. Powerful ocean currents are associated with them - in the Atlantic Ocean the Antilles Current and the Brazilian Current, in Pacific Ocean Mindanao and the East Australian, Mozambique Current in the Indian Ocean. The area of the planet blown by trade winds has a peculiar climate - mostly partly cloudy. warm weather with little rainfall. On land, this climate contributes to the formation of deserts and semi-deserts. In the Northern Hemisphere, trade winds are directed from the northeast, and in the Southern Hemisphere from the southeast to the equator.
Definition 3
Western winds- These are constant winds of temperate latitudes blowing from the tropics to the 60th parallel.
Tropical air normalizes the temperature of temperate latitudes and makes it favorable for human life. Temperate latitudes are where warm and cold air masses meet. Warm air masses come from the tropics, and cold air masses come from the polar regions. As a result of their contact, cyclones and anticyclones. Myself temperate zone is an area of low pressure, so quite strong air masses come here. Here the westerly transport of air masses dominates, half of them are formed in the north, and the other half are formed in the east, and they all blow in the same westerly direction. In general, westerly winds soften the weather - the summer will be cool with possible rain. Winter will be accompanied by thaws and heavy snowfalls. The north wind will bring cold, and the south wind will bring warmth. The east wind is less predictable - it can be either warm or cold, but large quantity There will be no precipitation either in summer or winter.
The polar type of climate forms two zones - Arctic and Antarctic. Polar air masses will be constant for this area of the planet for all year round. Arctic The polar wind blows quite strongly at moderate latitudes in a clockwise direction. It blows only in a southerly direction and comes to the northern coast of Eurasia and North America. Along with this wind comes a sharp cold snap. In the Southern Hemisphere, the polar wind is called Antarctic and blows only north counterclockwise, moving towards temperate latitudes. The wind is very strong and cold.
Seasonal winds
Definition 4
Seasonal are called periodic winds, the direction of which varies by half of the year.
One of these winds is monsoons.
Definition 5
Monsoons- These are winds that change their direction depending on the time of year.
Monsoons are persistent and cover vast areas. Their stability is related to the distribution of atmospheric pressure during each season. The cause of monsoons is the different heating of land and water throughout the year, which means that there is a winter monsoon and summer. When the monsoons change in spring and autumn, the stability of the wind regime is disrupted. winter monsoon blows from land to sea, because during this period the continent is cold, which means the pressure above it will be high. In summer, when the land warms up, the pressure becomes lower and wet air moves from the ocean to land - this summer monsoon. Dry, partly cloudy winter weather changes to rainy weather in summer.
In different areas of the planet, the nature of atmospheric circulation will be different. This determines the differences in the causes and nature of the monsoons, therefore they distinguish extratropical and tropical monsoons.
Extratropical Monsoons are typical for temperate and polar latitudes. The result of their formation is different pressures over land and sea according to the seasons of the year. As a rule, extratropical monsoons form on Far East, Northeast China, Korea.
Monsoons of tropical latitudes due to the fact that the Northern and Southern Hemispheres heat and cool differently depending on the seasons. This leads to the fact that according to the seasons of the year, atmospheric pressure zones relative to the equator shift to the hemisphere in which given time summer and trade winds penetrate there. The trade wind regime is replaced for the tropics by the winter monsoon. This change is facilitated by the westerly flow of air in a zone of low atmospheric pressure at the equator, which moves along with other zones. Tropical monsoons are persistent in the northern Indian Ocean.
On the coasts of seas and oceans, winds are formed, called breezes. These winds have local significance and during the day they blow from sea to land, and at night they change their direction to the opposite - from land to sea. As a result, a distinction is made between day and night breezes. During the daytime, land heats up faster than water and low atmospheric pressure is established above it. Over the water during the same period, the pressure will be higher because it heats up much more slowly. As a result, air from the sea begins to move to land. At night, low pressure is observed over the water, because it has not yet cooled down, and the air will move from land to sea.
The onshore breeze will change to a sea breeze shortly before noon, and the sea breeze will become onshore in the evening. Breezes can form along the shores of large lakes, large reservoirs, rec. From the coastline they penetrate tens of kilometers onto land and are especially frequent in the summer when the weather is clear and calm.
Frivolity in the modern dictionary is synonymous with inconstancy and changeability. But the trade winds completely destroy this statement. Unlike breezes, seasonal monsoons and especially winds caused by weather cyclones, they are constant. How are trade winds formed and why do they blow in a strictly defined direction? Where did this word “passat” come from in our language? Are these winds really that constant and where are they localized? You will learn about this and much more from this article.
The meaning of the word "trade winds"
In the days of the sailing fleet, wind was of paramount importance for navigation. When it always blew smoothly in the same direction, one could hope for a successful outcome dangerous journey. And the Spanish sailors dubbed this wind “viento de pasade” - favorable for movement. The Germans and Dutch included the word "pasade" in their nautical dictionary of navigation terms (Passat and passaat). And in the era of Peter the Great, this name penetrated into the Russian language. Although in our high latitudes trade winds are rare. Their main place of “habitat” is between the two tropics (Cancer and Capricorn). Trade winds are observed even further from them - up to the thirtieth degree. At a considerable distance from the equator, these winds lose their strength and are observed only in large open spaces, over the oceans. There they blow with a force of 3-4 points. Off the coast, trade winds transform into monsoons. And even further from the equator they give way to winds generated by cyclonic activity.
How are trade winds formed?
Let's do a little experiment. Apply a few drops to the ball. Now let's spin it like a spinning top. Take a closer look at the drops. Those of them that are closer to the axis of rotation remained motionless, and those located on the sides of the “tops” spread in the opposite direction. Now let's imagine that the ball is our planet. It rotates from west to east. This movement creates opposite winds. When a point is located close to the poles, it makes a smaller circle per day than the one located at the equator. Therefore, the speed of its movement around the axis is slower. No air currents arise from friction with the atmosphere in such subpolar latitudes. It is now clear that trade winds are the steady winds of the tropics. At the equator itself there is a so-called calm strip.
Direction of trade winds
It is easy to see from the drops on the ball that they spread in the direction opposite to the rotation. This is called But to say that trade winds are winds blowing from east to west would be wrong. In practice, air masses deviate from their main vector to the south. The same thing happens, only in mirror image, on the other side of the equator. That is, in the Southern Hemisphere, trade winds blow from southeast to northwest.
Why is the equator so attractive for air masses? In the tropics, as is known, a permanent area of high pressure is established. And at the equator, on the contrary, it is low. If we answer a child’s question about where the wind comes from, we will state a common natural history truth. Wind is the movement of air masses from layers with high pressure to an area with a lower one. The periphery of the tropics in science is called the “Horse Latitudes”. From there, the trade winds gallop into the “Calm Zone” above the equator.
Constant wind speed
So, we understand the distribution area of the trade winds. They form in both at a latitude of 25-30° and fade out near the calm zone somewhere around 6 degrees. The French believe that trade winds are “right winds” (vents alizes), very convenient for sailing. Their speed is low, but constant (five to six meters per second, sometimes it reaches 15 m/s). However, the power of these air masses is so great that they form trade wind currents. Born in hot regions, these winds contribute to the development of deserts such as the Kalahari, Namib and Atacama.
Are they that permanent?
Over continents, trade winds collide with local winds, sometimes changing their speed and direction. For example, in the Indian Ocean, due to the special configuration of the coast of Southeast Asia and climatic characteristics, the trade winds turn into seasonal monsoons. As you know, in summer they blow from the cool sea towards the heated land, and in winter - vice versa. However, the statement that trade winds are winds of tropical latitudes is not entirely true. In the Atlantic, for example, in the Northern Hemisphere they blow in winter and spring within 5-27° N latitude, and in summer and autumn 10-30° N latitude. Back in the 18th century, this strange phenomenon was given scientific explanation John Hadley, British astronomer. The calm strip does not stand at the equator, but moves after the Sun. Thus, by the date when our star is at its zenith over the Tropic of Cancer, the trade winds shift to the north, and in winter - to the south. The constant winds also vary in strength. Passat Southern Hemisphere more powerful. He encounters almost no obstacles in the form of land on his way. There it forms the so-called “roaring” forties latitudes.
Trade winds and tropical cyclones
To understand the mechanics of typhoon formation, you need to understand that two constant winds blow in each hemisphere of the Earth. Everything that we described above refers to the so-called lower trade winds. But the air, as is known, cools when ascending to a height (on average by one degree for every hundred meters of rise). Warm masses are lighter and rush upward. Cold air tends to sink. Thus, opposing trade winds arise in the upper layers of the atmosphere. blowing in the Northern Hemisphere from the southwest, and below the equator - from the northwest. inside the trade winds it sometimes changes the stable direction of the two layers. A zigzag swirling of warm, moisture-saturated and cold air masses occurs. In some cases, tropical cyclones gain hurricane strength. The same directional vector inherent in the trade winds carries them to the west, where they unleash their destructive force on coastal areas.
