General atmospheric circulation is a set of atmospheric currents of a large, “planetary” scale, which can be traced using synoptic maps. These include currents transporting air masses in extratropical latitudes, as well as trade winds and monsoons in the tropics.
With constant changes in the general circulation, many of its significant features are constantly maintained or repeated from year to year in the same seasons, creating climatic features of a particular area. These stable features of the general circulation can be especially well studied on climatological maps of the average distribution of pressure and wind. The pressure distribution should be taken into account in the analysis of the general circulation because air movement is connected with the pressure distribution by the baric law of the wind (which is also valid for average values).
In the equatorial belt, the pressure in the lower troposphere is lowered throughout the year ( equatorial depression ). On both sides of this belt, in the tropics and subtropics, pressure is increased year-round; only in the summer in these latitudes above the overheated continents (North Africa, South Asia, Mexico, Australia, South America and South Africa) the pressure in the lowest part of the troposphere is lowered. On climate maps in the tropics and subtropics are the so-called subtropical anticyclones ; the maximum values of pressure in them occur at latitudes of about 30–40 °. At sea level, these are the Azores anticyclone over the North Atlantic Ocean, the Honolulu anticyclone over the Northern Pacific Ocean and three anticyclones over the oceans of the southern hemisphere (in winter, the fourth is over Australia ). This does not mean, of course, that in each such region there is one and the same permanent anticyclone; the high-pressure region on the middle map only reflects the predominance of anticyclones in a given region over cyclones.
From subtropical anticyclones to the equator in the lower part of the troposphere winds of moderate strength are stable in the direction - trade winds , northeastern in the northern hemisphere and southeastern in the southern. Near the equator, trade winds of both hemispheres are found. The interface between them - tropical fronts (on the climate maps they correspond to the so-called equatorial calm zone ) - lie above the oceans in general, somewhat north of the equator. But over the Indian Ocean and the west of the Pacific Ocean, a tropical front separates the southeast trade wind from the continental monsoon in winter (the northern hemisphere) from the continental monsoon, blowing from the cooled mainland of Asia, and lies south of the equator, about 10 ° S. w. In summer, the southeastern trade wind passes through the equator to the heated continent of Asia, turning into the southwestern oceanic monsoon, and penetrates the Himalayas; the tropical front between the monsoon and the continental air masses is located accordingly above the mainland, at 25-30 ° N
In the upper half of the troposphere, high-pressure areas are shifted to the equator, and opposite trade winds are observed opposite winds that carry air from the equator to the subtropics: antipassat, southwest - in the northern hemisphere and northwest - in the southern .
Masses of tropical marine air flow out from oceanic anticyclones towards high latitudes, mainly with southwestern currents . Similarly, continental tropical air masses sometimes flow north from the continental subtropic regions (for example: Sahara, Arabia, southern North America), and in summer from the desert and steppe regions of higher latitudes (southern European territory of the Russian Federation, Kazakhstan, Central Asia, Mongolia , southern United States). At temperate latitudes, tropical air occurs with colder air masses of polar air. The interface between tropical and polar air, polar fronts, are most often observed at latitudes of about 45-50 °.
In each hemisphere, several polar fronts can be found simultaneously both over the oceans, especially in summer, and over the continents.
In summer, zones of formation of polar fronts above land are significantly shifted to higher latitudes. Over the oceans, zones of polar fronts in the summer are also somewhat shifted to higher latitudes.
The formation of cyclones at the polar fronts leads to the fact that the pressure in temperate latitudes is generally reduced. On average, the minimum pressure occurs at latitudes of about 60–65 °, since the centers of especially deep cyclonic disturbances — central cyclones — are located in polar air precisely at these latitudes.
In winter, cyclonic activity in temperate latitudes develops predominantly above the seas, and on the average climatological map one can see a deep Icelandic depression in the north of the Atlantic and an equally deep Aleutian depression in the north of the Pacific Ocean. On the contrary, stable anticyclones prevail over cold continents in winter; therefore, on the winter climatological map you can see a huge Asian (or Siberian) anticyclone and a much less powerful anticyclone over North America - the Canadian. In summer, oceanic depressions on the middle map weaken, but cyclonic activity on the continents intensifies, as a result of which an extensive depression appears over summer climate maps over Asia (Asian or South Asian) and less significant over North America.
In the zone from 35–40 to 60–65 °, along the polar (in the northern hemisphere – northern) periphery of subtropical anticyclones and along the equatorial (in the northern hemisphere – southern) periphery of extratropical depressions, air is transported from west to east. Therefore, the prevailing winds at these latitudes in both tropical and polar air will be the western quarter. But the meridional components in the temperate latitudes cause the transfer of polar air to the tropics and tropical to the pole. These meridional components appear as a result of cyclone formation on polar fronts: in the front of each cyclone, tropical air flows toward the pole — first at the surface of the earth, and then, in higher layers, above the polar air; in the rear of each cyclone, polar air flows toward the tropics.So the exchange of air between high and low latitudes of the earth.
Under latitudes of about 70–75 °, arctic fronts arise between polar air (air of temperate latitudes) and arctic air. The formation of cyclones, less intense, also occurs at these fronts, as a result of which the Arctic air invades lower latitudes, and the warmer polar air from temperate latitudes penetrates the Arctic.
In the Arctic itself, increased pressure prevails, especially in winter, which is reflected in climatic maps (Arctic anticyclone). The same is true in Antarctica. As a result, the prevailing winds in the Arctic in the lower layers will be eastern (along the southern periphery of the Arctic anticyclones). But in the upper half of the troposphere, pressure decreases in the meridional direction from the equator to the pole itself. Therefore, starting from an altitude of about 4 km, the prevailing wind direction in the Arctic will be western, as well as in tropical and temperate latitudes.
Signs of the clouds and air fronts
If you stand with your back to the wind, then the deterioration of the weather should be expected only on the left, but never on the right. Therefore, any cloud on the right does not carry any change in weather.
The surest signs of bad weather are usually clouds and wind.
If a warm front is approaching (warm air approaches cold, and cold air recedes), the main harbingers of bad weather are high cirrus clouds. They are visible at a distance of 100-200 km. They are 400-500 km ahead of the first rainfall and 12-16 hours earlier than the clouds of the lower tier, from which rain or snow falls.
If a cold front approaches (warm air recedes, and cold air spreads after it), then more often it is preceded by clouds in the form of small glomeruli, called “lamb” in everyday life. Precipitation can be predicted by the nature of cloudiness in no more than 3-5 hours, and more often the cloud appears so unexpectedly and moves so fast that it can be done in just 30-40 minutes.
Clouds - the forerunners of foam - always appear on the very edge of the horizon, thickening on one side of it. Spreading across the sky, they always remain the most dense on the side of the horizon where they first appeared.
Clutter randomly scattered across the sky is usually not a harbinger of bad weather.