Concepts of synoptic meteorology
All synoptic objects and processes are in close interconnection. Therefore, speaking, for example, about the air masses, we should already have an idea about the fronts that divide them; speaking of both, they must have an idea of cyclonic activity, which is a form of existence of air masses and fronts in extratropical latitudes; speaking of the formation of air masses, we should have an idea of the pressure systems, which are their centers, etc. Therefore, we offer a summary of the basic concepts and definitions of synoptic meteorology in the shortest form.
Fields of meteorological elements
The spatial distribution of one or another physical characteristic of air, scalar or vector, is called a field of this magnitude . A field of a scalar quantity, for example temperature or air pressure, is graphically represented by equiscal surfaces: isobaric - in the case of pressure, isothermal - in the case of temperature, isothermal - in the case of specific volume, etc. When intersected with the level surface, these surfaces give lines of equal values : isobars, isotherms, isosteres, etc.
A gradient of a scalar quantity is a vector expressing. a change in this value per unit distance in the direction of its greatest fall, i.e., normal to the equiscal surface. Horizontal gradient is the projection of the gradient in the horizon plane. A vector equal to the gradient in numerical value and inverse in direction is called the ascendant.
A vector field, for example, a wind field, is visually represented by vector lines, in the case of a wind field, by streamlines, and surfaces of equal numerical values of the vector. By a current line is meant a line passing through a given point of the field so that the velocity vector is directed tangentially to this line. It is possible to build streamlines in the horizontal plane, along the horizontal projections of the wind.
In a baric field (pressure field), the following main forms of pressure distribution or pressure systems, low pressure areas and high pressure areas are distinguished. The low-pressure region (N) with closed concentric isobars is called the cyclone , the low-pressure band with open isobars is the name of the hollow . The high-pressure region (B) with closed concentric isobars is called an anticyclone , and the high-pressure band with open isobars is called a ridge . Pressure has a minimum value in the center of the cyclone and grows to the periphery; in an anticyclone, the pressure has a maximum value in the center, and drops to the periphery. The movement of air in the northern hemisphere occurs in the cyclone counterclockwise and in the anticyclone clockwise. Moreover, in a free atmosphere, streamlines almost coincide with isobars, and at the surface of the earth they converge to the center of the cyclone and diverge from the center of the anticyclone in the form of spirals. The isobaric surfaces in the cyclone are bent downward in the form of funnels , and in the anticyclone they are convex upward in the form of domes .
The diameters of cyclones and anticyclones in extratropical latitudes are measured in thousands of kilometers, and the pressure in the center is reduced or increased by several tens of millibars in comparison with the periphery.
Air masses
Air masses are those basic structural elements that make up the troposphere.
Air mass (VM) is a certain amount of air in the troposphere, commensurate in size with the areas of continents and oceans, having common properties, i.e., sufficient uniformity or continuity in the distribution of the main meteorological elements in the horizontal direction. The air mass moves as a whole in one of the main currents of the general circulation of the atmosphere, or for a long time is located above the same area in a sedentary state (most often in a stable anticyclone). The horizontal dimensions of air masses are measured in thousands of kilometers. Vertically, they can stretch from 1-2 km to the upper boundary of the troposphere and even capture stratospheric layers.
Air masses are warm, cold, and local. A warm (HM) air mass is called if it enters a colder area, i.e., as a rule, at higher latitudes and especially on a colder underlying surface. Cold (XM) is called the air mass entering lower latitudes and especially on a warmer underlying surface.
A typical example of a warm mass is air moving in winter from the subtropical latitudes of the Atlantic Ocean to the chilled continent of Europe. A typical example of a cold mass is the air coming in late spring from the Barents Sea to the snow-free and warmed-up territory of the European part of the Russian Federation. Another typical example of cold mass in winter is air that has passed from the cooled mainland of North America to the warm waters of the Atlantic Ocean.
The warm mass is cooled from below , from a colder underlying surface. Therefore, small vertical temperature gradients are established in it, i.e., stable stratification , at least in the lower kilometer. The conditions for convection in a warm mass are unfavorable, and condensation of water vapor occurs mainly in the lower layers in the form of fogs and layered clouds, from which drizzle or fine snow sometimes falls.
The cold mass warms up from below and acquires high vertical temperature gradients, i.e., unstable stratification, at least in several lower kilometers. Therefore, intense convection develops in it with the formation of cumulus and storm clouds; precipitation is stormy and stormy and often accompanied by thunderstorms.
Thus, a characteristic warm mass is a stable mass, a characteristic cold mass is an unstable mass, in the sense of stability or instability of vertical stratification.
In addition to warm and cold masses, they also distinguish local masses (MM ), long located over the same area. They are either stable or unstable, depending on whether the underlying surface is cooled or heated.
The air mass in its center, that is, in the region of its formation, will be local; leaving him, she seems warm or cold. Then, having lingered for a long time in another area, it will again turn into a local one, etc.
By geographical origin, the following types of air masses are distinguished: arctic air (AB), coming to temperate latitudes from the arctic basin; polar air (PV), whose masses are formed in temperate (subpolar) latitudes; tropical air (TV), the masses of which are formed in the subtropical zone and flow both in the direction of the pole, and in the form of trade winds, in the direction of the equator; equatorial air (EV) flowing from the equator to the tropics. In each type, marine (m) and continental (k) air are distinguished.
The air masses are constantly being re-formed, as individuals, and lose their individuality. When moving to another geographical area, they change their character, transform; for example, AB, turning into temperate latitudes, turns into PV; sea air on land - in continental, etc.
The most stable, slowly changing properties of air masses are called conservative properties . Such conservative properties include, first of all, potential temperature, equivalent potential temperature, specific humidity.
Fronts
The front is the transition zone or, approximately, the interface between the air masses of the troposphere . This surface is inclined so that the colder air lies in the form of a flat wedge under the warmer. The order of magnitude of the slope of the frontal surface to the surface of the earth (i.e., the tangent of the angle of inclination) is 0.01. The line of intersection of the frontal surface with the earth's surface is called the frontline . The term "front *" is applied both to the front surface and to the front line.
The front is characterized by more or less sharp changes in the main meteorological elements, primarily temperature, during the transition from one air mass to another, i.e., their large horizontal gradients. The pressure on both sides of the front is the same, but when passing through the front, the direction of the pressure gradients changes, and often their magnitude. The width of the front zone is several tens of kilometers, and the vertical thickness is several hundred meters.
If the front moves toward cold air, it is called a warm front (TF) . In the opposite case, when moving towards warm air, the front is called the cold front (HF) . In the case of a warm front, there is usually an upward glide of warm air along a receding cold wedge, which leads to the formation in the warm air above the front surface of an extensive cloud system hundreds of kilometers wide with overcast sediments. In the case of a cold front, upward slip is developed in a narrower strip along the front; the cloud system has a smaller width, but precipitation is more stormy, stormy in nature, at least in part, since before the cold wedge advancing forward, the rise of warm air is especially intense.
Complex fronts are also observed, the so-called occlusion fronts , which arise when the surfaces of warm and cold fronts are closed.
Warm and cold fronts are sections of the main fronts, i.e., fronts between the air masses of the main types listed above. The following types of main fronts are distinguished: arctic, polar, and tropical fronts.
The southern border of the Arctic air mass is called the Arctic Front (AF ). The southern (in the northern hemisphere) border of the polar air mass is called the polar front (PF) . Consequently, the Arctic front is the boundary between the Arctic and polar air, the polar front is the boundary between the polar and tropical air. Finally, the boundary between tropical and equatorial air is called the tropical front (TF).
Secondary fronts are different from the main fronts, inside the main air mass, for example, separating continental polar air from marine polar.
Fronts in the troposphere constantly reappear and erode.
Cyclonic activity
Extratropical cyclones develop mainly in the air masses at the main fronts - polar and arctic.
In the first stage of development, in the stage of the so-called young cyclone, the main front passing through the center of the cyclone takes on the character of a warm front in the front part and the character of a cold front in the rear part The part of the cyclone located in warm air between these fronts is called the warm sector . In the second stage — an occluded cyclone — the warm and cold fronts in the cyclone region are closed into one occlusion front. This process of closing fronts is called occlusion, or cyclone occlusion. After occlusion, the cyclone decays more or less quickly.
Cyclones are areas of predominant upward air movement, both frontal and due to the general convergence of air currents in the lower layers. Therefore, the cyclone is generally characterized by significant cloud cover and precipitation. The change in air mass during the passage of the cyclone is often accompanied by sharp fluctuations in temperature. The wind strength in the cyclone area is increased.
Anticyclones in the lower layers develop inside homogeneous air masses; frontal surfaces do not intersect in the inner region of the anticyclone with the surface of the earth and do not have an ascending glide of warm air. On the contrary, descending air movements develop in the anticyclone, leading to the formation of temperature inversions in the free atmosphere. Due to the predominance of downward movements, cloud cover in anticyclones is generally small and there is no precipitation. However, in the anticyclone, the formation of fogs and layered clouds with drizzle in the lower layers is possible due to air cooling from the underlying surface.
A series of cyclones of several members with intermediate anticyclones between them usually appears on the same main front. A separate cyclone exists for several days, moving along the front at a speed of several tens of kilometers per hour in the direction of flow in warm air. Most often, cyclones move from the western half of the horizon to the eastern, and their average speed is 30–45 km / h. The vast and deep low-pressure region that arose as a result of the fusion of individual cyclones of the series, the so-called central cyclone, can remain inactive for many days in a row. The speeds and directions of movement of anticyclones are generally the same, but anticyclones can more often “stabilize *, that is, remain in a sedentary state for a long time over the same area.
General atmospheric circulation
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.