Looking at isobars on a synoptic map, we notice that in some places the isobars are denser, in others less frequent.

It is obvious that in the first places the atmospheric pressure changes in the horizontal direction more strongly, in the second - less. They also say: “faster” and “slower,” but one should not confuse the changes in space in question with changes in time.

You can accurately express how atmospheric pressure changes in the horizontal direction using the so-called horizontal pressure gradient, or horizontal pressure gradient. Chapter four talked about horizontal temperature gradients. Similarly, the horizontal pressure gradient is the change in pressure per unit distance in the horizontal plane (more precisely, on the surface of the level); in this case, the distance is taken in the direction in which the pressure decreases the most. And the direction of the strongest change in pressure at each point is the direction normal to the isobar at that point.

Thus, the horizontal pressure gradient is a vector, the direction of which coincides with the direction of the normal to the isobar in the direction of decreasing pressure, and the numerical value is equal to the derivative of the pressure in this direction. Let us denote this vector by the symbol - С R, and its numerical value -dp/dn, Where P- direction of the normal to the isobar.

Like any vector, the horizontal pressure gradient can be graphically represented by an arrow; in this case, an arrow directed normal to the isobar in the direction of decreasing pressure. In this case, the length of the arrow should be proportional to the numerical value of the gradient.

At different points of the pressure field, the direction and magnitude of the pressure gradient will, of course, be different. Where the isobars are concentrated, the change in pressure per unit distance normal to the isobar is greater; where the isobars are moved apart, it is smaller. In other words, the magnitude of the horizontal pressure gradient is inversely proportional to the distance between the isobars.

If there is a horizontal pressure gradient in the atmosphere, this means that the isobaric surfaces in a given part of the atmosphere are inclined to the level surface and, therefore, intersect with it, forming isobars. Isobaric surfaces are always inclined in the direction of the gradient, i.e., in the direction where the pressure decreases.

The horizontal pressure gradient is the horizontal component of the total pressure gradient. The latter is represented by a spatial vector, which at each point of an isobaric surface is directed along the normal to this surface towards the surface with a lower pressure value. The numerical value of this vector is –dp/dn; but here n- direction of the normal to the isobaric surface. The total pressure gradient can be decomposed into vertical and horizontal components, or into vertical and horizontal gradient s. It can also be decomposed into three components along the axes of rectangular coordinates X, Y, Z. Pressure changes with height much more strongly than in the horizontal direction. Therefore, the vertical pressure gradient is tens of thousands of times greater than the horizontal one. It is balanced or almost balanced by the force of gravity directed opposite to it, as follows from the basic equation of atmospheric statics. The vertical pressure gradient does not affect the horizontal movement of air. Further in this chapter we will talk only about the horizontal pressure gradient, calling it simply the pressure gradient.

Wind speed

As we already know from Chapter Two, wind is the movement of air relative to the earth's surface, and, as a rule, we mean the horizontal component of this movement. However, sometimes they speak of an upward or downward wind, taking into account the vertical component as well. Wind is characterized by a speed vector. In practice, wind speed refers only to the numerical value of speed; It is this that we will further call wind speed, and the direction of the speed vector - wind direction.

Wind speed is expressed in meters per second, kilometers per hour (especially for aviation services) and knots (nautical miles per hour). To convert speed from meters per second to knots, just multiply the number of meters per second by 2.

There is also an assessment of the speed (or, as they say in this case, strength) of the wind in points, the so-called Beaufort scale , according to which the entire range of possible wind speeds is divided into 12 gradations. This scale relates the strength of the wind to its various effects, such as the degree of rough seas, the swaying of branches and trees, the spread of smoke from chimneys, etc. Each gradation on the Beaufort scale has a specific name. Thus, zero on the Beaufort scale corresponds to calm, that is, a complete absence of wind. Wind at 4 points, according to Beaufort it is called moderate and corresponds to a speed of 5-7 m/sec; 7 points - strong, with a speed of 12-15 m/sec; at 9 points - a storm, with a speed of 18-21 m/sec; finally, a wind of 12 points Beaufort is already a hurricane, with a speed of over 29 m/sec.

A distinction is made between smoothed wind speed over a short period of time during which observations are made, and instantaneous wind speed, which generally fluctuates greatly and at times can be significantly lower or higher than the smoothed speed. Anemometers usually provide smoothed wind speed values, and this is what we will be talking about in what follows.

Near the earth's surface, we most often have to deal with winds whose speeds are on the order of 4-8 m/sec and rarely exceed 12-15 m/sec. But still, in storms and hurricanes of moderate latitudes, speeds can exceed 30 m/sec, and in some gusts reach 60 m/sec. In tropical hurricanes, wind speeds reach 65 m/sec, and individual gusts - up to 100 m/sec. In small-scale vortices (tornadoes, blood clots) speeds of more than 100 are possible. m/sec. In the so-called jet streams in the upper troposphere and lower stratosphere, the average wind speed over a long time and over a large area can reach 70-100 m/sec.

Wind speed at the earth's surface is measured by anemometers of various designs. Most often, they are based on the fact that wind pressure causes the receiving part of the device to rotate (cup anemometer, mill anemometer, etc.) or deflects it from the equilibrium position (Wild board). By the speed of rotation or the magnitude of the deviation, the wind speed can be determined. There are designs based on the manometric principle (Pitot tube). There are a number of designs of recording instruments - anemographs and (if the wind direction is also measured) anemorumgraphs. Instruments for measuring wind at ground stations are installed at a height of 10-15 m above the earth's surface. The wind they measured is called the wind at the earth's surface.

Direction of the wind

It is important to remember that when we talk about the direction of the wind, we mean the direction from which it blows. You can indicate this direction by naming either the point on the horizon from where the wind is blowing, or the angle formed by the direction of the wind with the meridian of the place, i.e. its azimuth. In the first case, there are 8 main directions of the horizon: north, northeast, east, southeast, south, southwest, west, northwest - and 8 intermediate directions between them: north-northeast, east-north- east, east-southeast, south-southeast, south-southwest, west-southwest, west-northwest, north-northwest (Fig. 68). 16 rhumbs indicating the direction from which the wind blows have the following abbreviations, Russian and international:

If the wind direction is characterized by its angle with the meridian, then the countdown is from north clockwise. Thus, north will correspond to 0° (360°), northeast 45°, east 90°, south 180°, west 270°. When observing the wind in high layers of the atmosphere, its direction is usually indicated in degrees, and when observing at ground-based meteorological stations - in horizon points.

The direction of the wind is determined by a weather vane rotating about a vertical axis. Under the influence of wind, the weather vane takes a position in the direction of the wind. The weather vane is usually connected to the Wild board.

As with speed, a distinction is made between instantaneous and smoothed wind direction. Instantaneous wind directions fluctuate significantly around a certain average (smoothed) direction, which is determined by observations from a weather vane.

However, the smoothed direction of the wind in each given place on the Earth is continuously changing, and in different places at the same time it is also different. In some places, winds of different directions have almost equal frequency over a long period of time, in others there is a well-defined predominance of some wind directions over others throughout the entire season or year. This depends on the conditions of the general circulation of the atmosphere and partly on local topographic conditions.

When climatologically processing wind observations, it is possible to construct for each given point a diagram representing the distribution of the frequency of wind directions along the main directions, in the form of a so-called wind rose (Fig. 69). From the origin of polar coordinates, directions are plotted along the horizon points (8 or 16) in segments, the lengths of which are proportional to the frequency of winds in a given direction. The ends of the segments can be connected by a broken line. The frequency of calms is indicated by the number in the center of the diagram (at the origin). When constructing a wind rose, you can also take into account the average wind speed in each direction by multiplying the repeatability of a given direction by it. Then the graph will show in conventional units the amount of air carried by the winds of each direction.

For presentation on climate maps, wind direction is generalized different ways. You can put wind roses on the map in different places. One can determine the resultant of all wind speeds (considered as vectors) at a given location for a given calendar month over a multi-year period and then take the direction of this resultant as the average wind direction. But more often the prevailing wind direction is determined. Namely, the quadrant with the greatest repeatability is determined. The midline of this quadrant is taken as the predominant direction.

Wind gusiness

The wind constantly and rapidly changes in speed and direction, fluctuating around some average values. The cause of these oscillations (pulsations, or fluctuations) of the wind is turbulence, which was discussed in Chapter Two. These vibrations can be recorded by sensitive recording instruments. Wind with pronounced fluctuations in speed and direction is called gusty. When the gusts are particularly strong, they speak of squally winds.

During ordinary station observations of the wind, the average (smoothed) direction and its average speed are determined over a period of time of the order of several minutes. When observing with a Wild vane, the observer must monitor the oscillations of the weather vane for two minutes and the oscillations of the Wild board for two minutes, and as a result determine the average (smoothed) direction and average (smoothed) speed during this time. A cup anemometer makes it possible to determine the average wind speed for any finite period of time.

However, the study of wind gusts is also of interest. Gustiness can be characterized by the ratio of the amplitude of wind speed fluctuations over a certain period of time to the average speed over the same time; in this case, either the average or the most frequently occurring amplitude is taken. By amplitude we mean the difference between successive maximum and minimum instantaneous speed. There are other characteristics of variability, including wind direction.

The greater the turbulence, the greater the gustiness. Consequently, it is more pronounced over land than over the sea; especially large in areas with difficult terrain; more in summer than in winter; has an afternoon maximum in the daily cycle.

In a free atmosphere, turbulence can cause aircraft to become loose. The chatter is especially great in highly developed convection clouds. But it increases sharply even in the absence of clouds in the zones of the so-called jet streams.

Vlad Merzhevich

The gradient is called smooth transition from one color to another, and there may be several colors themselves and transitions between them. With the help of gradients, the most bizarre web design effects are created, for example, pseudo-three-dimensionality, glare, background, etc. Also, with a gradient, elements look more attractive than monochromatic ones.

There is no separate property for adding a gradient, since it is considered a background image, so it is added through the background-image property or universal property background as shown in example 1.

Example 1: Gradient

Here the obscene idiom traditionally begins the prosaic image, but the language game does not lead to an active dialogical understanding.

The result of this example is shown in Fig. 1.

Rice. 1. Linear Gradient for Paragraph

In the simplest case with two colors, demonstrated in example 1, the position from which the gradient will begin is first written, then the start and end colors.

To record a position, you first write to , and then add the keywords top , bottom and left , right , as well as their combinations. The order of the words is not important, you can write to left top or to top left . In table 1 shows different positions and the type of gradient obtained for colors #000 and #fff, otherwise from black to white.

Table 1. Gradient types

Position		Description	View
to top	0deg	Down up.
to left	270deg	From right to left.
to the bottom	180deg	Top down.
to right	90deg	From left to right.
to top left		From the lower right corner to the upper left.
to top right		From the lower left to the upper right.
to bottom left		From the upper right corner to the lower left.
to bottom right		From top left to bottom right.

Instead of a keyword, you can specify the slope of the gradient line, which shows the direction of the gradient. First, the positive or negative value of the angle is written, then deg is added to it.

Zero degrees (or 360º) corresponds to the gradient from bottom to top, then the countdown is clockwise. The slope angle of the gradient line is shown below.

For top left and similar values, the gradient line's inclination angle is calculated based on the element's dimensions so as to connect two diagonally opposite corner points.

To create complex gradients, two colors will no longer be enough; the syntax allows you to add an unlimited number of them, listing colors separated by commas. You can use a transparent color (transparent keyword), as well as a semi-transparent color using the RGBA format, as shown in Example 2.

Example 2: Translucent colors

HTML5 CSS3 IE 9 IE 10 Cr Op Sa Fx

The result of this example is shown in Fig. 2.

Rice. 2. Gradient with translucent colors

To accurately position colors in a gradient, the color value is followed by its position in percentages, pixels, or other units. For example, record red 0%, orange 50%, yellow 100% means the gradient starts out red, then 50% orange, and then all the way yellow. For simplicity, extreme units like 0% and 100% can be omitted; they are assumed by default. Example 3 shows how to create a gradient button where the position of the second color out of three is set to 36%.

Example 3: Gradient Button

HTML5 CSS3 IE 9 IE 10 Cr Op Sa Fx

Press me gently

The result of this example is shown in Fig. 3.

Rice. 3. Gradient button

By setting the position of the color, you can get sharp transitions between colors, which ultimately gives a set of monochromatic stripes. So, for two colors you need to specify four colors, the first two colors are the same and start from 0% to 50%, the remaining colors are also the same and continue from 50% to 100%. In the example, 4 stripes are added as the background of the web page. Due to the fact that the extreme values ​​are automatically substituted, they can not be specified, so it is enough to write only two colors.

Example 4. Plain stripes

HTML5 CSS3 IE 9 IE 10 Cr Op Sa Fx

Typical European bourgeoisie and respectability are elegantly illustrated by the official language.

The result of this example is shown in Fig. 4. Note that one of the gradient colors is set to transparent, so it changes indirectly through the background color of the web page.

Rice. 4. Background of horizontal stripes

Gradients are quite popular among web designers, but adding them is complicated by different properties for each browser and specifying multiple colors. To make it easier for you to create gradients and insert them into your code, I recommend the site www.colorzilla.com/gradient-editor, which makes it easy to set up gradients and immediately get the desired code. Available ready-made templates(Presets), viewing the result (Preview), adjusting colors (Adjustments), final code (CSS) that supports IE through filters. For those who have worked in Photoshop or another graphic editor, creating gradients will seem like a piece of cake, but for others it will not be difficult to figure it out quickly. In general, I highly recommend it.

Atmospheric pressure varies in both vertical and horizontal directions, and each point in the atmosphere corresponds to a certain pressure. This means that the pressure forms a field, which is called pressure field. Such a field is visually represented in three-dimensional space by a system of surfaces of equal pressure values ​​- isobaric surfaces, and on a plane - by lines of equal pressure values ​​- isobars. Closed isobars represent cyclones and anticyclones. Cyclones are areas with low pressure in the center, anticyclones are areas with high pressure in the center (Fig. 6.13)

Rice. 6.13. Isobaric surfaces in the cyclone (H) and in the anticyclone (B) in a vertical section.

In addition, open baric systems are also distinguished - valleys, saddles and ridges. Stripes are called hollows low blood pressure between two areas of increased, ridges on the contrary, stripes relatively high blood pressure between areas of decreased. A saddle is distinguished between two hollows or ridges (Fig. 6.14)

Rice. 6.14. Isobars at sea level in various types pressure systems.

I-cyclone, II- anticyclone, III- hollow, IV- crest, V- saddle.

The change in atmospheric pressure in the horizontal direction is expressed using the horizontal baric gradient. A horizontal gradient is a vector that is directed normal to the isobar, in low pressure side and equal in value to the derivative of pressure along the normal. The horizontal pressure gradient is the change in pressure per unit distance in the horizontal plane (Fig. 6.15).

Pressure changes with height much faster than in the horizontal direction, so the vertical pressure gradient is tens of thousands of times greater than the horizontal one. Under actual atmospheric conditions, horizontal baric gradients are of the order of magnitude of 1-3 hPa for each meridian degree. Like the vertical pressure gradient, the horizontal gradient depends on temperature. Rice. 6.15. Isobars and horizontal pressure gradient. The arrows indicate the horizontal pressure gradient at three points of the pressure field.

The temperature in the atmosphere at the same altitude is different in different regions; therefore, there is a horizontal temperature (thermal) gradient that determines the change in air temperature per unit length normal to the isotherm. The presence of a horizontal thermal gradient determines the occurrence of a horizontal pressure gradient at a certain height, even if at the earth's surface we initially had the same pressure and a horizontal pressure gradient equal to zero. Let's look at how this happens. We have a certain area near the earth's surface with the same pressure, but with different temperatures; in one part of the area we have a cold air mass, in another warm. In cold air the pressure level is lower than in warm air, that is pressure will drop faster with altitude in a cold air mass, and at a certain altitude there will be a difference in pressure between the two air masses. It will be greater the higher we rise, that is, the horizontal baric gradient will increase with height and approach the horizontal thermal one. It means that in warm air masses the pressure at altitude will be increased, and in cold air masses it will be decreased (provided that the pressures at the surface are equal). An important conclusion follows from this provision: if a cyclone (a region of low pressure) exists in cold air with the lowest temperature in the central part, then baric gradients with height change their direction little and low pressure can be traced to high altitudes, that is, a cold cyclone is high(Fig. 6.16).

Rice. 6.16. High (cold) and low (warm) cyclone. Isobaric surfaces in vertical section.

Against, a cyclone in a warm air mass with a maximum temperature in the center quickly disappears with height, that is, it is low. In the overlying layers there will be an anticyclone above it.

For anticyclones the relationship is reversed, cold anticyclones are low and warm anticyclones are high(Fig. 6.17).

Rice. 6.17. Low (cold) and high (warm) anticyclone. Isobaric surfaces in vertical section.

The difference in atmospheric pressure between two areas, both at the earth's surface and above it, causes horizontal movement of air masses - wind. On the other hand, gravity and friction with the earth's surface hold air masses in place. Therefore, wind occurs only when the pressure difference is large enough to overcome air resistance and cause air movement. Obviously, the pressure difference must be related to a unit distance. The unit of distance used to be the 10 meridian, that is, 111 km. Currently, for simplicity of calculations, we agreed to take 100 km.

A horizontal pressure gradient is a pressure drop of 1 mb over a distance of 100 km normal to the isobar in the direction of decreasing pressure.

Wind speed is always proportional to the gradient: the greater the excess of air in one area compared to another, the stronger its outflow. On maps, the magnitude of the gradient is expressed by the distances between the isobars: the closer one is to the other, the greater the gradient and the stronger the wind.

In addition to the pressure gradient, the wind is affected by the rotation of the Earth, or the Coriolis force, centrifugal force and friction.

The rotation of the Earth (Coriolis force) deflects the wind in the northern hemisphere to the right (in the southern hemisphere to the left) from the direction of the gradient. The theoretically calculated wind, which is affected only by gradient and Coriolis forces, is called geostrophic. It blows tangentially to the isobars.

The stronger the wind, the greater its deflection due to the Earth's rotation. It increases with increasing latitude. Over land, the angle between the direction of the gradient and the wind reaches 45-50 0, and over the sea - 70-80 0; its average value is 60 0.

Centrifugal force acts on the wind in closed pressure systems - cyclones and anticyclones. It is directed along the radius of curvature of the trajectory towards its convexity.

The force of air friction on the earth's surface always reduces the wind speed. Wind speed is inversely proportional to the amount of friction. With the same pressure gradient over the sea, steppe and desert plains, the wind is stronger than over rugged hilly and forested terrain, and even more so mountainous. Friction affects the lower, approximately 1000-meter layer, called the friction layer. Higher up the winds are geostrophic.

The direction of the wind is determined by the side of the horizon from which it blows. To designate it, a 16-ray wind rose is usually adopted: N, CCW, NW, WNW, W, WSW, SW, SSW, S, SSE, SE, ESE, E, ENE, NE, NNE.

Sometimes the angle (rumb) between the wind direction and the meridian is calculated, with north (N) considered 0 0 or 360 0, east (E) - 90 0, south (S) - 180 0, west (W) - 270 0.

8.25 Causes and significance of the inhomogeneity of the Earth’s pressure field

For the geographic envelope, it is not the baric maximums and minimums themselves that are important, but the direction of those vertical air currents that create them.

The amount of atmospheric pressure shows the direction of vertical air movements - ascending or descending, and they either create conditions for moisture condensation and precipitation, or exclude these processes. There are two main types of connection between air humidity and its dynamics: cyclonic with ascending currents and anticyclonic with descending currents.

In rising currents, the air cools adiabatically, its relative humidity rises, water vapor condenses, clouds form and precipitation falls. Consequently, baric minimums are characterized by rainy weather and a humid climate. Condensation occurs gradually and at all altitudes. In this case, latent heat of vaporization is released, which causes further rise of air, its cooling and condensation of new portions of moisture, which entails the release of new portions of latent heat. Four mutually related processes occur simultaneously: 1) rising air, 2) cooling of air, 3) condensation of steam and 4) release of latent heat of vaporization. The root cause of all these processes is solar heat spent on water evaporation.

In descending air masses, adiabatic heating and a decrease in air humidity occur; clouds and precipitation cannot form. Consequently, pressure maxima, or anticyclones, are characterized by cloudless, clear and dry weather and a dry climate. From the surface of the oceans in the areas high pressure Significant evaporation occurs, the intensity of which is favored by cloudless skies. Moisture from here is carried away to other places, since the descending air must inevitably move to the sides. From the tropical highs it travels in the form of a trade wind to the equator.

The processes of assimilation of solar heat by the atmosphere, the dynamics of air masses and moisture circulation are mutually related and conditioned.

The circulation of the atmosphere and the heterogeneity of the pressure field are caused by two unequal reasons. The first and main one is the heterogeneity of the Earth's thermal field, the thermal difference between equatorial and polar latitudes. Indeed, there is a heater at the equator, and refrigerators at the poles. They create a first-order heat engine.

For thermal reasons, a fairly simple air circulation would be established on a non-rotating planet. At the equator, heated air rises, and rising currents near the earth's surface form a low-pressure belt called the equatorial pressure minimum. In the upper troposphere, isobaric surfaces rise and air flows towards the poles.

In polar latitudes, cold air sinks, areas of high pressure form near the earth's surface and the air returns to the equator.

The thermal difference between latitudes causes the transfer of air masses along the meridians or, as is commonly said in climatology, the meridional component of the atmospheric circulation.

Thus, the essence of the heat engine that causes atmospheric circulation is that part of the energy of solar radiation is converted into the energy of atmospheric movements. It is proportional to the temperature difference between the equator and the poles.

The second reason for atmospheric circulation is dynamic; it lies in the rotation of the planet. Air circulation directly between equatorial and polar latitudes is impossible, since the entire sphere in which the air moves rotates. Horizontal air flows both in the upper troposphere and near the earth's surface under the influence of the Earth's rotation certainly deviate to the right in the northern hemisphere and to the left in the southern hemisphere. This is how a zonal component of atmospheric circulation arises, directed from West to East and forming a west-east (western) transfer of air masses. On a rotating planet, west-east transport acts as the main type of atmospheric circulation.

Seasonal disturbances in the Earth's thermal field, caused by differences in the heating of the oceans and continents, cause fluctuations in atmospheric pressure above them. In winter, it is colder over Eurasia and North America than over the oceans at the same latitudes. Isobaric surfaces over the equatorial oceans are higher than over land. The air above flows from the oceans to the continents. The total mass of the air column over the continents increases. Extensive winter baric maxima are formed here - the Siberian maximum with a pressure of up to 1,040 mb and the slightly smaller North American maximum with a pressure of up to 1,022 mb. Over the oceans, the mass of the air column decreases and depressions form. This creates a second-order heat engine.

In summer, the thermal contrasts between land and sea decrease, the minimums and maximums seem to dissolve, the pressure equalizes or changes to the opposite of winter. In Siberia, for example, it drops to 1,006 mb.

Seasonal fluctuations in atmospheric pressure over land and sea create the so-called monsoon factor.

On the southern continents, in the January (summer for them) part of the year, pressure minima are formed, outlined by closed isobars.

The alternating six-month heating of the northern and southern hemispheres causes a shift of the entire pressure field of the Earth towards the summer hemisphere - in the northern part of the year in January, and in the southern part in July.

In the January part of the year, the equatorial minimum lies south of the equator; in July it is shifted north, reaching the northern tropics in South Asia. The Iran-Thar (South Asian) minimum is created over Iran and the Thar Desert. The pressure in it drops to 994 mb.

Looking at isobars on a synoptic map, we notice that in some places the isobars are denser, in others less frequent. It is obvious that in the first places the atmospheric pressure changes in the horizontal direction more strongly, in the second - less.

You can accurately express how atmospheric pressure changes in the horizontal direction using the so-called horizontal pressure gradient, or horizontal pressure gradient. The horizontal pressure gradient is the change in pressure per unit distance in the horizontal plane (more precisely, on the surface of the level); in this case, the distance is taken in the direction in which the pressure decreases the most.

Thus, the horizontal pressure gradient is a vector, the direction of which coincides with the direction of the normal to the isobar in the direction of decreasing pressure, and the numerical value is equal to the derivative of the pressure in this direction (G = -dp/dl).

Like any vector, the horizontal pressure gradient can be graphically represented by an arrow; in this case, an arrow directed normal to the isobar in the direction of decreasing pressure.

Where the isobars are concentrated, the change in pressure per unit distance normal to the isobar is greater; where the isobars are moved apart, it is smaller.

If there is a horizontal pressure gradient in the atmosphere, this means that the isobaric surfaces in a given part of the atmosphere are inclined to the level surface and, therefore, intersect with it, forming isobars.

In practice, the average pressure gradient for a particular section of the pressure field is measured on synoptic maps. Namely, they measure the distance between two adjacent isobars in a given area along a straight line. Then the pressure difference between the isobars (usually 5 mb) is divided by this distance, expressed in large units - 100 km. Under actual atmospheric conditions near the earth's surface, horizontal pressure gradients are on the order of several millibars (usually 1-3) per 100 km.

Pressure change with altitude

Atmospheric pressure decreases with altitude. This is due to two reasons. Firstly, the higher we are, the lower the height of the air column above us, and, therefore, the less weight presses on us. Secondly, with height the density of air decreases, it becomes more rarefied, that is, there are fewer gas molecules in it, and therefore it has less mass and weight.

International Standard Atmosphere (abbr. ISA, English ISA) is the conditional vertical distribution of temperature, pressure and air density in the Earth's atmosphere. The basis for calculating the ISA parameters is the barometric formula, with the parameters defined in the standard.

For ISA, the following conditions are accepted: air pressure at mean sea level at a temperature of 15 °C is equal to 1013 mb (101.3 kN/mI or 760 mm Hg), the temperature decreases vertically with an increase in altitude by 6.5 °C by 1 km to the level of 11 km (the conditional height of the beginning of the tropopause), where the temperature becomes equal to? 56.5 ° C and almost stops changing.