Linear Gradient. baric field
Looking at the isobars on the synoptic map, we notice that in some places the isobars are thicker, in others - less often. It is obvious that in the first places the atmospheric pressure changes in the horizontal direction more strongly, in the second - weaker.
To accurately express how atmospheric pressure changes in the horizontal direction, you can use the so-called horizontal baric 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 level surface); in this case, the distance is taken in the direction in which the pressure decreases most strongly.
Thus, the horizontal baric gradient is a vector whose direction 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 pressure along this direction (G = -dp/dl).
Like any vector, the horizontal baric gradient can be represented graphically by an arrow; in this case, an arrow directed along the normal to the isobar in the direction of decreasing pressure.
Where the isobars are condensed, the change in pressure per unit distance along the normal to the isobar is greater; where the isobars are moved apart, it is smaller.
If there is a horizontal baric gradient in the atmosphere, this means that the isobaric surfaces in a given section of the atmosphere are inclined to the level surface and, therefore, intersect with it, forming isobars.
In practice, the average baric gradient is measured on synoptic maps for a particular section of the baric field. 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 baric gradients are on the order of a few millibars (usually 1-3) per 100 km.
Change in pressure with height
Atmospheric pressure decreases with height. This is due to two reasons. Firstly, the higher we are, the lower the height of the air column above us, and, therefore, less weight presses on us. Secondly, with height, the density of air decreases, it becomes more rarefied, that is, it has fewer gas molecules, and therefore it has less mass and weight.
International Standard Atmosphere (abbr. ISA, eng. ISA) is a conditional vertical distribution of temperature, pressure and air density in the Earth's atmosphere. The basis for calculating the parameters of the ISA 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 1013 mb (101.3 kN/m² or 760 mmHg), the temperature decreases vertically with an increase in altitude by 6.5 °C by 1 km to the level of 11 km (conditional altitude of the beginning of the tropopause), where the temperature becomes equal to −56.5 °C and almost stops changing.
Vlad Merzhevich
A gradient is a smooth transition from one color to another, and there can 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 prettier than plain ones.
There is no separate property to add the gradient, because it is considered a background image, so it is added via the background-image property or the background generic property, as shown in example 1.
Example 1 Gradient
Here, the obscene idiom traditionally begins a prose image, but the language game does not lead to an active-dialogical understanding.
The result of this example is shown in Fig. one.
Rice. 1. Linear gradient for a paragraph
In the simplest case with two colors shown in example 1, first write the position from which the gradient will start, then the start and end colors.
To record a position, first write to , and then add the keywords top , bottom and left , right , as well as their combinations. Word order is not important, you can write to left top or to top left . In table. 1 shows the different positions and type of the resulting gradient for the colors #000 and #fff, otherwise from black to white.
| Position | Description | View | |
|---|---|---|---|
| to top | 0deg | Upwards. | |
| left | 270deg | From right to left. | |
| 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 corner to the upper right. | ||
| to bottom left | From the top right corner to the bottom left. | ||
| to bottom right | From top left to bottom right. |
Instead of a keyword, it is allowed to set the slope of the gradient line, which shows the direction of the gradient. First, a positive or negative value of the angle is written, then deg is added together.
Zero degrees (or 360º) corresponds to a gradient from bottom to top, then the countdown is clockwise. The slope angle of the gradient line is shown below.
For the top left value and similar values, the angle of the gradient line is calculated based on the dimensions of the element 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 the colors separated by commas. In this case, you can use a transparent color (the transparent keyword), as well as a translucent color using the RGBA format, as shown in example 2.
Example 2: Translucent Colors
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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, the entry red 0%, orange 50%, yellow 100% means that the gradient starts from red, then 50% goes to orange, and then all the way to yellow. For simplicity, extreme units like 0% and 100% can be omitted, they are assumed by default. Example 3 shows the creation of a gradient button in which the position of the second color of the three is set to 36%.
Example 3: Gradient Button
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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, four colors must be specified, the first two colors are the same and start from 0% to 50%, the remaining colors are also the same among themselves and continue from 50% to 100%. Example 4 adds stripes as the background of the web page. Due to the fact that the extreme values are substituted automatically, they can be omitted, so it is enough to write just two colors.
Example 4. Plain stripes
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Typical European bourgeoisness and integrity gracefully illustrates 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 their addition is complicated by different properties for each browser and specifying many colors. To make it easier for you to create gradients and insert them into your code, I recommend www.colorzilla.com/gradient-editor, which makes it easy to set up gradients and get the code you need right away. There are ready-made templates (Presets), preview of the result (Preview), color settings (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 trifling matter, the rest will not be difficult to quickly figure it out. In general, I highly recommend it.
Consider in the atmosphere a rectangular parallelepiped with ribs dx, dy, dz(Fig. 5.12) . We are interested in the change in pressure in the horizontal direction, i.e. along the axis X.
Let the pressure isobar R directed parallel to the axis y, along the edge. Parallel to her along the rib SW passes an isobar with pressure ( p+dp). Recall that atmospheric pressure is characterized by a force acting per unit surface area, normal to the latter. In what follows, we neglect temporal changes in pressure, i.e. we consider its change only in space.
Fig / 5.12. To the calculation of the force of the horizontal pressure gradient
So, on the left side of AA "D" D, the atmospheric pressure is equal to R. The pressure on the opposite face of BB"C"C is . Since the force acting on the entire face is equal to the product of atmospheric pressure and its area, we write the expression for the force:
left pdydz,
· on right .
As a result, the volume dxdydz force is acting dFx), equal to
According to Newton's second law, the force dFx and the mass of the considered volume
dm = pdxdydz (5.2)
related to each other (the ratio of force to mass is equal to acceleration a):
whence, in view of (5.1) and (5.2)
We got the expression for the acceleration a, which creates the force of the baric gradient. Its value, according to (5.3), is equal to the force of the baric gradient per unit mass of an elementary volume of air. The minus sign in formulas (5.1) and (5.4) indicates that the force and acceleration of the baric gradient are directed in the direction of decreasing pressure. Moreover, the force and acceleration of the baric gradient act in the direction of the most rapid decrease in pressure. This direction is the direction of the normal to the isobar at the considered point of application of the force.
In (5.4) the expression is equal to the numerical value of the baric gradient. The horizontal baric gradient can be graphically represented by an arrow pointing normal to the isobar in the direction of decreasing pressure. The length of the arrow should be proportional to the numerical value of the gradient (Fig. 5.13). In other words, the magnitude of the horizontal baric gradient is inversely proportional to the distance between the isobars.
Obviously, where the isobars are condensed, the baric gradient, i.e. the change in pressure per unit distance along the normal to the isobar is greater. Where the isobars are moved apart, the baric gradient is smaller.
Rice. 5.13. The arrows indicate the horizontal baric gradient at three points in the baric field.
Isobaric surfaces are always inclined in the direction of the gradient, i.e. in the direction where the pressure decreases (Fig. 5.13).
The vertical baric gradient (see Chap. 1) is tens of thousands of times larger than the horizontal one. In what follows, only the horizontal baric gradient will be discussed. To determine the average baric gradient for a section of the baric field, the pressure is measured along the normal to the isobars at two points located at a distance corresponding to one degree of the meridian (111 km). The pressure gradient is numerically equal to the pressure difference and has the dimension of mb/111 km (or hPa/111 km). In the atmosphere near the Earth's surface, the order of magnitude of horizontal baric gradients is several millibars (usually 1–3) per meridian degree (111 km).
Rice. 5.14. Vertical section of isobaric surfaces. Arrow – direction of the horizontal baric gradient; double line - level surface
For example, let the distance between adjacent isobars be 2 cm on a synoptic map at a scale of 1: 10,000,000. The step of the isolines is 5 mb. For the specified scale, 2 cm on the map corresponds to 200 km in kind. Therefore, the pressure difference per 100 km will be 5/2= 2.5 mb/100 km. For a distance of 111 km, this difference = 2.75 mb/111 km.
If only the force of the horizontal baric gradient acted in the atmosphere, then the air would move uniformly accelerated, with an acceleration that can be calculated using formula (5.4). The acceleration at real pressure gradients is small, on the order of 0–0.3 cm/s 2 . Nevertheless, with an increase in the duration of the action of the baric gradient force, the wind speeds would increase indefinitely. In reality, wind speeds rarely exceed 10 m/s or more. Consequently, there are also other forces that balance the force of the baric gradient (more on this in the next chapter).
Change in baric gradient with height associated with uneven temperature distribution. Following S.P. Khromov, imagine that the baric gradient at the earth's surface is zero, i.e. the pressure at all points is the same (Fig. 5.15). In this case, the temperature in one part of the considered area is higher, in the other it is lower. G the horizontal temperature (thermal) gradient, by definition, T, is always directed along the normal to the isotherm (line of equal temperatures) in the direction where the temperature increases.
Recall that pressure decreases with altitude the faster the lower the air temperature. It follows that isobaric surfaces with uneven temperature distribution cannot be horizontal. Even if the surface isobaric surface is horizontal, then each overlying isobaric surface will be raised above the underlying surface in cold air less, in warm air more. This means that the overlying surfaces will be inclined from warm air to cold air (Fig. 5.15). Thus, although the horizontal baric gradient is zero near the earth's surface, there is such a gradient in the overlying layers.
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Cold Heat
Rice. 5.15. Relationship between horizontal temperature and pressure gradients
Moreover, whatever the horizontal baric gradient at the earth's surface, with height it will approach the horizontal temperature gradient in its direction. At a sufficiently high altitude, the horizontal baric gradient will closely coincide in direction with the average horizontal temperature gradient in the air layer from the lower level to the upper one. From fig. 5.15 it follows that in warm regions of the atmosphere the pressure at a given height will be increased, and in cold regions it will be reduced.
The difference in atmospheric pressure between two areas both at the earth's surface and above it causes a horizontal movement of air masses - the wind. On the other hand, gravity and friction on the earth's surface hold air masses in place. Therefore, wind occurs only at a pressure drop that is large enough to overcome air resistance and cause it to move. Obviously, the pressure difference must be related to the distance unit. As a unit of distance, they used to take 10 meridian, that is, 111 km. At present, for simplicity of calculations, we agreed to take 100 km.
The horizontal baric gradient is a pressure drop of 1 mb over a distance of 100 km along the normal to the isobar in the direction of decreasing pressure.
The wind speed is always proportional to the gradient: the greater the excess 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 baric gradient, the rotation of the Earth, or the Coriolis force, centrifugal force and friction act on the wind.
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 the forces of the gradient and Coriolis, is called geostrophic. It blows tangentially to the isobars.
The stronger the wind, the greater its deflection due to the rotation of the Earth. 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 baric 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 forest terrain, and even more so mountainous. Friction affects the lower, approximately 1000-meter layer, called the friction layer. Above, 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-beam wind rose is usually taken: C, NW, NW, WNW, W, WSW, SW, SSW, S, SSE, SE, ESE, B, NE, NE, NNE.
Sometimes the angle (rhumb) between the direction of the wind and the meridian is calculated, with north (N) considered as 0 0 or 360 0, east (E) - for 90 0, south (S) - 180 0, west (W) - 270 0.
8.25 Causes and significance of the inhomogeneity of the Earth's baric field
For the geographic envelope, it is not the pressure maxima and minima themselves that are important, but the direction of those vertical air currents that create them.
The size 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 relationship between air humidity and its dynamics: cyclonic with ascending currents and anticyclonic with descending currents.
In ascending currents, the air cools adiabatically, its relative humidity rises, water vapor condenses, clouds form and precipitation falls. Consequently, rainy weather and a humid climate are characteristic of baric minima. Condensation occurs gradually and at all altitudes. In this case, the latent heat of vaporization is released, which causes a further rise in air, its cooling and condensation of new portions of moisture, which entails the release of new portions of latent heat. At the same time, four mutually connected processes are going on: 1) air rise, 2) air cooling, 3) steam condensation and 4) release of latent heat of vaporization. The root cause of all these processes is the solar heat spent on the evaporation of water.
In descending air masses, adiabatic heating and a decrease in air humidity occur; clouds and precipitation cannot form. Consequently, baric maxima, or anticyclones, are characterized by cloudless, clear and dry weather and a dry climate. Significant evaporation occurs from the surface of the oceans in areas of high pressure, the intensity of which is favored by a cloudless sky. The moisture from here is carried away to other places, since the descending air must inevitably move to the sides. From tropical highs, it goes 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 connected and conditioned.
The circulation of the atmosphere and the inhomogeneity of the baric 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 the 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 circulation of air would be established on a non-rotating planet. At the equator, heated air rises, rising currents near the earth's surface form a low-pressure belt called the equatorial baric minimum. In the upper troposphere, isobaric surfaces rise and air flows towards the poles.
In the polar latitudes, cold air descends, 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 they say in climatology, the meridional component of atmospheric circulation.
Thus, the essence of the heat engine that causes the circulation of the atmosphere lies in the fact 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 the 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, will certainly deviate to the right in the northern hemisphere and to the left in the southern hemisphere. This is how the zonal component of the atmospheric circulation arises, directed from West to East and forming the west-east (western) transport of air masses. On a rotating planet, west-east transport acts as the main type of atmospheric circulation.
Seasonal perturbations of the Earth's thermal field, due to differences in the heating of the oceans and continents, cause fluctuations in atmospheric pressure over them. In winter over Eurasia and North America it is colder than over the oceans in the same latitudes. The isobaric surfaces over the equators of the oceans are higher than over the land. The air above flows from the oceans to the continents. The total mass of the air column over the continents is increasing. Extensive winter baric maxima are formed here - the Siberian maximum with a pressure of up to 1,040 mb and the somewhat 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 is how a second-order heat engine is created.
In summer, thermal contrasts between land and sea decrease, minima and maxima seem to dissolve, 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, baric minima are formed, outlined by closed isobars.
The alternating semi-annual heating of the northern and southern hemispheres causes a shift of the entire baric field of the Earth towards the summer hemisphere - in the January part of the northern year, and in the July part of the southern one.
The equatorial minimum in the January part of the year lies south of the equator, in July it is shifted to the north, reaching the northern tropic in South Asia. Iran-Tara (South Asian) minimum is created over Iran and the Thar desert. The pressure in it drops to 994 mb.
Horizontal baric gradient
1. Looking at the isobars on the synoptic map, we notice that in some places the isobars are thicker, in others - less often. It is obvious that in the first places the atmospheric pressure changes in the horizontal direction more strongly, in the second - weaker. They also say:<быстрее>and<медленнее>, but the changes in space in question should not be confused with changes in time.
To accurately express how atmospheric pressure changes in the horizontal direction, you can use the so-called horizontal baric gradient, or horizontal pressure gradient. Chapter 4 discussed the horizontal temperature gradient. Similarly, the change in pressure per unit distance in a horizontal plane (more precisely, on a level surface) is called a horizontal pressure gradient. In this case, the distance is taken in the direction in which the pressure decreases the most, and such a direction at each point is the direction along the normal to the isobar at the given point.
Thus, the horizontal baric gradient is a vector whose direction 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 pressure along this direction. We denote this vector by the symbol -s/p, and its numerical value (modulus) -dr/dp, where p is the normal to the isobar.
Like any vector, the horizontal baric gradient can be graphically represented by an arrow, in this case an arrow directed along the normal to the isobar in the direction of decreasing pressure. The length of the arrow should be proportional to the numerical value of the gradient (Fig. 58).
Rice. 58. Isobars and horizontal baric gradient (arrows) at three points in the baric field.
Rice. 59. Isobaric surfaces in a vertical section and the direction of the horizontal baric gradient. The double line is the level surface.
At different points in the baric field, the direction and modulus of the baric gradient will, of course, be different. Where the isobars are condensed, the change in pressure per unit distance along the normal to the isobar is greater; where the isobars are moved apart, it is smaller. In other words, the modulus of the horizontal baric gradient is inversely proportional to the distance between the isobars.
If there is a horizontal baric gradient in the atmosphere, it means that the isobaric surfaces in a given section 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., where the pressure decreases (Fig. 59).
2. The horizontal baric gradient is the horizontal component of the total baric gradient. The latter is represented by a spatial vector, which at each point of the isobaric surface is directed along the normal to this surface towards the surface with a lower pressure value. The modulus of this vector is - dr/dp, but here n is the normal to the isobaric surface. The total baric gradient can be decomposed into vertical and horizontal components, or into vertical and horizontal gradients. You can decompose it into three components along the axes of rectangular coordinates X, Y, Z.
Pressure changes with height much more than in the horizontal direction. Therefore, the vertical baric 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 baric gradient does not affect the horizontal movement of air. Later in this chapter, we will only talk about the horizontal baric gradient, simply calling it the baric gradient.
3. In practice, the average baric gradient is measured on synoptic maps for one or another section of the baric field. Namely, the distance Ap is measured between two adjacent isobars in a given section along a straight line, which is quite close to the normals of both isobars. Then the pressure difference between the Ap isobars (usually 5 hPa) is divided by this distance, expressed in large units - hundreds of kilometers or meridian degrees (111 km). The average baric gradient will be represented by the ratio of finite differences Ap/An hPa/degree meridian. Instead of a meridian degree, 100 km is now more often taken. The baric gradient in the free atmosphere can be determined from the distance between isohypses on baric topography maps. Under actual atmospheric conditions near the earth's surface, horizontal baric gradients are of the order of several hectopascals (typically 1-3) per meridian degree.