How much does air weigh? Physical properties of air: density, viscosity, specific heat capacity Change in air density with altitude

DEFINITION

Atmospheric air is a mixture of many gases. Air has a complex composition. Its main components can be divided into three groups: constant, variable and random. The former include oxygen (the oxygen content in the air is about 21% by volume), nitrogen (about 86%) and the so-called inert gases (about 1%).

Content components practically does not depend on where globe a sample of dry air was taken. The second group includes carbon dioxide (0.02 - 0.04%) and water vapor (up to 3%). The content of random components depends on local conditions: near metallurgical plants, noticeable amounts of sulfur dioxide are often mixed into the air, in places where organic residues decompose - ammonia, etc. In addition to various gases, the air always contains more or less dust.

Air density is a value equal to the mass of gas in the Earth's atmosphere divided by a unit volume. It depends on pressure, temperature and humidity. There is a standard value for air density - 1.225 kg/m 3, corresponding to the density of dry air at a temperature of 15 o C and a pressure of 101330 Pa.

Knowing from experience the mass of a liter of air at normal conditions(1.293 g), we can calculate the molecular weight that air would have if it were an individual gas. Since a gram molecule of any gas occupies a volume of 22.4 liters under normal conditions, the average molecular weight of air is equal to

22.4 × 1.293 = 29.

This number - 29 - should be remembered: knowing it, it is easy to calculate the density of any gas relative to air.

Density of liquid air

When sufficiently cooled, the air turns into a liquid state. Liquid air can be stored for quite a long time in vessels with double walls, from the space between which the air is pumped out to reduce heat transfer. Similar vessels are used, for example, in thermoses.

Liquid air that evaporates freely under normal conditions has a temperature of about (-190 o C). Its composition is not constant, since nitrogen evaporates more easily than oxygen. As the nitrogen is removed, the color of the liquid air changes from bluish to pale blue (the color of liquid oxygen).

In liquid air they easily transform into solid state ethyl alcohol, diethyl ether and many gases. If, for example, carbon dioxide is passed through liquid air, it turns into white flakes similar in appearance. appearance to the snow. Mercury immersed in liquid air becomes hard and malleable.

Many substances cooled by liquid air dramatically change their properties. Thus, chink and tin become so brittle that they easily turn into powder, a lead bell makes a clear ringing sound, and a frozen rubber ball shatters if dropped on the floor.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

Exercise	Determine how many times heavier than air is hydrogen sulfide H 2 S.
Solution	The ratio of the mass of a given gas to the mass of another gas taken in the same volume, at the same temperature and the same pressure is called the relative density of the first gas to the second. This value shows how many times the first gas is heavier or lighter than the second gas. The relative molecular weight of air is taken to be 29 (taking into account the content of nitrogen, oxygen and other gases in the air). It should be noted that the concept of “relative molecular weight air" is used conditionally, since air is a mixture of gases. D air (H 2 S) = M r (H 2 S) / M r (air); D air (H 2 S) = 34 / 29 = 1.17. M r (H 2 S) = 2 × A r (H) + A r (S) = 2 × 1 + 32 = 2 + 32 = 34.
Answer	Hydrogen sulfide H 2 S is 1.17 times heavier than air.

Air is an intangible quantity, it cannot be touched or smelled, it is everywhere, but for humans it is invisible; finding out how much air weighs is not easy, but possible. If the surface of the Earth, as in a children's game, is drawn into small squares measuring 1x1 cm, then the weight of each of them will be equal to 1 kg, that is, 1 cm 2 of atmosphere contains 1 kg of air.

Can this be proven? Quite. If you build a scale from an ordinary pencil and two balloons, having secured the structure to the thread, the pencil will be in balance, since the weight of the two inflated balls is the same. Once one of the balloons is pierced, the advantage will be in the direction of the inflated balloon, because the air from the damaged balloon has escaped. Accordingly, simple physical experience proves that air has some weight. But, if you weigh the air on a flat surface and in the mountains, then its mass will turn out to be different - mountain air is much lighter than the one we breathe near the sea. Reasons different weights some:

The weight of 1 m 3 of air is 1.29 kg.

• the higher the air rises, the more rarefied it becomes, that is, high in the mountains, the air pressure will not be 1 kg per cm 2, but half as much, but the content of oxygen necessary for breathing also decreases by exactly half, which can cause dizziness, nausea and ear pain;
• water content in the air.

The air mixture includes:

1.Nitrogen – 75.5%;

2. Oxygen – 23.15%;

3. Argon – 1.292%;

4. Carbon dioxide – 0.046%;

5. Neon – 0.0014%;

6. Methane – 0.000084%;

7. Helium – 0.000073%;

8. Krypton – 0.003%;

9. Hydrogen – 0.00008%;

10. Xenon – 0.00004%.

The amount of ingredients in the air may change and, accordingly, the mass of air also undergoes changes in the direction of increase or decrease.

• air always contains water vapor. The physical law is that the higher the air temperature, the more water it contains. This indicator is called air humidity and affects its weight.

What is the weight of air measured in? There are several indicators that determine its mass.

How much does a cube of air weigh?

At a temperature of 0° Celsius, the weight of 1 m 3 of air is 1.29 kg. That is, if you mentally allocate a space in a room with a height, width and length equal to 1 m, then this air cube will contain exactly this amount of air.

If air has weight and weight that is quite noticeable, why does a person not feel heaviness? This physical phenomenon, like atmospheric pressure, implies that every inhabitant of the planet is pressed by an air column weighing 250 kg. The average palm area of ​​an adult is 77 cm2. That is, in accordance with physical laws, each of us holds 77 kg of air in the palm of our hand! This is equivalent to the fact that we constantly carry 5 pound weights in each hand. IN real life Even a weightlifter cannot do this, however, each of us copes with such a load easily, because atmospheric pressure presses from both sides, both outside the human body and from the inside, that is, the difference is ultimately zero.

The properties of air are such that it affects the human body differently. High in the mountains, due to lack of oxygen, people experience visual hallucinations, and great depth, the combination of oxygen and nitrogen in a special mixture - “laughing gas” can create a feeling of euphoria and a feeling of weightlessness.

Knowing these physical quantities, you can calculate the mass of the Earth’s atmosphere - the amount of air that is held in near-Earth space forces of gravity. The upper boundary of the atmosphere ends at an altitude of 118 km, that is, knowing the weight of m 3 of air, you can divide the entire surface area into air columns, with a base of 1x1 m, and add up the resulting mass of such columns. Ultimately, it will be equal to 5.3 * 10 to the fifteenth power of tons. The weight of the planet's air armor is quite large, but it is only one millionth of total mass globe. The Earth's atmosphere serves as a kind of buffer that protects the Earth from unpleasant cosmic surprises. From solar storms alone that reach the surface of the planet, the atmosphere loses up to 100 thousand tons of its mass per year! So invisible and reliable shield- air.

How much does a liter of air weigh?

A person does not notice that he is constantly surrounded by transparent and almost invisible air. Is it possible to see this intangible element of the atmosphere? Visually, the movement of air masses is broadcast daily on the television screen - warm or cold front brings long-awaited warming or heavy snowfall.

What else do we know about air? Probably, the fact that it is vitally necessary for all living beings living on the planet. Every day a person inhales and exhales about 20 kg of air, a quarter of which is consumed by the brain.

The weight of air can be measured in different physical units, including liters. The weight of one liter of air will be equal to 1.2930 grams, at a pressure of 760 mm Hg. column and a temperature of 0°C. In addition to the usual gaseous state, air can also be found in liquid form. For the transition of a substance into a given physical state it will require exposure to enormous pressure and very low temperatures. Astronomers suggest that there are planets whose surfaces are completely covered with liquid air.

The sources of oxygen necessary for human existence are the Amazon forests, which produce up to 20% of this important element on the entire planet.

Forests are truly the “green” lungs of the planet, without which human existence is simply impossible. Therefore the living indoor plants in an apartment are not just a piece of furniture, they purify the indoor air, the pollution of which is tens of times higher than outside.

Clean air has long become a shortage in megacities; air pollution is so great that people are ready to buy clean air. “Air sellers” first appeared in Japan. They produced and sold clean air in cans, and any resident of Tokyo could open a can of clean air for dinner and enjoy its freshest aroma.

Air purity has a significant impact not only on human health, but also on animal health. In polluted areas of equatorial waters, near human-populated areas, dozens of dolphins are dying. The cause of death for mammals is a polluted atmosphere; in autopsies of animals, the lungs of dolphins resemble the lungs of miners, clogged with coal dust. The inhabitants of Antarctica, penguins, are also very sensitive to air pollution if the air contains large number harmful impurities, they begin to breathe heavily and intermittently.

For a person, clean air is also very important, so after working in the office, doctors recommend taking daily hour-long walks in the park, forest, or outside the city. After such “air” therapy, the body’s vitality is restored and well-being significantly improves. The recipe for this free and effective medicine has been known since ancient times; many scientists and rulers considered daily walks in the fresh air a mandatory ritual.

For a modern city dweller, air treatment is very relevant: a small portion of life-giving air, weighing 1-2 kg, is a panacea for many modern ailments!

Many may be surprised by the fact that air has a certain non-zero weight. Exact value This weight is not so easy to determine, since it is greatly influenced by factors such as chemical composition, humidity, temperature and pressure. Let's take a closer look at the question of how much air weighs.

What is air

Before answering the question of how much air weighs, it is necessary to understand what this substance is. Air is a gaseous shell that exists around our planet, and which is a homogeneous mixture of various gases. Air contains the following gases:

• nitrogen (78.08%);
• oxygen (20.94%);
• argon (0.93%);
• water vapor (0.40%);
• carbon dioxide (0.035%).

In addition to the gases listed above, the air also contains minimum quantities neon (0.0018%), helium (0.0005%), methane (0.00017%), krypton (0.00014%), hydrogen (0.00005%), ammonia (0.0003%).

It is interesting to note that these components can be separated by condensing air, that is, turning it into a liquid state by increasing pressure and decreasing temperature. Since each component of air has its own condensation temperature, in this way it is possible to isolate all components from the air, which is used in practice.

Air weight and factors that affect it

What prevents you from answering exactly the question of how much a cubic meter of air weighs? Of course, there are a number of factors that can greatly influence this weight.

Firstly, this is the chemical composition. Above are the data for the composition clean air However, at present this air in many places on the planet is highly polluted, and accordingly its composition will be different. Thus, near large cities the air contains more carbon dioxide, ammonia, methane than in rural air.

Secondly, humidity, that is, the amount of water vapor contained in the atmosphere. The more humid the air, the less it weighs, other things being equal.

Thirdly, temperature. This is one of important factors, the lower its value, the higher the air density, and, accordingly, the greater its weight.

Fourthly, atmospheric pressure, which directly reflects the number of air molecules in a certain volume, that is, its weight.

To understand how the combination of these factors affects the weight of air, let's give a simple example: the mass of one meter of cubic dry air at a temperature of 25 ° C, located near the surface of the earth, is 1.205 kg, if we consider a similar volume of air near the surface of the sea at a temperature of 0 ° C, then its mass will already be equal to 1.293 kg, that is, it will increase by 7.3%.

Change in air density with altitude

As altitude increases, air pressure drops, and its density and weight decrease accordingly. Atmospheric air at pressures observed on Earth can, to a first approximation, be considered an ideal gas. This means that air pressure and density are mathematically related to each other through the equation of state ideal gas: P = ρ*R*T/M, where P is pressure, ρ is density, T is temperature in Kelvin, M is the molar mass of air, R is the universal gas constant.

From the above formula, you can obtain a formula for the dependence of air density on height, taking into account that the pressure varies according to the law P = P 0 +ρ*g*h, where P 0 is the pressure at the surface of the earth, g is the acceleration of gravity, h is the height . Substituting this formula for pressure into the previous expression and expressing the density, we obtain: ρ(h) = P 0 *M/(R*T(h)+g(h)*M*h). Using this expression, you can determine the density of air at any altitude. Accordingly, the weight of air (it would be more correct to say mass) is determined by the formula m(h) = ρ(h)*V, where V is the given volume.

In the expression for the dependence of density on height, it can be noted that temperature and gravitational acceleration also depend on height. The last dependence can be neglected if we're talking about about heights of no more than 1-2 km. As for temperature, its dependence on height is well described by the following empirical expression: T(h) = T 0 -0.65*h, where T 0 is the air temperature near the earth's surface.

In order not to constantly calculate the density for each altitude, below we provide a table of the dependence of the main characteristics of air on altitude (up to 10 km).

Which air is the heaviest

By considering the main factors that determine the answer to the question of how much air weighs, you can understand which air will be the heaviest. In short, cold air always weighs more than warm air, since the density of the latter is lower, and dry air weighs more than humid air. The last statement is easy to understand, since it is 29 g/mol, and the molar mass of a water molecule is 18 g/mol, that is, 1.6 times less.

Determination of air weight under given conditions

Now let's solve a specific problem. Let's answer the question of how much air weighs, occupying a volume of 150 liters, at a temperature of 288 K. Let's take into account that 1 liter is a thousandth of a cubic meter, that is, 1 liter = 0.001 m 3. As for the temperature of 288 K, it corresponds to 15 ° C, that is, it is typical for many areas of our planet. Next you need to determine the air density. You can do this in two ways:

1. Calculate using the above formula for an altitude of 0 meters above sea level. In this case, the value obtained is ρ = 1.227 kg/m 3
2. Look at the table above, which was built based on T 0 = 288.15 K. The table contains the value ρ = 1.225 kg/m 3.

Thus, we have two numbers that agree well with each other. The slight difference is due to an error of 0.15 K in determining the temperature, and also to the fact that air is still not an ideal gas, but a real gas. Therefore, for further calculations, we will take the average of the two obtained values, that is, ρ = 1.226 kg/m 3.

Now, using the formula for the relationship between mass, density and volume, we get: m = ρ*V = 1.226 kg/m 3 * 0.150 m 3 = 0.1839 kg or 183.9 grams.

You can also answer how much a liter of air weighs under given conditions: m = 1.226 kg/m3 * 0.001 m3 = 0.001226 kg or approximately 1.2 grams.

Why don't we feel the air pressing on us?

How much does 1 m3 of air weigh? A little more than 1 kilogram. The entire atmospheric table of our planet puts pressure on a person with its weight of 200 kg! This is a fairly large mass of air that could cause a lot of trouble to a person. Why don't we feel it? This is explained by two reasons: firstly, inside the person himself there is also internal pressure, which counteracts external atmospheric pressure, and secondly, air, being a gas, exerts pressure in all directions equally, that is, pressures in all directions balance each other.

The basic physical properties of air are considered: air density, its dynamic and kinematic viscosity, specific heat, thermal conductivity, thermal diffusivity, Prandtl number and entropy. The properties of air are given in tables depending on the temperature at normal atmospheric pressure.

Air density depending on temperature

A detailed table of dry air density values ​​is presented at different temperatures and normal atmospheric pressure. What is the density of air? The density of air can be determined analytically by dividing its mass by the volume it occupies. under given conditions (pressure, temperature and humidity). You can also calculate its density using the formula of the ideal gas equation of state. To do this you need to know absolute pressure and air temperature, as well as its gas constant and molar volume. This equation allows you to calculate the dry density of air.

In practice, to find out what the density of air is at different temperatures, it is convenient to use ready-made tables. For example, the given table of density values atmospheric air depending on its temperature. Air density in the table is expressed in kilograms per cubic meter and is given in the temperature range from minus 50 to 1200 degrees Celsius at normal atmospheric pressure (101325 Pa).

Air density depending on temperature - table

t, °С	ρ, kg/m 3	t, °С	ρ, kg/m 3	t, °С	ρ, kg/m 3	t, °С	ρ, kg/m 3
-50	1,584	20	1,205	150	0,835	600	0,404
-45	1,549	30	1,165	160	0,815	650	0,383
-40	1,515	40	1,128	170	0,797	700	0,362
-35	1,484	50	1,093	180	0,779	750	0,346
-30	1,453	60	1,06	190	0,763	800	0,329
-25	1,424	70	1,029	200	0,746	850	0,315
-20	1,395	80	1	250	0,674	900	0,301
-15	1,369	90	0,972	300	0,615	950	0,289
-10	1,342	100	0,946	350	0,566	1000	0,277
-5	1,318	110	0,922	400	0,524	1050	0,267
0	1,293	120	0,898	450	0,49	1100	0,257
10	1,247	130	0,876	500	0,456	1150	0,248
15	1,226	140	0,854	550	0,43	1200	0,239

At 25°C, air has a density of 1.185 kg/m3. When heated, the air density decreases - the air expands (its specific volume increases). With increasing temperature, for example, to 1200°C, a very low air density is achieved, equal to 0.239 kg/m 3, which is 5 times less than its value at room temperature. In general, reduction during heating allows a process such as natural convection to take place and is used, for example, in aeronautics.

If we compare the density of air relative to , then air is three orders of magnitude lighter - at a temperature of 4°C, the density of water is 1000 kg/m3, and the density of air is 1.27 kg/m3. It is also necessary to note the value of air density under normal conditions. Normal conditions for gases are those at which their temperature is 0°C and the pressure is equal to normal atmospheric pressure. Thus, according to the table, air density under normal conditions (at NL) is 1.293 kg/m 3.

Dynamic and kinematic viscosity of air at different temperatures

When performing thermal calculations, it is necessary to know the value of air viscosity (viscosity coefficient) at different temperatures. This value is required to calculate the Reynolds, Grashof, and Rayleigh numbers, the values ​​of which determine the flow regime of this gas. The table shows the values ​​of the dynamic coefficients μ and kinematic ν air viscosity in the temperature range from -50 to 1200°C at atmospheric pressure.

The viscosity coefficient of air increases significantly with increasing temperature. For example, the kinematic viscosity of air is equal to 15.06 10 -6 m 2 /s at a temperature of 20°C, and with an increase in temperature to 1200°C, the viscosity of air becomes equal to 233.7 10 -6 m 2 /s, that is, it increases 15.5 times! The dynamic viscosity of air at a temperature of 20°C is 18.1·10 -6 Pa·s.

When air is heated, the values ​​of both kinematic and dynamic viscosity increase. These two quantities are related to each other through the air density, the value of which decreases when this gas is heated. An increase in the kinematic and dynamic viscosity of air (as well as other gases) when heated is associated with a more intense vibration of air molecules around their equilibrium state (according to MKT).

Dynamic and kinematic viscosity of air at different temperatures - table

t, °С	μ·10 6 , Pa·s	ν·10 6, m 2 /s	t, °С	μ·10 6 , Pa·s	ν·10 6, m 2 /s	t, °С	μ·10 6 , Pa·s	ν·10 6, m 2 /s
-50	14,6	9,23	70	20,6	20,02	350	31,4	55,46
-45	14,9	9,64	80	21,1	21,09	400	33	63,09
-40	15,2	10,04	90	21,5	22,1	450	34,6	69,28
-35	15,5	10,42	100	21,9	23,13	500	36,2	79,38
-30	15,7	10,8	110	22,4	24,3	550	37,7	88,14
-25	16	11,21	120	22,8	25,45	600	39,1	96,89
-20	16,2	11,61	130	23,3	26,63	650	40,5	106,15
-15	16,5	12,02	140	23,7	27,8	700	41,8	115,4
-10	16,7	12,43	150	24,1	28,95	750	43,1	125,1
-5	17	12,86	160	24,5	30,09	800	44,3	134,8
0	17,2	13,28	170	24,9	31,29	850	45,5	145
10	17,6	14,16	180	25,3	32,49	900	46,7	155,1
15	17,9	14,61	190	25,7	33,67	950	47,9	166,1
20	18,1	15,06	200	26	34,85	1000	49	177,1
30	18,6	16	225	26,7	37,73	1050	50,1	188,2
40	19,1	16,96	250	27,4	40,61	1100	51,2	199,3
50	19,6	17,95	300	29,7	48,33	1150	52,4	216,5
60	20,1	18,97	325	30,6	51,9	1200	53,5	233,7

Note: Be careful! Air viscosity is given to the power of 10 6 .

Specific heat capacity of air at temperatures from -50 to 1200°C

A table of the specific heat capacity of air at various temperatures is presented. The heat capacity in the table is given at constant pressure (isobaric heat capacity of air) in the temperature range from minus 50 to 1200°C for dry air. What is the specific heat capacity of air? The specific heat capacity determines the amount of heat that must be supplied to one kilogram of air at constant pressure to increase its temperature by 1 degree. For example, at 20°C, to heat 1 kg of this gas by 1°C in an isobaric process, 1005 J of heat is required.

The specific heat capacity of air increases with increasing temperature. However, the dependence of the mass heat capacity of air on temperature is not linear. In the range from -50 to 120°C, its value practically does not change - under these conditions, the average heat capacity of air is 1010 J/(kg deg). According to the table, it can be seen that temperature begins to have a significant effect from a value of 130°C. However, air temperature affects its specific heat capacity much less than its viscosity. Thus, when heated from 0 to 1200°C, the heat capacity of air increases only 1.2 times - from 1005 to 1210 J/(kg deg).

It should be noted that the heat capacity of humid air is higher than that of dry air. If we compare air, it is obvious that water has a higher value and the water content in air leads to an increase in specific heat capacity.

Specific heat capacity of air at different temperatures - table

t, °С	C p , J/(kg deg)	t, °С	C p , J/(kg deg)	t, °С	C p , J/(kg deg)	t, °С	C p , J/(kg deg)
-50	1013	20	1005	150	1015	600	1114
-45	1013	30	1005	160	1017	650	1125
-40	1013	40	1005	170	1020	700	1135
-35	1013	50	1005	180	1022	750	1146
-30	1013	60	1005	190	1024	800	1156
-25	1011	70	1009	200	1026	850	1164
-20	1009	80	1009	250	1037	900	1172
-15	1009	90	1009	300	1047	950	1179
-10	1009	100	1009	350	1058	1000	1185
-5	1007	110	1009	400	1068	1050	1191
0	1005	120	1009	450	1081	1100	1197
10	1005	130	1011	500	1093	1150	1204
15	1005	140	1013	550	1104	1200	1210

Thermal conductivity, thermal diffusivity, Prandtl number of air

The table presents such physical properties of atmospheric air as thermal conductivity, thermal diffusivity and its Prandtl number depending on temperature. Thermophysical properties of air are given in the range from -50 to 1200°C for dry air. According to the table, it can be seen that the indicated properties of air depend significantly on temperature and the temperature dependence of the considered properties of this gas is different.

Air density is physical quantity, characterizing the specific gravity of air under natural conditions or the mass of gas in the Earth’s atmosphere per unit volume. The value of air density is a function of the height of the measurements taken, its humidity and temperature.

The air density standard is taken to be 1.29 kg/m3, which is calculated as the ratio of its molar mass(29 g/mol) to the molar volume, the same for all gases (22.413996 dm3), corresponding to the density of dry air at 0°C (273.15°K) and a pressure of 760 mm mercury(101325 Pa) at sea level (that is, under normal conditions).

Not long ago, information about air density was obtained indirectly through observations of auroras, the propagation of radio waves, and meteors. Since the advent of artificial Earth satellites, air density began to be calculated using data obtained from their braking.

Another method is to observe the spreading of artificial sodium vapor clouds created by weather rockets. In Europe, the air density at the Earth's surface is 1.258 kg/m3, at an altitude of five km - 0.735, at an altitude of twenty km - 0.087, at an altitude of forty km - 0.004 kg/m3.

There are two types of air density: mass and weight ( specific gravity).

Weight density determines the weight of 1 m3 of air and is calculated by the formula γ = G/V, where γ is weight density, kgf/m3; G is the weight of air, measured in kgf; V is the volume of air, measured in m3. It has been established that 1 m3 of air under standard conditions(barometric pressure 760 mmHg, t=15°С) weighs 1.225 kgf, based on this, the weight density (specific gravity) of 1 m3 of air is equal to γ ​​= 1.225 kgf/m3.

It should be taken into account that air weight is a variable quantity and changes depending on various conditions, such as geographic latitude and the force of inertia that occurs when the Earth rotates around its axis. At the poles the weight of air is 5% greater than at the equator.

Mass density of air is the mass of 1 m3 of air, denoted Greek letterρ. As you know, body weight is a constant quantity. The unit of mass is considered to be the mass of a platinum iridide weight, which is located in the International Chamber of Weights and Measures in Paris.

Air mass density ρ is calculated using the following formula: ρ = m / v. Here m is the mass of air, measured in kg×s2/m; ρ is its mass density, measured in kgf×s2/m4.

The mass and weight densities of air depend on: ρ = γ / g, where g is the gravitational acceleration coefficient equal to 9.8 m/s². It follows that the mass density of air under standard conditions is 0.1250 kg × s2/m4.

As barometric pressure and temperature change, the density of the air changes. Based on the Boyle-Marriott law, the greater the pressure, the greater the air density. However, as pressure decreases with altitude, air density also decreases, which introduces its own adjustments, as a result of which the law of vertical pressure changes becomes more complex.

The equation that expresses this law of pressure change with height in an atmosphere at rest is called basic equation of statics.

It states that with increasing altitude the pressure changes downward and when rising to the same height, the greater the decrease in pressure. more power gravity and air density.

Changes in air density play an important role in this equation. As a result, we can say that the higher you rise, the less pressure will drop when rising to the same height. Air density depends on temperature as follows: in warm air the pressure decreases less intensely than in cold air, therefore, at the same height in warm air mass pressure is higher than in cold.

With changing values ​​of temperature and pressure, the mass density of air is calculated by the formula: ρ = 0.0473xB / T. Here B is the barometric pressure, measured in mm of mercury, T is the air temperature, measured in Kelvin.

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Density is also determined by air humidity. The presence of water pores leads to a decrease in air density, which is explained by the low molar mass of water (18 g/mol) against the background of the molar mass of dry air (29 g/mol). Humid air can be considered as a mixture ideal gases, in each of which the combination of densities allows one to obtain the required density value for their mixture.

This kind of interpretation makes it possible to determine density values ​​with an error level of less than 0.2% in the temperature range from −10 °C to 50 °C. Air density allows you to obtain the value of its moisture content, which is calculated by dividing the density of water vapor (in grams) contained in the air by the density of dry air in kilograms.

The basic equation of statics does not allow us to solve constantly arising practical problems in the real conditions of a changing atmosphere. Therefore, it is solved under various simplified assumptions that correspond to the actual real conditions, by putting forward a number of particular assumptions.

The basic equation of statics makes it possible to obtain the value of the vertical pressure gradient, which expresses the change in pressure during ascent or descent per unit height, i.e., the change in pressure per unit vertical distance.

Instead of a vertical gradient, they often use its inverse value - the pressure level in meters per millibar (sometimes an outdated version of the term “pressure gradient” is also used - barometric gradient).

Low air density determines low resistance to movement. Many land animals have evolved to take advantage of the environmental benefits of this property. air environment, due to which they acquired the ability to fly. 75% of all species of land animals are capable of active flight. They are mostly insects and birds, but there are also mammals and reptiles.

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