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This is the mass (weight) of 1 cubic meter of a mixture of air and water vapor at a certain temperature and relative humidity. Specific volume is the volume of air and water vapor per 1 kg of dry air.

Moisture and heat content

To determine the heat consumption for moisture, you need to know the value heat content of moist air. This value is understood as contained in a mixture of air and water vapor. It is numerically equal to the sum:

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 temperature at normal atmospheric pressure.

Air density depending on temperature

A detailed table of dry air density values ​​at various temperatures and normal atmospheric pressure is presented. What is the density of air? The density of air can be determined analytically by dividing its mass by the volume it occupies. at 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 table below shows the density of 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).

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). As the temperature increases, 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 air density at 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).

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.

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.

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 the pressures observed on Earth, it 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 - pressure, ρ - density, T - temperature in kelvins, M - molar mass 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 altitude, it can be noted that temperature and gravitational acceleration also depend on altitude. 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 due to two reasons: firstly, there is also internal pressure within the person himself, which counteracts the external atmospheric pressure, secondly, air, being a gas, exerts pressure in all directions equally, that is, pressures in all directions balance each other.