Air density is physical quantity, which characterizes the specific mass 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, its humidity and temperature.

A value equal to 1.29 kg/m3 is taken as the air density standard, which is calculated as the ratio of its molar mass (29 g/mol) to the molar volume, which is 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 mmHg (101325 Pa) at sea level (that is, under normal conditions).

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

Another method is to observe the spreading of artificial clouds of sodium vapor created by meteorological 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).

The weight density determines the weight of 1 m3 of air and is calculated by the formula γ = G/V, where γ is the weight density, kgf/m3; G is the weight of air, measured in kgf; V is the volume of air, measured in m3. Determined that 1 m3 of air under standard conditions(barometric pressure 760 mmHg, t=15°C) 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 the weight of air is a variable and changes depending on various conditions, such as geographical latitude and the force of inertia that occurs when the Earth rotates around its axis. At the poles, the weight of air is 5% more than at the equator.

The mass density of air is the mass of 1 m3 of air, denoted by the Greek letter ρ. As you know, body weight is a constant value. A unit of mass is considered to be the mass of a weight made of platinum iridide, which is located in the International Chamber of Weights and Measures in Paris.

Mass air density ρ is calculated from 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 density of air are dependent: ρ = γ / g, where g is the free fall acceleration coefficient equal to 9.8 m/s². Whence it follows that the mass density of air under standard conditions is 0.1250 kg×s2/m4.

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

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

It says that with increasing altitude, the pressure changes to a smaller side and when ascending to the same height, the decrease in pressure is the greater, the more more strength gravity and air density.

An important role in this equation belongs to changes in air density. As a result, we can say that the higher you climb, the less pressure will drop when you rise to the same height. The density of air depends on temperature as follows: in warm air, the pressure decreases less intensively than in cold air, therefore, at the same height in warm air mass the pressure is higher than in the cold.

With changing values ​​of temperature and pressure, the mass density of air is calculated by the formula: ρ = 0.0473xV / 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 of ideal gases, in each of which the combination of densities allows one to obtain the required density value for their mixture.

Such a kind of interpretation allows density values ​​to be determined with an error level of less than 0.2% in the temperature range from −10 °C to 50 °C. The density of air allows you to get 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 solving constantly arising practical tasks in 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 the vertical gradient, the reciprocal of it is often used - the baric step in meters per millibar (sometimes there is still an outdated version of the term "pressure gradient" - the barometric gradient).

The low air density determines a slight resistance to movement. Many terrestrial animals, in the course of evolution, used the ecological benefits of this property of the air environment, due to which they acquired the ability to fly. 75% of all land animal species are capable of active flight. For the most part, these are insects and birds, but there are mammals and reptiles.

Video on the topic "Determination of air density"

The main 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 versus temperature

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

On practice, to find out what is the density of air at different temperatures, it is convenient to use ready-made tables. For example, the given table of atmospheric air density values ​​depending on its temperature. The 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/m 3 . When heated, the density of air decreases - the air expands (its specific volume increases). With an increase in temperature, for example, up 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, the decrease in 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 with respect to, then air is lighter by three orders of magnitude - at a temperature of 4 ° C, the density of water is 1000 kg / m 3, and the density of air is 1.27 kg / m 3. It is also necessary to note the value of air density under normal conditions. Normal conditions for gases are those under 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 NU) 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, Rayleigh numbers, the values ​​of which determine the flow regime of this gas. The table shows the values ​​of the coefficients of dynamic μ and kinematic ν air viscosity in the temperature range from -50 to 1200°C at atmospheric pressure.

The viscosity of air increases significantly with increasing temperature. For example, the kinematic viscosity of air is 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 the 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 interconnected through the value of 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) during heating is associated with a more intense vibration of air molecules around their equilibrium state (according to the MKT).

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

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

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 value of 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 as its temperature rises. 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 the temperature begins to have a significant effect from a value of 130°C. However, air temperature affects its specific heat capacity much weaker than its viscosity. So, 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 moist 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 the air leads to an increase in specific heat.

Thermal conductivity, thermal diffusivity, Prandtl number of air

The table shows such physical properties of atmospheric air as thermal conductivity, thermal diffusivity and its Prandtl number depending on temperature. The 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. The exact value of this weight is not so easy to determine, since it is strongly influenced by factors such as chemical composition, humidity, temperature and pressure. Let us consider in more detail 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 gas envelope 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, 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 if air is condensed, that is, it is turned into a liquid state by increasing pressure and decreasing temperature. Since each component of the 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, a number of factors that can greatly influence this weight.

First, it is the chemical composition. Above are the data for the composition of clean air, however, at present this air is heavily polluted in many places on the planet, respectively, its composition will be different. Thus, near large cities, the air contains more carbon dioxide, ammonia, methane than the air in rural areas.

Secondly, humidity, that is, the amount of water vapor that is contained in the atmosphere. The more wet air, the less it weighs ceteris paribus.

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

Fourth, Atmosphere 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 take a simple example: the mass of one meter of dry cubic air at a temperature of 25 ° C, located near the surface of the earth, is 1.205 kg, if we consider the same volume of air near the sea surface 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 height

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

From the above formula, you can get the formula for the dependence of air density on height, given that the pressure changes according to the law P \u003d P 0 + ρ * g * h, where P 0 is the pressure at the earth's surface, g is the acceleration of gravity, h is the height . Substituting this formula for pressure into the previous expression, and expressing the density, we get: ρ(h) = P 0 *M/(R*T(h)+g(h)*M*h). Using this expression, you can determine the density of air at any height. Accordingly, the weight of air (more correctly, mass) is determined by the formula m(h) = ρ(h)*V, where V is a given volume.

In the expression for the dependence of density on height, one can notice that the temperature and acceleration of free fall also depend on height. The last dependence can be neglected if we are talking about heights of no more than 1–2 km. As for temperature, its dependence on altitude 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 present a table of the dependence of the main air characteristics 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 moist 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.

Determining the weight of air 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 regions of our planet. The next step is to determine the density of the air. 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 ρ \u003d 1.227 kg / m 3 is obtained
2. Look at the table above, which is built on the basis of T 0 \u003d 288.15 K. The table contains the value ρ \u003d 1.225 kg / m 3.

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

Now, using the formula for the relationship of mass, density and volume, we get: m \u003d ρ * V \u003d 1.226 kg / m 3 * 0.150 m 3 \u003d 0.1839 kg or 183.9 grams.

You can also answer how much a liter of air weighs under given conditions: m \u003d 1.226 kg / m 3 * 0.001 m 3 \u003d 0.001226 kg or approximately 1.2 grams.

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

How much does 1 m3 of air weigh? A little over 1 kilogram. The entire atmospheric table of our planet puts pressure on a person with its weight of 200 kg! This is enough big mass air, which 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 inside the person himself, 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.

What is the density of air at 150 degrees Celsius in kg/m3, g/cm3, g/ml, lb/m3 . Do not forget that such a physical quantity, a characteristic of air, as its density in kg / m3 (the mass of a unit volume of atmospheric gas, where 1 m3, 1 cubic meter, 1 cubic meter, 1 cubic centimeter, 1 cm3, 1 milliliter, 1 ml or 1 lb) depends on several parameters. Among the parameters describing the conditions for determining the air density (specific gravity of air gas), I consider the following to be the most important and must be taken into account:

1. Temperature air gas.
2. Pressure at which the density of the air gas was measured.
3. Humidity air gas or the percentage of water in it.

If you are interested in the second case air density at T = 150 degrees C, then excuse me, but I have no desire to copy tabular data, a huge special reference book for air density at various pressures. I cannot yet decide on such a colossal amount of work, and I do not see the need for it. See reference book. Narrow profile information or rare special data, density values, should be sought in primary sources. So smarter.

It is more realistic, and probably more practical from our point of view, to indicate what is the density of air at 150 degrees Celsius, for a situation where the pressure is given by a constant and is atmospheric pressure(under normal conditions - the most popular question). By the way, do you remember what normal atmospheric pressure is? What does it equal? Let me remind you that normal atmospheric pressure is considered to be 760 mm Hg, or 101325 Pa (101 kPa), in principle, this is normal conditions corrected for temperature. Meaning, what is the density of air in kg/m3 at a given temperature air gas you will see, find, learn in table 1.

However, it must be said that the values ​​indicated in the table air density values ​​at 150 degrees in kg/m3, g/cm3, g/ml, will not be true for any atmospheric, but only for dry gas. As soon as we change the initial conditions and change the humidity of the air gas, it will immediately have different physical properties. And its density (weight of 1 cubic meter of air in kilograms) at given temperature in degrees C (Celsius) (kg/m3) will also differ from the dry gas density.

Physics at every step Perelman Yakov Isidorovich

How much does the air in the room weigh?

Can you say at least approximately what kind of load is the air that your room contains? A few grams or a few kilograms? Are you able to lift such a load with one finger, or would you barely keep it on your shoulders?

Now, perhaps, there are no longer people who think, as the ancients believed, that air weighs nothing at all. But even now many cannot say how much a certain volume of air weighs.

Remember that a liter mug of air of the density that it has near earth's surface at normal room temperature, it weighs about 1.2 g. Since a cubic meter contains 1 thousand liters, a cubic meter of air weighs a thousand times more than 1.2 g, namely 1.2 kg. It is now easy to answer the question posed earlier. To do this, you just need to find out how many cubic meters are in your room, and then the weight of the air contained in it will be determined.

Let the room have an area of ​​10 m 2 and a height of 4 m. In such a room there are 40 cubic meters of air, which weighs, therefore, forty times 1.2 kg. This will be 48 kg.

So, even in such a small room, the air weighs a little less than yourself. It would not be easy for you to carry such a load on your shoulders. And the air of a room twice as large, loaded onto your back, could crush you.