Dependence of air humidity on volume. Air humidity is an important indicator

DEFINITION

Absolute air humidity is the amount of water vapor per unit volume of air:

The SI unit of measurement for absolute humidity is

Air humidity is a very important parameter environment. It is known that most of the Earth’s surface is occupied by water (the World Ocean), from the surface of which evaporation continuously occurs. In different climatic zones the intensity of this process varies. It depends on the average daily temperature, the presence of winds and other factors. Thus, in certain places the process of water vaporization is more intense than its condensation, and in some cases it is the other way around.

The human body actively reacts to changes in air humidity. For example, the process of sweating is closely related to the temperature and humidity of the environment. At high humidity, the processes of moisture evaporation from the surface of the skin are practically compensated by the processes of its condensation, and the removal of heat from the body is disrupted, which leads to disturbances in thermoregulation; At low humidity, moisture evaporation processes prevail over condensation processes and the body loses too much fluid, which can lead to dehydration.

In addition, the concept of humidity is the most important evaluation criterion weather conditions, which everyone knows from weather forecasts.

Absolute air humidity gives an idea of ​​the specific water content in the air by mass, but this value is inconvenient from the point of view of the susceptibility of humidity by living organisms. A person does not feel the mass amount of water in the air, but its content relative to the maximum possible value. To describe the reaction of living organisms to changes in the content of water vapor in the air, the concept of relative humidity is introduced.

Relative humidity

DEFINITION

Relative humidity- This physical quantity, showing how far water vapor in the air is from saturation:

where is the density of water vapor in the air (absolute humidity); density of saturated water vapor at a given temperature.

Dew point

DEFINITION

Dew point is the temperature at which water vapor becomes saturated.

Knowing the dew point temperature can give you an idea of ​​the relative humidity. If the dew point temperature is close to the ambient temperature, then the humidity is high ( When the temperatures coincide, fog is formed). On the contrary, if the values ​​of the dew point and air temperature at the time of measurement differ greatly, then we can talk about a low content of water vapor in the atmosphere.

When something is brought into a warm room from the cold, the air above it cools, becomes saturated with water vapor, and water droplets condense on the item. Subsequently, the item warms up to the room temperature, and all condensation evaporates.

Another, no less familiar example is the fogging of glass in a house. Many people experience condensation on their windows in winter. This phenomenon is influenced by two factors - humidity and temperature. If a normal double-glazed window is installed and the insulation is carried out correctly, and there is condensation, it means there is high humidity in the room; Possibly poor ventilation or exhaust.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

The saturated vapor pressure of water increases greatly with increasing temperature. Therefore, with isobaric (that is, at constant pressure) cooling of air with a constant vapor concentration, a moment (dew point) comes when the vapor is saturated. In this case, the “extra” steam condenses in the form of fog, dew or ice crystals. The processes of saturation and condensation of water vapor play a huge role in atmospheric physics: processes of cloud formation and formation atmospheric fronts are largely determined by the processes of saturation and condensation; the heat released during the condensation of atmospheric water vapor provides the energy mechanism for the emergence and development of tropical cyclones (hurricanes).

Relative humidity is the only hygrometric indicator of air that allows direct instrumental measurement.

Relative Humidity Estimation

The relative humidity of a water-air mixture can be estimated if its temperature is known ( T) and dew point temperature ( Td), By the following formula:

Where P s- pressure saturated steam for the corresponding temperature, which can be calculated using the Arden Buck formula:

Approximate calculation

Relative humidity can be approximately calculated using the following formula:

That is, with every degree Celsius difference between the air temperature and the dew point temperature, the relative humidity decreases by 5%.

Additionally, relative humidity can be estimated using a psychrometric chart.

Supersaturated water vapor

In the absence of condensation centers, when the temperature decreases, a supersaturated state may form, that is, the relative humidity becomes more than 100%. Ions or aerosol particles can act as condensation centers; it is on the condensation of supersaturated vapor on ions formed during the passage of a charged particle in such a vapor that the operating principle of the Wilson chamber and diffusion chambers is based: water droplets condensing on the formed ions form a visible trace (track ) charged particle.

Another example of the condensation of supersaturated water vapor is the contrails of aircraft, which occur when supersaturated water vapor condenses on soot particles from engine exhaust.

Means and methods of control

To determine air humidity, instruments called psychrometers and hygrometers are used. August's psychrometer consists of two thermometers - dry and wet. A wet thermometer shows a lower temperature than a dry thermometer because its reservoir is wrapped in a cloth soaked in water, which cools it as it evaporates. The intensity of evaporation depends on the relative humidity of the air. Based on the readings of dry and wet thermometers, the relative humidity of the air is found using psychrometric tables. IN Lately Integrated humidity sensors (usually with voltage output) have become widely used, based on the property of some polymers to change their electrical characteristics (such as the dielectric constant of the medium) under the influence of water vapor contained in the air.

Comfortable air humidity for humans is determined by documents such as GOST and SNIP. They regulate that in winter the optimal humidity for a person in a room is 30-45%, in summer – 30-60%. The data on SNIP are slightly different: 40-60% for any time of year, the maximum level is 65%, but for very humid regions - 75%.

To determine and confirm the metrological characteristics of instruments for measuring humidity, special reference (model) installations are used - climatic chambers (hygrostats) or dynamic generators of gas humidity.

Meaning

Relative air humidity is an important environmental indicator of the environment. If the humidity is too low or too high, a person becomes fatigued quickly, and perception and memory deteriorate. Human mucous membranes dry out, moving surfaces crack, forming microcracks into which viruses, bacteria, and microbes directly penetrate. Low relative humidity (up to 5-7%) in apartments and offices has been observed in regions with prolonged low negative outdoor temperatures. Typically, a duration of up to 1-2 weeks at temperatures below −20 ° C leads to drying out of the premises. A significant deteriorating factor in maintaining relative humidity is air exchange at low negative temperatures. The greater the air exchange in the rooms, the faster low (5-7%) relative humidity is created in these rooms.

Ventilating rooms in cold weather to increase humidity is blunder- this is the most effective method achieve the opposite. The reason for the widespread misconception is the perception of relative humidity figures, known to everyone from weather forecasts. These are percentages of a certain number, but this number is different for a room and a street! You can find out this number from the table linking temperature and absolute humidity. For example, 100% humidity of street air at −15 °C means 1.6 g of water per cubic meter, but the same air (and the same grams) at +20 °C means only 8% humidity.

Food products, Construction Materials and even many electronic components can be stored in a strictly defined range of relative humidity. Many technological processes occur only with strict control of the water vapor content in the air of the production premises.

The humidity in the room can be changed.

Humidifiers are used to increase humidity.

The functions of dehumidifying (reducing humidity) of air are implemented in most air conditioners and in the form of separate devices - air dehumidifiers.

In floriculture

The relative air humidity in greenhouses and residential premises used for cultivating plants is subject to fluctuations, which is determined by the time of year, air temperature, the degree and frequency of watering and spraying of plants, the presence of humidifiers, aquariums or other containers with an open water surface, ventilation and heating systems. Cacti and many succulent plants tolerate dry air more easily than many tropical and subtropical plants.
As a rule, for plants whose homeland is wet rainforests, the optimal relative air humidity is 80-95% (in winter it can be reduced to 65-75%). For plants of warm subtropics - 75-80%, cold subtropics - 50-75% (Levy, cyclamen, cineraria, etc.)
When keeping plants in residential areas, many species suffer from dry air. First of all, this affects

Let us now consider the device and principle of operation psychrometer– a more accurate device for measuring air humidity. The psychrometer has two thermometers: dry and wet. They are so called because the end of one of the thermometers is in the air, and the end of the second is tied with a piece of gauze immersed in water (see picture). The evaporation of water from the surface of the gauze leads to a decrease in its temperature. The second, “dry” thermometer shows the normal air temperature. The temperature values ​​measured by a psychrometer can be converted into relative air humidity using the table (see below).

Let's look at an example. Let's say the room temperature is 20 °C, and the wet bulb thermometer shows 15 °C. That is, the difference in thermometer readings is 5 °C. In the table, along line “20”, we move to column “5”. We read the number there: 59. Consequently, the relative humidity in the room where the psychrometer hangs is exactly 59%.

If there is little water vapor in the air where the psychrometer is located, then vaporization from the surface of the gauze is intense. According to the formula Q=rm (see § 6-d), the heat “taken” from the water on the gauze is spent on this, and it is cooled according to the formula $Q=C\cdot m\cdot \Delta t^o$ (see § 6-c). That's why a wet thermometer shows a lower temperature than a dry bulb. If the air is so humid that the water vapor it contains is saturated, then there will be no evaporation of water from the surface of the gauze. Therefore, both thermometers will show equal temperatures, and this will mean that the relative humidity is 100%.

Check your understanding of the material:

1. The purpose of this paragraph is to consider...
2. Air humidity is important not only for human health, but also for...
3. Why is it important that the water vapor in the air is not (almost) saturated?
4. The new physical quantity we introduce should show...
5. Relative air humidity is calculated by the ratio of the density of water vapor in the air to...
6. The hair hygrometer is...
7. The hygrometer reacts to changes in relative air humidity...
8. A hygrometer allows (is capable of) measuring the relative humidity of the air, since...
9. The convenience of using a hygrometer for measuring humidity is that its needle...
10. Instead of a hygrometer, a psychrometer is often used as...
11. Why does the right thermometer of a psychrometer usually show a lower temperature?
12. A specially compiled so-called psychrometric table is used to...
13. If the air temperature in the room is 30 °C, and the wet bulb temperature is 25 °C, then...
14. Under what condition does the evaporation of water from the surface of the gauze occur quickly?
15. The wet gauze, and with it the right thermometer, cools down, as...
16. Under what condition will both thermometers show equal temperatures?

AIR HUMIDITY. DEW POINT.

DEVICES FOR DETERMINING AIR HUMIDITY.

1.Atmosphere.

The atmosphere is the gaseous shell of the Earth, consisting mainly of nitrogen (more than 75%), oxygen (slightly less than 15%) and other gases. About 1% of the atmosphere is water vapor. Where does it come from in the atmosphere?

Larger share of area globe occupy seas and oceans, from the surface of which water constantly evaporates at any temperature. Water is also released during the respiration of living organisms.

The amount of water vapor contained in the air affects the weather, human well-being, technological processes in production, the safety of exhibits in the museum, and the safety of grain in storage facilities. Therefore, it is very important to control the degree of air humidity and the ability, if necessary, to change it in the room.

2.Absolute humidity.

Absolute humidity air is the amount of water vapor contained in 1 m 3 of air (water vapor density).

or , Where

m is the mass of water vapor, V is the volume of air that contains water vapor. P is the partial pressure of water vapor, μ is the molar mass of water vapor, T is its temperature.

Since density is proportional to pressure, absolute humidity can also be characterized by the partial pressure of water vapor.

3.Relative humidity.

The degree of humidity or dryness of the air is affected not only by the amount of water vapor contained in it, but also by the temperature of the air. Even if the amount of water vapor is the same, at a lower temperature the air will appear more humid. This is why a cold room feels damp.

This is explained by the fact that at higher temperatures the air can contain a greater maximum amount of water vapor, and is contained in the air when vapor is rich. That's why, maximum amount of water vapor, which may contain in 1 m 3 of air at a given temperature is called saturated vapor density at a given temperature.

The dependence of density and partial pressure of saturated vapor on temperature can be found in physical tables.

Taking this dependence into account, we came to the conclusion that a more objective characteristic of air humidity is relative humidity.

Relative humidity is the ratio of absolute air humidity to the amount of steam that is necessary to saturate 1 m 3 of air at a given temperature.

ρ is the vapor density, ρ 0 is the saturated vapor density at a given temperature, and φ is the relative air humidity at a given temperature.

Relative humidity can also be determined through partial vapor pressure

P is the partial pressure of vapor, P 0 is the partial pressure of saturated vapor at a given temperature, and φ is the relative humidity of the air at a given temperature.

4. Dew point.

If air containing water vapor is isobarically cooled, then at a certain temperature the water vapor becomes saturated, since with decreasing temperature the maximum possible density of water vapor in the air at a given temperature decreases, i.e. the density of saturated steam decreases. As the temperature drops further, excess water vapor begins to condense.

Temperature, at which a given water vapor contained in the air becomes saturated is called dew point.

This name is associated with a phenomenon observed in nature - dew fall. Dew loss is explained as follows. During the day, the air, earth and water in various bodies of water warm up. Consequently, there is intense evaporation of water from the surface of reservoirs and soil. The water vapor contained in the air is unsaturated at daytime temperatures. At night, and especially in the morning, the temperature of the air and the surface of the earth drops, water vapor becomes saturated, and excess water vapor condenses on various surfaces.

Δρ is the excess moisture that is released when the temperature drops below the dew point.

Fog has the same nature. Fog is tiny droplets of water formed as a result of steam condensation, not on the surface of the earth, but in the air. The droplets are so small and light that they can be suspended in the air. Light rays are scattered on these droplets, and the air becomes opaque, i.e. visibility becomes difficult.

With rapid cooling of air, steam, becoming saturated, can, bypassing the liquid phase, immediately turn into a solid. This explains the appearance of frost on trees. Some interesting optical phenomena in the sky (for example, a halo) are caused by the passage of solar or lunar rays through cirrus clouds consisting of tiny ice crystals.

5. Instruments for determining humidity.

The simplest instruments for determining humidity are hygrometers of various designs (condensation, film, hair) and a psychrometer.

Operating principle condensation hygrometer is based on measuring the dew point and determining the absolute humidity in the room from it. Knowing the temperature in the room and the saturated vapor density corresponding to this temperature, we find the relative humidity of the air.

Action film and hair hygrometers associated with changes in the elastic properties of biological materials. As humidity increases, their elasticity decreases, and the film or hair stretches to a greater length.

Psychrometer consists of two thermometers, one of which has a reservoir with alcohol wrapped in a damp cloth. Since moisture is constantly evaporating from the fabric and, therefore, heat is being removed, the temperature shown by this thermometer will always be lower. The less wet air indoors, the more intense evaporation occurs, the thermometer with a wet tank cools more and shows a lower temperature. Based on the temperature difference between dry and wet thermometers, using the appropriate psychrometric table, the relative air humidity in a given room is determined.

The amount of moisture contained in one cubic meter of air. Due to its small value, it is usually measured in g/m³. But due to the fact that at a certain air temperature it can only contain a maximum maximum amount of moisture (with increasing temperature this maximum possible amount of moisture increases, with decreasing air temperature the maximum possible amount of moisture decreases), the concept of relative humidity was introduced.

Relative humidity

An equivalent definition is the ratio of the mole fraction of water vapor in the air to the maximum possible at a given temperature. Measured as a percentage and determined by the formula:

where: - relative humidity of the mixture (air) in question; - partial pressure of water vapor in the mixture; - equilibrium saturated vapor pressure.

The saturated vapor pressure of water increases greatly with increasing temperature. Therefore, with isobaric (that is, at constant pressure) cooling of air with a constant vapor concentration, a moment comes (dew point) when the vapor is saturated. In this case, the “extra” steam condenses in the form of fog or ice crystals. The processes of saturation and condensation of water vapor play a huge role in atmospheric physics: the processes of cloud formation and the formation of atmospheric fronts are largely determined by the processes of saturation and condensation; the heat released during the condensation of atmospheric water vapor provides the energy mechanism for the emergence and development of tropical cyclones (hurricanes).

Relative Humidity Estimation

The relative humidity of a water-air mixture can be estimated if its temperature is known ( T) and dew point temperature ( Td). When T And Td expressed in degrees Celsius, then the following expression is true:

where the partial pressure of water vapor in the mixture is estimated:

and the wet vapor pressure of water in the mixture at temperature is estimated:

Supersaturated water vapor

In the absence of condensation centers, when the temperature decreases, a supersaturated state may form, that is, the relative humidity becomes more than 100%. Ions or aerosol particles can act as condensation centers; it is on the condensation of supersaturated vapor on ions formed during the passage of a charged particle in such a vapor that the principle of operation of the Wilson chamber and diffusion chambers is based: droplets of water condensing on the formed ions form a visible trace (track) of the charged particles.

Another example of the condensation of supersaturated water vapor is the contrails of aircraft, which occur when supersaturated water vapor condenses on soot particles from engine exhaust.

Means and methods of control

To determine air humidity, instruments called psychrometers and hygrometers are used. August's psychrometer consists of two thermometers - dry and wet. A wet thermometer shows a lower temperature than a dry thermometer because its reservoir is wrapped in a cloth soaked in water, which cools it as it evaporates. The intensity of evaporation depends on the relative humidity of the air. Based on the readings of dry and wet thermometers, the relative humidity of the air is found using psychrometric tables. Recently, integrated humidity sensors (usually with voltage output) have become widely used, based on the property of some polymers to change their electrical characteristics (such as the dielectric constant of the medium) under the influence of water vapor contained in the air.

To increase the relative humidity in residential areas, electric humidifiers, trays filled with wet expanded clay, and regular spraying are used.

Notes

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See what “Relative humidity” is in other dictionaries:

RELATIVE HUMIDITY, a measure of the quantitative content of water vapor in the air. The ratio of the actual vapor pressure to the saturated vapor pressure at which water typically condenses is expressed as a percentage. Humidity is measured by HYGROMETER... Scientific and technical encyclopedic Dictionary - The percentage ratio of the elasticity of water vapor contained in a unit volume of air to the elasticity of saturating vapor at the same temperature... Dictionary of Geography

Relative humidity- 16. Relative humidity D. Relative Feuchtigkeit E. Relative humidity F. Humidite relative The ratio of the partial pressure of water vapor to the pressure of saturated vapor at the same pressure and temperature Source ... Dictionary-reference book of terms of normative and technical documentation

The ratio of the elasticity of water vapor contained in the air to the elasticity of saturated vapor at the same temperature; expressed as a percentage. * * * RELATIVE HUMIDITY RELATIVE HUMIDITY, the ratio of the elasticity of water vapor (see ELASTICITY ... ... encyclopedic Dictionary

relative humidity- drėgnis statusas T sritis Standartizacija ir metrologija apibrėžtis Drėgmės ir ją sugėrusios medžiagos masių arba tūrių dalmuo, dažniausiai išreikštas procentais. atitikmenys: engl. relative humidity vok. relative Feuchte, f; relative… … Penkiakalbis aiškinamasis metrologijos terminų žodynas

relative humidity- santykinis drėgnis statusas T sritis chemija apibrėžtis Drėgmės ir drėgnos medžiagos, kurioje ji yra, masių arba tūrių santykis (%). atitikmenys: engl. relative humidity rus. relative humidity... Chemijos terminų aiškinamasis žodynas

relative humidity- drėgnis statusas T sritis fizika atitikmenys: engl. relative humidity vok. relative Feuchte, f; relative Feuchtigkeit, f rus. relative humidity, f pranc. humidité relative, f … Fizikos terminų žodynas