What is absolute and relative humidity. Absolute and relative humidity. Heat balance equation

One of the very important indicators in our atmosphere. It can be either absolute or relative. How is absolute humidity measured and what formula should be used for this? You can find out about this by reading our article.

Air humidity - what is it?

What is humidity? This is the amount of water that is contained in any physical body or medium. This indicator directly depends on the very nature of the medium or substance, as well as on the degree of porosity (if we are talking about solids). In this article, we will talk about a specific type of humidity - about the humidity of the air.

From the course of chemistry, we all know very well that atmospheric air consists of nitrogen, oxygen, carbon dioxide and some other gases, which make up no more than 1% of the total mass. But besides these gases, the air also contains water vapor and other impurities.

Air humidity is understood as the amount of water vapor that is currently (and in a given place) contained in the air mass. At the same time, meteorologists distinguish two of its values: these are absolute and relative humidity.

Air humidity is one of the most important characteristics of the Earth's atmosphere, which affects the nature of local weather. It should be noted that the value of atmospheric air humidity is not the same - both in the vertical section and in the horizontal (latitudinal) section. So, if in subpolar latitudes the relative indicators of air humidity (in the lower layer of the atmosphere) are about 0.2-0.5%, then in tropical latitudes - up to 2.5%. Next, we will find out what absolute and relative humidity are. Also consider what difference exists between these two indicators.

Absolute humidity: definition and formula

Translated from Latin, the word absolutus means "full". Based on this, the essence of the concept of "absolute air humidity" becomes obvious. This value, which shows how many grams of water vapor is actually contained in one cubic meter of a particular air mass. As a rule, this indicator is denoted by the Latin letter F.

G/m 3 is the unit of measurement in which absolute humidity is calculated. The formula for its calculation is as follows:

In this formula, the letter m denotes the mass of water vapor, and the letter V denotes the volume of a particular air mass.

The value of absolute humidity depends on several factors. First of all, this is the air temperature and the nature of advection processes.

Relative Humidity

Now consider what relative humidity is. This is a relative value that shows how much moisture is contained in the air in relation to the maximum possible amount of water vapor in this air mass at a particular temperature. The relative humidity of the air is measured as a percentage (%). And it is this percentage that we can often find out in weather forecasts and weather reports.

It is also worth mentioning such an important concept as the dew point. This is the phenomenon of the maximum possible saturation of the air mass with water vapor (the relative humidity of this moment is 100%). In this case, excess moisture condenses, and precipitation, fog or clouds form.

Methods for measuring air humidity

Women know that you can detect the increase in humidity in the atmosphere with the help of your puffy hair. However, there are other, more accurate, methods and technical devices. These are the hygrometer and the psychrometer.

The first hygrometer was created in the 17th century. One of the types of this device is precisely based on the properties of the hair to change its length with changes in the humidity of the environment. Today, however, there are also electronic hygrometers. A psychrometer is a special instrument that has a wet and dry thermometer. By the difference in their indicators and determine the humidity at a particular point in time.

Air humidity as an important environmental indicator

It is believed that the optimum for the human body is a relative humidity of 40-60%. Humidity indicators also greatly affect the perception of air temperature by a person. So, at low humidity it seems to us that the air is much colder than in reality (and vice versa). That is why travelers in the tropical and equatorial latitudes of our planet experience the heat and heat so hard.

Today, there are special humidifiers and dehumidifiers that help a person regulate the humidity of the air in enclosed spaces.

Finally...

Thus, the absolute humidity of the air is the most important indicator that gives us an idea of ​​the state and characteristics of the air masses. In this case, it is necessary to be able to distinguish this value from relative humidity. And if the latter shows the proportion of water vapor (in percent) that is present in the air, then absolute humidity is the actual amount of water vapor in grams in one cubic meter of air.

The following values ​​are used to characterize air humidity: absolute, maximum and relative humidity, saturation deficit, dew point.

absolute humidity call the amount of water vapor in grams contained at a given time in 1 m³ of air.

Maximum humidity is the amount of water vapor in grams contained in 1 m³ of air at the moment of full saturation.

relative humidity is the ratio of absolute humidity to the maximum, expressed as a percentage.

Saturation deficiency is the difference between maximum and absolute humidity.

Dew point - the temperature at which the absolute humidity is at its maximum.

When assessing air humidity, the value of relative humidity is of the greatest importance.

Relative humidity can be measured with a hygrometer or psychrometer. basis hygrometer is a defatted human hair connected through a block with an arrow that moves along a scale. The hair lengthens with increasing humidity and becomes shorter when it decreases.

Psychrometers consist of two identical thermometers (mercury or alcohol), the tank of one of them is covered with a cloth, which is pre-moistened with distilled water. As the water evaporates, the tank cools. The temperature difference is used to judge the humidity of the air, since the intensity of evaporation depends on the degree of saturation of the surrounding air with water vapor. Two types of psychrometers are used: stationary (August) and aspiration (Assman).

August psychrometer used in stationary conditions (at meteorological stations, in hospitals), placing it in places where the device is not exposed to thermal radiation and wind.

Absolute humidity is calculated using the Regnot formula:

K \u003d f - a (t c - t c) x B,

where To- absolute humidity, mm Hg;

f- maximum air humidity at wet bulb temperature (determined according to Table 1.6);

a- psychometric coefficient equal to 0.0001;

t s - dry bulb temperature;

t in - wet bulb temperature;

B- atmospheric pressure at the time of observation, mm Hg

In the Assmann psychrometer thermometer tanks are protected by double metal shields from radiant heat. There are ventilation ducts around the tanks, through which air is sucked in at a constant speed (4 m/s). To measure humidity, a cloth-wrapped thermometer is moistened with distilled water, then the fan spring is wound up and the device is placed at the desired point. Dry and wet thermometer readings are recorded 4–5 minutes after the fan is started.

… how does the relative humidity of the air affect the drying parameters of water-based paints and varnishes?

Relative air humidity - has a significant impact on both the speed and the completeness of drying of a water-based paint and varnish coating.

Relative humidity is a parameter that determines how much more water the air is willing to take in in the form of steam.

Relative Humidity

Relative humidity is the ratio of the amount of water vapor in the air to the maximum possible amount of vapor at a given temperature.

From the definition, at least it becomes clear that air can only contain a limited amount of water and this amount depends on temperature.

When the air humidity is 100%, this means that the maximum possible amount of water vapor is in the air and the air cannot take more. In other words, the evaporation of water under these conditions is impossible.

The lower the relative humidity of the air, the more water can be converted into steam and the higher the rate of evaporation. But this process is not endless - if evaporation occurs in a closed space (for example, there is no hood in the dryer), then at some point the evaporation will stop.

Absolute humidity

The table shows the values ​​of the absolute humidity of air with a relative humidity of 100% in the temperature range of interest to us and the behavior of the relative humidity parameter with increasing temperature.

From the above data, it can be seen that while maintaining the value of absolute humidity, with increasing temperature, the value of relative humidity decreases.

The value of the maximum absolute humidity at a certain temperature makes it possible to calculate the efficiency of the dryer, or more precisely, the inefficiency of the dryer without forced ventilation.

Let's say we have a dryer - a room 7 by 4 and a height of 3 meters, which is 84 cubic meters. And suppose that we want to dry 100 pieces of PVC window profiles or 160 facade panels of glass or fiber cement panels in the size of 600 by 600 mm in this room; which is about 60 sq.m. surfaces.

To paint such a surface, 6 liters of paint will be used; Approximately 2 liters of water must evaporate for the paint to dry completely. At the same time, according to the table, at a temperature of 20 ° C, 84 cubic meters. air can contain a maximum of 1.5 liters of water.

That is, even if the air initially had zero absolute humidity, water-based paint in this room will not dry out without forced ventilation.

Relative humidity reduction

Since the complete evaporation of water is a necessary condition for the polymerization of a water-based paint coating, the value of the relative humidity of the air has a significant effect on the drying rate and even on the performance of the polymer coating.

But it's not as scary as it might seem. For example, if you bring in outside air that has 100% relative humidity and a temperature of 5°C and heat it up to 15°C, the air will only have 53% relative humidity.

Moisture has not disappeared from the air, that is, the absolute humidity has not changed, but the air is ready to take in twice as much water as at a low temperature.

That is, there is no need to use dehumidifiers or condensers to obtain acceptable parameters for drying the paintwork - it is enough to raise the temperature above the ambient temperature.

The greater the temperature difference between the outside air and the air fed into the dryer, the lower the relative humidity of the latter.

Humidity is an important characteristic of the environment. But not everyone fully understands what is meant by the weather reports. and absolute humidity are related concepts. It is not possible to understand the essence of one without understanding the other.

Air and moisture

Air contains a mixture of substances in the gaseous state. The first is nitrogen and oxygen. Their total composition (100%) contains approximately 75% and 23% by weight, respectively. About 1.3% argon, less than 0.05% is carbon dioxide. The remainder (missing about 0.005% in total) is xenon, hydrogen, krypton, helium, methane, and neon.

There is also a constant amount of moisture in the air. It enters the atmosphere after the evaporation of water molecules from the world's oceans, from moist soil. In a closed space, its content may differ from the external environment and depends on the presence of additional sources of income and consumption.

For a more accurate definition of physical characteristics and quantitative indicators, two concepts are used: relative humidity and absolute humidity. In everyday life, excess is formed when drying clothes, in the process of cooking. People and animals excrete it with respiration, plants as a result of gas exchange. In production, a change in the ratio of water vapor can be associated with condensation during temperature changes.

Absolute and features of the use of the term

How important is knowing the exact amount of water vapor in the atmosphere? These parameters are used to calculate weather forecasts, the possibility of precipitation and its volume, and the paths of movement of fronts. Based on this, the risks of cyclones and especially hurricanes, which can pose a serious danger to the region, are determined.

What is the difference between the two concepts? In common, both relative humidity and absolute humidity indicate the amount of water vapor in the air. But the first indicator is determined by calculation. The second one can be measured by physical methods with the result in g/m 3 .

However, with a change in ambient temperature, these indicators change. It is known that the maximum amount of water vapor that can be contained in the air is absolute humidity. But for modes +1°C and +10°C these values ​​will be different.

The dependence of the quantitative content of water vapor in the air on temperature is displayed in the relative humidity indicator. It is calculated using a formula. The result is expressed as a percentage (an objective indicator of the maximum possible value).

Influence of environmental conditions

How will the absolute and relative humidity of the air change with an increase in temperature, for example, from +15°C to +25°C? With its increase, the pressure of water vapor increases. This means that more water molecules will fit in a unit volume (1 m3). As a result, the absolute humidity also increases. The relative will then decrease. This is because the actual water vapor content remained at the same level, but the maximum possible value increased. According to the formula (dividing one by the other and multiplying the result by 100%), the result will be a decrease in the indicator.

How will absolute and relative humidity change with decreasing temperature? What happens when you decrease from +15°C to +5°C? This will reduce the absolute humidity. Accordingly, in 1 m3. the air mixture of water vapor can fit a smaller amount as much as possible. The calculation according to the formula will show an increase in the final indicator - the percentage of relative humidity will increase.

Significance for a person

In the presence of an excess amount of water vapor, stuffiness is felt, with a lack, dryness of the skin and thirst are felt. Obviously, the humidity of raw air is higher. With an excess, excess water is not retained in a gaseous state and passes into a liquid or solid medium. In the atmosphere, it rushes down, this is manifested by precipitation (fog, frost). Indoors, a layer of condensate forms on interior items, and dew forms on the grass surface in the morning.

The rise in temperature is easier to bear in a dry room. However, the same mode, but at a relative humidity above 90%, causes a rapid overheating of the body. The body struggles with this phenomenon in the same way - heat is released with sweat. But in dry air, it quickly evaporates (dries) from the surface of the body. In a humid environment, this practically does not occur. The most suitable (comfortable) mode for a person is 40-60%.

What is it for? In bulk materials in wet weather, the dry matter content per unit volume decreases. This difference is not so significant, but with large volumes it can “result” into a really determined amount.

Products (grain, flour, cement) have an acceptable moisture threshold at which they can be stored without loss of quality or technological properties. Therefore, monitoring indicators and maintaining them at an optimal level are mandatory for storage facilities. By reducing the humidity in the air, it is also achieved to reduce it in the product.

Devices

In practice, actual humidity is measured with hygrometers. There used to be two approaches. One is based on changing the extensibility of the hair (human or animal). The other one is based on the difference between the readings of thermometers in a dry and humid environment (psychrometric).

In a hair hygrometer, the arrow of the mechanism is connected with a hair stretched on a frame. It changes physical properties depending on the humidity of the surrounding air. The arrow deviates from the reference value. Her movements are tracked on the applied scale.

Relative humidity and absolute humidity of the air, as you know, depend on the ambient temperature. This feature is used in the psychrometer. When determining, the readings of two adjacent thermometers are taken. The flask of one (dry) is under normal conditions. In the other (wet) it is wrapped in a wick, which is connected to a reservoir of water.

Under such conditions, the thermometer measures the environment, taking into account the evaporating moisture. And this indicator depends on the amount of water vapor in the air. The difference is determined. The value of relative humidity is determined by special tables.

Recently, sensors that use changes in the electrical characteristics of certain materials have become more widely used. To confirm the results and verify the instruments, there are reference settings.

General information

Humidity depends on the nature of the substance, and in solids, in addition, on the degree of fineness or porosity. The content of chemically bound, so-called constitutional water, for example, hydroxides, which is released only during chemical decomposition, as well as crystalline hydrated water, is not included in the concept of humidity.

Units of measurement and features of the definition of the concept of humidity

• Moisture is usually characterized by the amount of water in a substance, expressed as a percentage (%) of the original mass of the wet substance ( mass humidity) or its volume ( bulk moisture).

• Humidity can also be characterized by moisture content, or absolute humidity- the amount of water per unit mass of the dry part of the material. This definition of moisture is widely used to assess the quality of wood.

This value cannot always be accurately measured, because in some cases it is impossible to remove all unconstitutional water and weigh the object before and after this operation.

• Relative humidity characterizes the moisture content relative to the maximum amount of moisture that can be contained in a substance in a state of thermodynamic equilibrium. Relative humidity is usually measured as a percentage of the maximum.

Methods of determination

Titrator Karl Fischer.

Establishing the moisture content of many products, materials, etc. is important. Only at a certain humidity many bodies (grain, cement, etc.) are suitable for the purpose for which they are intended. The vital activity of animal and plant organisms is possible only at certain limits of humidity and relative humidity of the air. Humidity can introduce a significant error in the weight of the item. Kilograms of sugar or grains with 5% and 10% moisture content will contain different amounts of dry sugar or grains.

Moisture measurement is determined by drying the moisture and titrating the moisture according to Karl Fischer. These methods are primary. In addition to them, many others have been developed that are calibrated according to the results of moisture measurements by primary methods and according to standard moisture samples.

Air humidity

Air humidity is a value that characterizes the content of water vapor in various parts of the Earth's atmosphere.

Humidity - the content of water vapor in the air; one of the most significant characteristics of weather and climate.

Humidity in the earth's atmosphere varies widely. Thus, near the earth's surface, the content of water vapor in the air averages from 0.2% by volume in high latitudes to 2.5% in the tropics. The vapor pressure in the polar latitudes is less than 1 mb in winter (sometimes only hundredths of a mb) and in summer below 5 mb; in the tropics, it increases to 30 mb, and sometimes more. In subtropical deserts, vapor pressure is reduced to 5-10 mb.

Absolute air humidity (f) is the amount of water vapor actually contained in 1m³ of air:

f = (mass of water vapor in the air)/(volume of moist air)

Commonly used absolute humidity unit: (f) = g/m³

Relative humidity (φ) is the ratio of its current absolute humidity to the maximum absolute humidity at a given temperature (see table)

φ = (absolute humidity)/(maximum humidity)

Relative humidity is usually expressed as a percentage. These quantities are related to each other by the following relation:

φ = (f×100)/fmax

Relative humidity is very high in the equatorial zone (average annual up to 85% or more), as well as in polar latitudes and in winter inside the continents of middle latitudes. In summer, monsoon regions are characterized by high relative humidity. Low values ​​of relative humidity are observed in subtropical and tropical deserts and in winter in monsoon regions (up to 50% and below).

Humidity decreases rapidly with altitude. At a height of 1.5-2 km, the vapor pressure is on average half that at the earth's surface. The troposphere accounts for 99% of the atmospheric water vapor. On average, over every square meter of the earth's surface, the air contains about 28.5 kg of water vapor.

Literature

Usoltsev V. A. Measurement of air humidity, L., 1959.

Gas humidity measurement values

The following quantities are used to indicate the moisture content in the air:

Absolute air humidity is the mass of water vapor contained in a unit volume of air, i.e. density of water vapor contained in the air, [g/m³]; in the atmosphere ranges from 0.1-1.0 g/m³ (over the continents in winter) to 30 g/m³ or more (in the equatorial zone); maximum air humidity (saturation limit) the amount of water vapor that can be contained in the air at a certain temperature in thermodynamic equilibrium (maximum value of air humidity at a given temperature), [g/m³]. With an increase in air temperature, its maximum humidity increases; vapor pressure pressure exerted by water vapor contained in the air (water vapor pressure as part of atmospheric pressure), [Pa]; humidity deficit difference between saturated vapor pressure and vapor pressure [Pa], i.e. between maximum and absolute air humidity [g/m³]; relative humidity ratio of vapor pressure to saturated vapor pressure, i.e. absolute air humidity to maximum [% relative humidity]; dew point temperature of a gas at which the gas is saturated with water vapor °C . The relative humidity of the gas is 100%. With a further influx of water vapor or when air (gas) is cooled, condensate appears. Thus, although dew does not fall at −10 or −50°C, it does