Humidity is an important characteristic of the environment. But not everyone fully understands what is meant by the values of indicators presented in weather reports. Relative humidity 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 a gaseous state. First of all, it is nitrogen and oxygen. Their total composition (100%) contains approximately 75% and 23% by weight, respectively. About 1.3% of argon, less than 0.05%, is carbon dioxide. The remainder (the missing mass fraction of about 0.005% in total) is accounted for by xenon, hydrogen, krypton, helium, methane and neon.
Also in the air constantly contains some amount of moisture. It enters the atmosphere after the evaporation of water molecules from the oceans, from moistened soil. In a confined space, its content may differ from the external environment and depends on the availability of additional sources of income and consumption.
For a more accurate determination of physical characteristics and quantitative indicators, two concepts are used: relative humidity and absolute humidity. In everyday life, excess water vapor is formed when drying clothes, during cooking. People and animals secrete it with respiration, plants as a result of gas exchange. In production, a change in the ratio of water vapor may be associated with condensation at a temperature differential.
Absolute and relative humidity: features of the use of the term
How important is knowledge of the exact amount of water vapor in the atmosphere? These parameters are used to calculate weather forecasts, the possibility of precipitation and their volume, and the ways of moving fronts. Based on this, the risks of cyclones and especially hurricanes that can pose a serious danger to the region are determined.
What is the difference between the two concepts? In general, both relative humidity and absolute humidity show the water vapor content in the air. But the first indicator is determined by calculation. The second can be measured by physical methods with a result in g / m 3 .
However, with a change in ambient temperature, these indicators change. It is known that a certain amount of water vapor can be contained in air as much as possible - absolute humidity. But for the 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 index. It is calculated by the formula. The result is expressed as a percentage (an objective indicator of the maximum possible value).
Environmental impact
How will the absolute and relative humidity of air change with increasing temperature, for example, from + 15 ° C to + 25 ° C? With its increase, the pressure of water vapor rises. So, in a unit volume (1 m3) water molecules will fit more. Consequently, absolute humidity also grows. Relative will decrease. This is because the actual water vapor content remained at the same level, and the maximum possible value increased. According to the formula (dividing one by the other and multiplying the result by 100%), the result is a decrease in the indicator.
How will absolute and relative humidity change with decreasing temperature? What happens when decreasing from + 15 ° C to + 5 ° C? The absolute humidity will decrease. Accordingly, in 1 m3 air mixture of water vapor can fit a smaller amount. Calculation by the formula will show an increase in the final indicator - the percentage of relative humidity will increase.
Value for man
If there is an excessive amount of water vapor, stuffiness is felt, with a lack, dry skin and thirst are felt. Obviously, the humidity of the raw air is higher. With excess, excess water is not kept in a gaseous state and passes into a liquid or solid medium. In the atmosphere, it rushes down, it is manifested by precipitation (fog, frost). Indoors, a layer of condensation forms on the interior, on the grass surface in the morning dew.
An increase in temperature is easier to carry in a dry room. However, the same mode, but with a relative humidity above 90%, causes rapid overheating of the body. The body fights 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 happen. The most suitable (comfortable) mode for a person is 40-60%.
Relative and absolute humidity measurement
Why is this necessary? 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 "pour out" 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 the optimal level are mandatory for storage. By reducing humidity in the air, they are also reducing in production.
Devices
In practice, the actual humidity is measured by hygrometers. There used to be two approaches. One is based on a change in the extensibility of the hair (human or animal). The other is on the difference in the readings of thermometers in a dry and wet environment (psychrometric).
In a hair hygrometer, the arrow of the mechanism is connected with the hair stretched on the frame. It, depending on the humidity of the surrounding air, changes its physical properties. The arrow deviates from the reference value. Her movements are tracked on the scale.
Relative humidity and absolute humidity are known to depend on the ambient temperature. This feature is used in a psychrometer. When determining the readings of two adjacent thermometers are taken. A flask of one (dry) is in normal conditions. In another (wet), it is shrouded in a wick, which is connected to a tank 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 in readings is determined. The value of relative humidity is determined by special tables.
Recently, sensors using changes in the electrical characteristics of certain materials have become more widely used. To confirm the results and verification of devices, there are reference settings.