Relative humidity and absolute humidity: features of measurement and definition

2024 Author: Henry Conors | [email protected]. Last modified: 2024-02-12 02:55

Humidity is an important characteristic of the environment. But not everyone fully understands what is meant by the values \u200b\u200bof indicators given 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 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 (the missing mass fraction of about 0.005% in total) is xenon, hydrogen, krypton, helium, methane and neon.

Also, there is always some amount of moisture in the air. It enters the atmosphere after the evaporation of water molecules from the world's oceans, withmoist 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 water vapor 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 may be due to condensation due to temperature changes.

Absolute and relative air humidity: 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/m3.

However, as the ambient temperature changes, these figures change. It is known that the maximum amount of water vapor can be contained in the air - 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 the 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. Calculation according to the formula will show an increase in the final indicator - the percentage of relative humidity will increase.

Meaning for a person

In the presence of an excess amount of water vapor, stuffiness is felt, with a lack - it is feltdry skin and thirst. 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, dew on the surface of the grass in the morning.

Rise in temperature is easier to bear in a dry environment. However, the same mode, but at a relative humidity above 90%, causes a 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 occur. The most suitable (comfortable) mode for a person is 40-60%.

Measuring relative and absolute humidity

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, they also reduce it in products.

Instruments

In practice, actual humidity is measured with hygrometers. Previously there were twoapproach. One is based on changing the extensibility of the hair (human or animal). The other is based on the difference between thermometer readings in a dry and humid environment (psychrometric).

In a hair hygrometer, the needle of the mechanism is connected to the hair stretched on the 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, 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. The other (wet) has it wrapped in a wick, which is connected to a reservoir of water.

In 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 instruments, there are reference settings.