Relative humidity and absolute humidity: features of measurement and determination

Humidity is an important characteristic of the environment. But not everyone understands what is meant by the values of the indicators given in the weather reports. Relative humidity and absolute humidity are related concepts. Understand the essence of one without understanding the other is not possible.

Air and Moisture

Air contains a mixture of substances in the gaseous state. First of all, this 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) falls on the share of xenon, hydrogen, krypton, helium, methane and neon.

Also in the air, there is always a certain amount of moisture. In the atmosphere, it falls 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 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, during cooking. People and animals sing it out with breathing, plants as a result of gas exchange. In production, a change in the water vapor ratio may be due to condensation at a temperature drop.

Absolute and relative humidity of air: features of the use of the term

How important is the knowledge of the exact amount of water vapor in the atmosphere? According to these parameters, weather forecasts, the possibility of precipitation and their volume, the ways of moving fronts are calculated. On the basis of this, risks of cyclones and especially hurricanes, which may pose a serious threat to the region, are determined.

What is the difference between the two concepts? The common thing is that both relative humidity and absolute humidity show the content 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 the ambient temperature, these values change. It is known that in air, a certain amount of water vapor can be contained 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 air on temperature is displayed in the relative humidity index. It is calculated by the formula. The result is expressed as a percentage (objective indicator from 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 water vapor pressure rises. So, in a unit of volume (1 cubic meter.) Of water molecules will fit more. Consequently, absolute humidity also grows. Relative in this case will decrease. This is because the actual water vapor content remained at the same level, and the maximum possible value increased. By the formula (dividing one into another and multiplying the result by 100%) the result is a decrease in the indicator.

How will the absolute and relative humidity change when the temperature is lowered? What happens when you decrease from + 15 ° C to + 5 ° C? Absolute humidity will decrease. Accordingly, in 1 cubic meter. Air mixture of water vapor as much as possible can fit a smaller amount. Calculation of the formula will show an increase in the final indicator - the percentage of relative humidity will increase.

Meaning for man

If there is an excessive amount of water vapor, the 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 does not remain in the gaseous state and passes into a liquid or solid medium. In the atmosphere, it rushes downwards, this is manifested by precipitation (fog, frost). In the room, a layer of condensation forms on the interior items, on the grass surface in the morning dew.

The temperature rise is easier to carry in a dry room. However, the same regime, but with a relative humidity above 90% causes a rapid overheating of the body. The organism fights this phenomenon equally - there is a release of heat 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%.

Relative and absolute humidity measurement

Why is it necessary? In bulk materials in wet weather, the dry matter content per unit volume decreases. This difference is not so significant, but at high volumes it can "pour out" into a real quantity.

The products (grain, flour, cement) have an acceptable humidity threshold at which it can be stored without loss of quality or technological properties. Therefore, monitoring indicators and maintaining them at the optimal level are mandatory for storage facilities. Reduction of humidity in the air is achieved by reducing it in production.

Devices

In practice, the actual humidity is measured by hygrometers. Previously, there were two approaches. One is based on the change in the extensibility of the hair (human or animal). The other is on the difference in thermometer readings in a dry and wet environment (psychrometric).

In the hair hygrometer the arrow of the mechanism is connected with the hair stretched on the frame. Depending on the humidity of the ambient air, it changes physical properties. The arrow deviates from the reference value. Its movements are tracked on a scale.

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

Under these conditions, the thermometer measures the medium 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 relative humidity value is determined by special tables.

Recently, more use has sensors that use changes in the electrical characteristics of certain materials. To confirm the results and reconcile the devices, there are reference installations.