What is humidity? What is the unit?

A physical quantity indicating the degree of atmospheric dryness. At a certain temperature, the less water vapor in a certain volume of air, the drier the air; The more water vapor, the wetter the air. The degree of dryness and wetness of air is called "humidity". In this sense, it is often expressed by physical quantities such as absolute humidity, relative humidity, relative humidity, mixing ratio, saturation deficit and dew point. If the weight of liquid water in wet steam accounts for the percentage of the total weight of steam, it is called the humidity of steam.

summary

The higher the temperature of air, the stronger its ability to contain water vapor. Although water vapor can react with some components in the air (such as salt in suspended dust) or be absorbed by porous particles, these processes or reactions account for a small proportion. On the contrary, most water vapor can be dissolved in the air. Dry air can generally be regarded as an ideal gas, but with the increase of water vapor composition, its ideality is getting lower and lower. At this time, only van der Waals equation can describe its performance.

Theoretically, the statement that "the water vapor in the air is saturated" is incorrect, because the water vapor saturation in the air has nothing to do with the composition of the air itself, but only with the temperature of the water vapor. At the same temperature, the saturation of water vapor in vacuum is actually as high as that in air. But for the sake of simplicity, people (even in the scientific community) use the words "dissolved water vapor in the air" or "saturated water vapor in the air". We also use these common words in this article.

If the temperature of saturated air drops below the dew point and there are condensation nuclei (such as fogging agents) in the air (there are always condensation nuclei in nature), the water in the air will condense. Many phenomena such as dew and fog on cold surfaces such as clouds, condensed water, window panes, and steam emitted by people in cold air are formed in this way. Occasionally (or artificially caused in the laboratory) water vapor can be below the dew point without condensation. This phenomenon is called supersaturation.

The dissolved amount of water vapor in the air varies with temperature. One cubic meter of air can dissolve 9.4 1 g of water at 10℃ and 30.38 g of water at 30℃.

measure

Several quantities of hygrometer are used to indicate the humidity of air. The most commonly used are listed below:

Steam pressure

absolute humidity

relative humidity

Specific humidity

dew point

The instrument used to measure humidity is called a hygrometer.

▲ Absolute humidity

Absolute humidity is the mass of water vapor contained in a certain volume of air, and its unit is generally gram/cubic meter. Maximum absolute humidity is the highest humidity in saturated state. Absolute humidity is meaningful only when there is temperature, because the amount of humidity that can be accommodated in the air varies with temperature and at different heights, because the volume of air varies with height. However, the closer the absolute humidity is to the highest humidity, the smaller its change with height.

The following formula is used to calculate absolute humidity:

e·m

ρw =───=─

Rw TV

These symbols are:

E–Vapor pressure in Pascal.

Rw-gas constant of water = 461.52j/(kg k)

T-temperature, in Kelvin.

M—— the mass of water dissolved in air, in grams.

V-the volume of air, in cubic meters.

▲ Relative humidity

The hygrometer is recording the relative humidity. Relative humidity is the ratio of absolute humidity to maximum humidity, and its value shows how high the saturation of water vapor is. Air with a relative humidity of 100% is saturated air. The relative humidity is that 50% air contains half of water vapor, reaching the saturation point of air at the same temperature. Water vapor in the air with relative humidity exceeding 100% will generally condense out. With the increase of temperature, the more water can be contained in the air, that is to say, the relative humidity will decrease with the increase of temperature for the same amount of water vapor. Therefore, it is necessary to provide temperature data and relative humidity. Dew point can also be calculated by relative humidity and temperature.

The following formula is used to calculate the relative humidity:

ρw e s

φ =─── 100%=─ 100%=─ 100%

ρw, maximum E S

These symbols are:

ρ W-absolute humidity, in grams per cubic meter.

ρ w, max-maximum humidity, in grams per cubic meter.

E–Vapor pressure in Pascal.

E—— saturated vapor pressure in Pascal.

S-specific humidity, in grams/kg.

S—— Maximum specific humidity, in gram/kg.

3. Specific humidity

Specific humidity is the ratio of the mass of water dissolved in air to the mass of wet air. If there is no condensation or evaporation, the specific humidity of enclosed air at different heights is the same. The symbol of the highest specific humidity in saturated state is S.

For the formula for calculating specific humidity, please refer to resources.

See Resources for similar maximum specific humidity.

The symbols used are:

MX–mass, in grams.

ρ x-density, in grams per cubic meter.

Vtotal-–The total volume of humid air in cubic meters.

Rw-gas constant of water, in joule/(kilogram kelvin)

Rl—— gas constant of dry air, in joule/(kilokelvin)

T–temperature, kelvin

M molar mass of water =18.5438+0.528g/mol.

–Molar mass of dry air =28.9634 g/mol

E–Vapor pressure in Pascal.

P-air pressure, in Pascal.

E—— saturated vapor pressure in Pascal.

Significance and use

Air humidity plays an important role in many aspects, mainly in theory in meteorology, climatology and meteorology, but it has little effect in practical application.

▲ Meteorology and Hydrology

When it rains, the air is very humid. In meteorology and hydrology, humidity is an important data to determine evaporation and transpiration. It plays a decisive role in the formation of different climatic zones. Water vapor in the atmosphere is also essential in the water cycle. Water can move quickly on the surface of the earth through steam. Water forms precipitation, clouds and other phenomena in the atmosphere, which determine the weather and climate of the earth.

Relative humidity is commonly used in weather forecast. Reflects the possibility of rain and fog. In hot weather, high relative humidity will make people (and other animals) feel hotter because it hinders the evaporation of sweat. Humans can therefore calculate the heat index.

▲ medicine

In medicine, the relationship between air humidity and breathing is very close. Under certain humidity, oxygen can easily enter the blood through alveoli. The average person feels most comfortable at 45-55% relative humidity. The relative humidity of overheated and unventilated rooms is generally low, which may be harmful to the skin and irritating to the mucosa. Excessive humidity affects people's perspiration function of regulating body temperature, and people will feel stuffy. Generally speaking, people feel better in high temperature and low humidity (such as desert) than in low temperature and high humidity (such as rainforest). The humidity of anesthetic gas is very important when anesthesia is performed by breathing. Anesthetic gases used in medicine are generally stored without water. If humidity is not added during use, it will cause evaporation and water loss in human lungs.

▲ biology

In biology, especially in ecology, air humidity is a very critical quantity. It determines the composition of an ecosystem. Opening and closing of stomata on plant leaves and respiration of plants. Some animals, such as snails, can only absorb oxygen when their skin has a certain humidity.

▲ Storage and production

In the warehouse where fruits are stored, humidity determines the maturity of fruits. Excessive humidity may lead to corrosion in warehouses where metals are stored. Many other commodities, such as chemicals, cigarettes, wine, sausages, wood, artworks, integrated circuits, etc. It must also be stored at a certain humidity or zero humidity. Therefore, in many warehouses, museums, libraries, computer centers and some factories (such as microelectronics industry), there are air conditioning devices to control indoor humidity.

▲ Agriculture and Forestry

Low humidity in foggy forests will lead to water loss and yield reduction of soil and plants in agriculture.

Humidity is also a very critical quantity in forestry and forest industry. In the sawmill, people often water the wood piled there. Wood itself has its own humidity, and its humidity in the air gradually approaches the humidity around the air. The change of humidity in this kind of wood will lead to the change of wood volume, which is very important for forest industry.

Generally speaking, wood should be stored in such a way that air can directly contact it from all directions to avoid wood deformation or mildew. When laying the floor, it is best to leave the wood of the floor in the house for a day or two to make it the same as the humidity in the house, otherwise the wood of the floor may stretch or shrink after laying.

▲ architecture

Dew point is a very important quantity in architectural physics. If the temperature in the building is different, the water in the humid air flowing from the high temperature part to the low temperature part may condense. These places may take shape, and architectural design must take this phenomenon into account. In addition, relative humidity is an important index to measure the indoor thermal environment of buildings. When the subjective thermal sensation of human body is neutral, the wind speed is not more than 0. 15m/s and the relative humidity is 50%, which is also a benchmark for indoor thermal environment design.