What is humidity?

Question 1: What is the unit of humidity? In metrology, humidity is defined as "the amount of image state". Humidity in daily life refers to relative humidity, expressed in %rh. In a word, it is the percentage of saturated water vapor (saturated water vapor pressure) of gas (usually air) under the same conditions as its air.

History of humidity measurement

For a long time, humidity and temperature are closely related to life, but it is difficult to express them quantitatively. The history of hygrometer can be traced back to Libra in China (BC 179) as the earliest hygrometer. Thermometers can be traced back to recorded thermometers in Greek times. )

Absolute humidity (absolute humidity)

The gas in unit volume (1m3) contains the mass (g) of water vapor.

Representative: d = g/m3

However, even if the amount of water vapor is the same, the gas volume will change due to the change of temperature and pressure, that is, the absolute humidity D will also change. D is the volume reference.

relative humidity

The ratio of vapor pressure (e) in a gas to its saturated vapor pressure (es) is expressed as a percentage.

Average value: RH = e/es× 100%

However, the change of temperature and pressure leads to the change of saturated water vapor pressure, and rh will also change accordingly.

Saturated vapor pressure

The amount of water vapor contained in gas is limited, and the state that reaches the limit can be called saturation, and the water vapor pressure at this time is called saturated water vapor pressure. This physical quantity also changes with the change of temperature and pressure. Even at the same humidity below 0℃, the values of saturated vapor pressure (esw) stored in water and saturated vapor pressure (esi) stored in ice are different, and the saturated vapor pressure (esw) stored in water is usually used. The saturated steam pressure gauge JIS-Z-8806 corresponding to each temperature is recorded at the end of the volume.

dew point

Gases with higher temperatures also contain more water vapor. After the gas is cooled, even if the amount of water vapor contained in it remains unchanged, the relative humidity will increase. When it reaches a certain temperature, the relative humidity will reach 100% saturation. At this time, if the temperature continues to drop, some water vapor will condense into dew. The temperature at this time is the dew point temperature. When the dew point is lower than 0℃, it is frost point.

Unpleasant index THI (temperature and humidity index)

The popular term "unhappy index" of living environment was first used by the US Meteorological Bureau in 1959. Expressed as: THI= (dry bulb temperature TD+ wet bulb temperature TW) × 0.72+40.6. This data 70 ~ 75 is half unhappy, and over 80 is basically that all employees are unhappy. Recently, there are some unhappy indicators in the market.

Effective temperature (effective temperature)

Unhappiness index is a simple expression of the index that the human body can perceive. With the recent development of air conditioning technology, in addition to temperature and humidity, other items that can be perceived by human body, such as wind speed, have been introduced, thus creating this term. There is little difference with the unhappy index, and its change is close.

Equivalent temperature

Terms including effective temperature (temperature, humidity, airflow) and radiation elements.

Mixing ratio x (humidity mixing ratio)

For air (dry air) with steam below 1kg, water vapor is contained in the proportion of Xkg, and its mass ratio x (kg/kg) is the mixing ratio. Even if the temperature, pressure and volume change, the mixing ratio will not change as long as the amount of water vapor remains unchanged. Therefore, for the convenience of calculation, the mixing ratio is called absolute humidity in industry. X is the weight standard.

Air map

That is, a line graph showing the properties of air (moisture) containing water vapor. The horizontal axis represents enthalpy (I) and the vertical axis represents mixing ratio (X). All states of air represented by the point 1 in the figure are called state points. Knowing this state point, we can calculate the dry-bulb temperature, wet-bulb temperature, Ludian temperature, mixing ratio, relative humidity and enthalpy of air in its state.

※? Heat function (kcal/kg) ... sensible heat of dry air+latent heat of steam ... >; & gt

Question 2: What is the concept of humidity in weather forecast? The percentage of the actual water vapor density in the air and the saturated water vapor density at the same temperature is called the "relative humidity" of short air.

Question 3: What is the manifestation of humidity? Generally refers to the relative humidity.

Symbol %RH

Question 4: What do absolute humidity and relative humidity mean respectively? Humidity is 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 impact 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.

Absolute humidity The mass of water vapor contained in air per unit volume is called "absolute humidity" of air. It is the representation of the physical quantity of atmospheric humidity. It is usually expressed in grams of water vapor contained in 1 m3 air. The pressure of water vapor increases with the density of water vapor, so the absolute humidity in the air can also be expressed by the pressure of water vapor. Because the value of water vapor density is very close to the value of saturated water vapor pressure at the same temperature expressed by millimeter high mercury column, the humidity of air is often calculated by millimeter high mercury column of water vapor

Relative Humidity The percentage of the actual water vapor density contained in the air to the saturated water vapor density at the same temperature is called the "relative humidity" of the air. The degree of dryness and wetness of the air is related to the degree that the water vapor contained in the air is close to saturation, but not directly related to the absolute amount of water vapor contained in the air. For example, when the pressure of water vapor contained in the air is equal to1606.24pa (12.79mm Hg), at noon in hot summer, the temperature is around 35℃, and people do not feel wet, so it is far from reaching the saturated pressure of water vapor, and the water in the object can continue to evaporate. In the cold autumn, people will feel wet around 15℃, because the water vapor pressure has reached supersaturation, and water can not only evaporate, but also condense into water, so we call the actual density of water vapor in the air ρ 1 and the percentage of saturated water vapor density ρ2 at the same temperature ρ1ρ 2×100. It can also be expressed by water vapor pressure ratio:

For example, the pressure of water vapor in air is 1606.24Pa( 12.79 mm Hg), the saturated vapor pressure at 35℃ is 5938.52Pa(44.55 mm Hg), and the relative humidity of air.

At 15℃, the saturated vapor pressure is1606.24pa (12.79mmhg) and the relative humidity is 100%.

There is no functional relationship between absolute humidity and relative humidity. For example, the higher the temperature, the faster the water evaporates, so the water vapor in the air also increases accordingly. Therefore, in a day, the absolute humidity at noon is often greater than at night. In a year, the absolute humidity in summer is higher than that in winter. However, because the water vapor saturation pressure of air also changes with temperature, it may be that the relative humidity at noon is smaller than that at night, and the relative humidity in winter is larger than that in summer. Because the saturated vapor pressure at a certain temperature can be found out from the table of "saturated vapor pressure at different temperatures", as long as the absolute humidity or relative humidity is known, the relative humidity or absolute humidity can be calculated.

Question 5: What unit should be used for humidity? Air humidity refers to the degree of air humidity, which can be expressed by relative humidity (RH), and it is defined by dew point temperature. Relative humidity refers to the percentage of water vapor density actually contained in the air and saturated water vapor density at the same temperature. The optimum relative humidity for human body to feel comfortable indoors is 49% ~ 5 1%. Too low or too high relative humidity is not good for human body.

Question 6: 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:

Vapor pressure, Pascal

The gas constant of Rw C water is 461.52j/(kg k).

T C kelvin temperature.

The mass of water dissolved in air by m C, in grams.

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

Among them, symbols are divided into >>