Humidity states the amount of water vapor in the air. When it rains, the atmosphere is very humid because there is a lot of water vapor in the air. Conversely, if the water vapor in the air is very little, the air is arid. The amount of water vapor in the air is expressed by the relative humidity.
The relative humidity is the ratio of the partial pressure of steam to the pressure of saturated vapor of water at a certain temperature (steam is water vapor). Relative humidity is expressed in percent, mathematically formulated:
There is a new term, namely partial pressure. Partial pressure is the pressure given by every gas in the air. The air consists of various types of gas. There are nitrogen (78%), oxygen (21%), argon (0.90%), carbon dioxide, water vapor etc. The amount of partial pressure from each gas in the air is called total pressure (total pressure = atmospheric pressure = air pressure). If there is no water vapor in the air, then the partial pressure of water vapor = 0. Conversely, the partial pressure of water vapor is maximum if the partial pressure of water vapor = the pressure of steam is saturated with water. Water saturated vapor pressure depends on temperature.
If the partial pressure of steam = saturated vapor pressure (relative humidity = 100%), then the air becomes saturated with water vapor. When the air becomes saturated with water vapor, the water vapor in the air almost reaches its maximum value. If the partial pressure of the water vapor > saturated vapor pressure (relative humidity > 100%), then air becomes supersaturated. When the air becomes supersaturated, the air is unable to withstand water vapor. Because the air is unable to withstand the water vapor, then the excess of the water vapor will condense into water. The temperature at which water vapor condenses into dew is called the dew point.
