Humidity is the amount of water vapor in the air, measured as mass of water per unit volume or mass of air, or as a percentage of the maximum amount the air would support without condensation, or indirectly via the dew point.

Saturation of the air occurs when the water vapor pressure reaches the vapor pressure of liquid water at the temperature concerned; this rises rapidly with temperature. The physiologically tolerable humidity level falls rapidly with temperature, as humidity inhibits cooling by evaporation of sweat.

Types of humidity

Absolute humidity

Absolute humidity is the ratio of the mass of water vapor to the volume occupied by a mixture of water vapor and dry air. It is measured in grams of water vapor per cubic meter of air.

Conversion to relative humidity (RH) can only be made when the temperature is known. For instance 15 g/m3 at 17.5 °C equals 100% RH, while at 25 °C it equals 65% RH, and at 30 °C it equals 50% RH.

Relative humidity

Relative humidity is the amount of water vapour present in air expressed as a percentage of the amount needed for saturation at the same temperature.

If the air is gradually cooled while maintaining the moisture content constant, the relative humidity will rise until it reaches 100%. This temperature, at which the moisture content in the air will saturate the air, is called the dew point.

Specific humidity

Specific humidity is the ratio of the mass of water vapor in air to the total mass of the mixture of air and water vapor.

Mixing ratio

Mixing ratio is a measure of humidity in terms of the mass of water vapor for a given mass of dry air. Its units are typically grams of water vapor per kilogram of dry air.

Notice the difference between mixing ratio and specific humidity: the latter includes the water vapor in the air in the denominator while mixing ratio measures water vapor per mass of dry air.

Since water vapor comprises only a few percent of the mass of air, the values for specific humidity and mixing ratio are very close for a given parcel of air. The mixing ratio is not affected by changes in pressure and temperature and is a measure commonly used by meteorologists.

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