Humidity and Dew Point

About this topic

Humidity is the amount of water vapour in the air. Absolute humidity is the mass of vapour per volume; relative humidity compares it to the maximum the air can hold at that temperature, as a percentage. Warm air holds more vapour. The dew point is the temperature at which air becomes saturated and condenses.

When the temperature falls to the dew point, dew, fog or cloud forms. High humidity makes it feel hotter because sweat evaporates slowly. Hygrometers and psychrometers measure it.

An example makes dew point practical: on a summer evening the air is 25 degrees C with a dew point of 20 degrees C. As the temperature drops to 20 degrees overnight, the relative humidity reaches 100 percent and dew forms on grass, or fog if the air is calm. Relative humidity depends on temperature: the same amount of vapour is 80 percent at 15 degrees but only about 40 percent at 28 degrees, which is why indoor air in winter feels dry once it is heated.

Meteorologists often prefer dew point, since it is a direct measure of moisture. A dew point above about 20 degrees feels muggy and oppressive. The heat index combines temperature and humidity to indicate how hot it feels, and is important for health in heat waves. Humidity also affects wood, paper, mould growth and food storage, and a psychrometer uses wet and dry bulb thermometers, with evaporative cooling showing the difference. A common misconception is that 100 percent relative humidity means it is raining or that the air is full of water; it means saturation at that temperature. Also, warm air does not hold water like a sponge; it is more that evaporation and condensation balance.