Relative Humidity and Dewpoint

The relative humidity is the amount of water in the air relative to the total (saturation) amount the air can hold at a given temperature. Saturation is the extent to which something is dissolved or absorbed compared with the maximum possible, usually expressed as a percentage. Try putting a spoon of sugar in your tea or coffee, now put another. Keep going until the sugar no longer dissolves. It is now fully saturated, it cannot hold any more sugar. Air that is saturated can no longer hold any more water vapour and is said to have 100 percent relative humidity. Air that has high relative humidity will cause a reduction in aircraft performance.

Relative humidity is the amount of water in the air relative to the total amount the air can hold at a given temperature.

Dewpoint

The dewpoint is the temperature to which air must be cooled to become saturated with water vapour. It is provided by aviation weather reports in degrees Celsius. When analyzing a weather report, the smaller the difference between the reported temperature and dewpoint, the more humid the air is. This difference is known as the temperature dew point spread. When the temperature of the air is reduced to the dew point, the air is completely saturated and moisture begins to condense out of the air in the form of fog, dew, frost or clouds.

There are several ways in which the air can be cooled to its dewpoint, these are listed below:

Warm Air Over Cold Surface: When warm air moves over a cold surface, the air temperature drops and may reach its saturation point
Mixing of Cold and Warm Air: The saturation point may be reached when cold air and warm air mix
Night Time Cooling: When air cools at night through contact with the colder ground, it may reach its saturation point
Lifted Air: When air is lifted or is forced upward in the atmosphere, it may cool to its dew point and become saturated