Finally, equal numbers are escaping and returning; the vapour is then saturated, and its pressure is known as the saturation vapour pressure, ew.
Saturation deficit, an index of humidity typically characterized by the difference between the saturation vapour pressure and the actual vapour pressure of a volume of air.
Precipitable water vapour has, on the average, a vapour pressure of 0.0025 atmosphere, which amounts to 15 percent of the saturation vapour pressure.
where e is the actual vapour pressure for a given volume of air and ew is the saturation vapour pressure of the volume at a given temperature.
Since the saturation vapour pressure of liquid water is always greater than or equal to the saturation vapour pressure of ice, ice crystals will grow at the expense of the liquid water.
The ratio of observed water vapour pressure to the saturation vapour pressure at the same temperature is the relative humidity of the air.
The saturation vapour pressure will rise with rising temperature, and thus the same relative humidity U will correspond to a greater saturation deficit and increased evaporation at warm temperatures.
The saturation deficit can be expressed as and, because the saturation vapour pressure ew rises with rising temperature, the same relative humidity will correspond to a greater saturation deficit and evaporation at warm temperatures.
Its location is fixed, however, by the boiling point of 100 °C (212 °F), where the saturation vapour pressure of water vapour is 1,013 mb (1 standard atmosphere), the standard pressure of the atmosphere at sea level.
