Eva Crossan of Stirling, South Otago, asks :-

How does humidity work? How can there be more moisture in the air in summer than in winter when it is wetter weather?

Blair Blakie, a physicist researching ultra-cold gases at the University of Otago, responded.

Humidity is a measure of the amount of water in air. Humidity increases when water is added to the air through evaporation and is reduced by cooling the air and causing the moisture to condense (this is how a dehumidifier works). The most commonly used measure of humidity is relative humidity; this is the amount of water in the air as compared to a sample of air (at the same temperature) that is saturated with water vapour.

Relative humidity is not so useful for comparing the water content of air samples at different temperatures, because the saturation level changes significantly with temperature. It takes much more water to saturate hot air than cold air.

As an example of the seasonal conditions in New Zealand, the following table contains NIWA data on the average temperature, humidity and rainfall in Auckland.

Winter Summer Temperature 11C 19C Relative Humidity 88% 77% Rainfall 135mm 77mm Absolute Humidity 9 g/m3 12.5 g/m3

The higher rainfall during winter is enhanced by the high relative humidity, since as the relative humidity reaches 100% vapour will tend to condense into liquid water. However, despite having lower relative humidity, the warmer summer air has more water vapour. This is revealed by comparing the absolute humidity: the mass of water within a cubic metre of air. The absolute humidity is estimated in the last row of the table, showing that summer air has roughly 40% more water vapour than winter.