Extra - April 27, 2012
Humidity problems
Importance of vapour pressure

By Philip Eden


A brisk north wind on Sunday 15th and Monday 16th April brought exceptionally dry air across many parts of England and Wales. This triggered some interest in forecast offices around the country, but not many people outside the meteorological world would even have noticed. If, by chance, you had hung out a line of washing that day you might have remarked how quickly it dried, but that was about the extent of its impact on our lives.


Humidity is particularly difficult to explain because the maximum amount of water vapour that can be held in the air is dependent on the temperature of the air. If that maximum amount is reached, the air is said to be "saturated", and it is usually either raining or foggy; but the actual quantity of water vapour in saturated air at 21C is twice that at 10C, four times that at 0C, and ten times that at minus 10C. This also explains, at least in part, why rain falls more intensely in summer than in winter, and why tropical downpours are so much heavier than Arctic snowfalls.


Some correspondents have urged me to introduce "relative humidity" figures into my weather forecasts. I decline to do so simply because relative humidity (RH) tells you relatively little about how humid it feels. This measure gives you the ratio, expressed as a percentage, of the amount of water vapour in the air at any one time to the maximum possible amount at a given temperature. Thus saturated air has an RH of 100 per cent. The only use I can think of for relative humidity in a forecast is that it gives some indication of how quickly that line of washing will dry.


Physiologically, human beings react to the actual quantity of water vapour in the air. This is normally measured as the "vapour pressure", reported in millibars, just like air pressure.


Let us look at how the single RH value of 50 per cent changes with the temperature. At 0C it gives a vapour pressure (VP) of 3.1 mbar and might subjectively be described as "crisp". At 10C the VP has risen to 6.2 mbar and the weather might be called "brisk" or "bracing". At 20C the VP is 11.6 mbar which most of us would call "comfortable", but at 30C the VP is 21 millibars and we would find it very oppressive, while at 35C the VP is 28 mbar and we would be at risk from heatstroke.


By Philip Eden


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