Tuesday 28 June 2011, 16:23
Someone on Twitter asked me earlier "what is the dew point temperature?"
Well, the dew point is the temperature at which air can no longer hold all of its water vapour and some of the water vapour must condense into liquid water.
The dew point is always lower than or equal to the air temperature. If the air temperature cools to the dew point, or if the dew point rises to equal the air temperature, then dew, fog or clouds begins to form. At this point the relative humidity is 100%.
During the summer, the dew point temperature is a good measure of how humid it feels. A higher dew point means there is more water vapour available in the atmosphere to produce showers and thunderstorms.
My idea of a perfect summer's day would be a temperature of 25 Celsius with a dew point of 10 Celsius.
This would feel very pleasant with a relative humidity of 52%. Yesterday, though, the dew point was high - around 18 Celsius, so it felt humid and sticky.
Today, though, dew points are lower and it feels fresher because the air over us has come from the Atlantic rather than the Bay of Biscay, so it feels more comfortable.
You can follow me on Twitter @derektheweather