Guide to dewpoint.....

[Paul]

Dewpoint:

The strict definition for this is:-The temperature to which the air must be cooled in order that it is saturated with respect to a water surface and for this to occur at its existing pressure and humidity content.

Dewpoint may be measured indirectly from wet- bulb and dry-bulb temperatures with the aid of humidity tables. More often now it is produced by some kind of calculator or computer programme taking these basics into consideration.

Dew forms on clear nights when there is little or no wind at the surface. It can occur on summer nights but is most prevalent during the long nights from autumn through to spring. Dew forms on surfaces whose temperature falls to below the dewpoint of the air in contact with it.

From what has already been said it should be clear that the dewpoint is always below the surface temperature. As in many things to do with Meteorology there is a caveat. Remember that the definition is 'become saturated with respect to a water surface'. There are a very small number of occasions when, with temperatures falling, the relationship becomes to that with respect to ice. In these few instances, briefly, the dewpoint may be a fraction higher than the air temperature.

Dewpoint is really a measure of how much water vapour there is at any one time. Obviously the higher the air temperature then the higher the dewpoint can be. Compare the temperature in winter say of 3C with that on a hot summer day, maybe 25C. Obviously much more water vapour is possible with higher temperatures. This is partially responsible for the intensity of thunderstorms in summer compared to those of winter, and to the Tropics having much more intense downpours than in Temperate latitudes.

The frost point is that temperature at which the air is saturated with respect to an ice surface.

Dew point is closely associated with humidity. Thus in warm frontal zones the humidity is high. The arrival of a cold air mass will usually bring a sharp drop in the dewpoint. Dewpoint is an important tool for forecasters to use when forecasting many weather phenomena, be it thunderstorms, human comfort levels, or the likelihood of snow or frost.

by John Holmes