Question submitted by Sharyn Pountney
(West Vancouver, BC)
Sharyn, the dew point is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation point. When the dew point temperature falls below freezing it is often called the frost point, as the water vapor no longer creates dew but instead creates frost or hoarfrost by deposition.
The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates the dew point is equal to the current temperature and the air is maximally saturated with water. When the dew point remains constant and temperature increases, relative humidity will decrease.
The dew point really won't be mentioned much in a forecast, but you may here the odd broadcaster mentioning the reading during their weather presentation, as an indication of how close a site may be to becoming saturated or "fogged in".
And in regards to the "DEW line": The Distant Early Warning Line, also known as the DEW Line or Early Warning Line, was a system of radar stations in the far northern Arctic region of Canada. I was lucky enough to work on the DEW line before it was essentially disbanded in the late 1990's.