Dew point, Frost point

The temperature at which the change of state occurs as the air is cooled is called the dew point temperature when the change of state occurs at a temperature above freezing, or the frost point temperature when the change of state occurs at a temperaturebelow freezing.

Whether dew or frost is formed depends on the amount of moisture in the air, the greater the amount of water vapor, the higher the condensation temperature. In a hot summer day, it is not uncommon to have a dew point of around 20'C. But in the cold winter day, the dew point can be below freezing, in which case it is called the frost point temperature.

You may also refer to the follwoing web-page for explanation of dew point and frost point:

http://www.theweatherprediction.com/habyhints/347/