Concepts Related To Dew Point Temperature


All gases can be "forced" to become liquid (condense) by the process of cooling.  For conditions on the earth, only one gas can be condensed by cooling, water vapor. The temperature at which such condensation would occur is called the DEW POINT TEMPERATURE.


The difference between the dew point temperature and the actual temperature is related to RELATIVE HUMIDITY.  When the temperature is cooled to the dewpoint then the relative humidity is 100%.  If there is a large difference between the temperature and the dewpoint temperature, then the relative humidity is very low (e.g., 10%).


It can be shown that the dew point temperature is also related to the total number of water vapor molecules present.  In essence, the dew point temperature also is a rough measure of how much water vapor is present.  This is why dew point temperatures are characteristically higher over oceans and in areas that have access to air flowing from the oceans.


Another very important aspect of the dew point temperature at the surface of the earth is that it can be used as a rough approximation for how prone the atmosphere is to develop thunderstorms. Roughly speaking, dew point temperatures during the warm season of 60F or greater will be found in areas with developing thunderstorms. However, this is the "weakest" rule of thumb, since several other factors relate to the tendency for thunderstorms to occur, such as temperatures at the ground and aloft.



Rules of Thumb:


The higher the dew point temperature, the greater the amount of water vapor is present (source for clouds).


The smaller the difference between the temperature and the dew point temperature, the higher the relative humidity (the closer the atmosphere is to a state in which water vapor would condense).