The dewpoint temperature is the temperature at which the air can no longer "hold"
all of the water vapor which is mixed with it, and some of the water vapor 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 begin to form. At this point where the dew point temperature
equals the air temperature, the relative humidity is 100%.
If there is then further cooling of the air, say because the air parcel is rising to higher (and thus colder) levels in the atmosphere,
even more water vapor must condense out as additional dew, fog, or cloud, so that the dew point temperature
then falls along with the air temperature. This is how precipitation forms...when water vapor is removed from the air so rapidly that the liquid
water drops grow to a size where they fall out of the cloud.
While relative humidity is (as its name suggests) a relative measure of how humid the air is, the dewpoint temperature
is an absolute measure of how much water vapor is in the air. In very warm, humid conditions, the dewpoint temperature
often reaches 75 to 77 degrees F, and sometimes exceeds 80 degrees. No matter how hot the temperature gets, a dewpoint temperature of
(say) 75 deg. F always represents the same amount of water vapor in the air in absolute terms (but different relative humidities).
During the summer, the dewpoint temperature -- not the relative humidity -- is usually a better measure of how humid it feels outside.
It is also a good measure of how much water vapor "fuel" is available to showers and thunderstorms, with a
higher dewpoint representing more water vapor available for conversion to rain.
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