fog forms at the dewpoint temperature

What is the dewpoint temperature?

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.
Interesting facts:
SOUPY AIR: When the dewpoint approaches 75 degrees F, most people can "feel" the thickness of the air as they breathe, since the water vapor content is so high (about 20 grams of water vapor per kilogram of dry air, or 2% of the air's mass).

(page last updated 12/13/2019)
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