What's a "dew point"?


My husband and I went out to eat about 6pm this eve.(Thurs). We finished about 7PM and then we came out to our car we noticed that the dew had already fallen. Why was the dew-fall so early this evening?


Good question.  In simplest terms, "dew fall" occurs when the air temperature and the dewpoint temperature are equal.

Remember, the dewpoint temperature is the temperature at which the air becomes saturated (relative humidity becomes 100%). Saturation is the point at which the air is holding all the water vapor that it can hold.

On most days, the dewpoint does not change very much, while the air temperature may change 20-30 degrees.

The air temperature is always greater than or equal to the dewpoint temperature.  When the air temperature is greater than the dewpoint, the relative humidity is less than 100% -- the greater the "spread" between the air temperature and the dewpoint temperature, the lower the relative humidity.

If the air temperature becomes equal to the dewpoint, the air is saturated (holding all the water vapor that it can hold), and the relative humidity becomes 100%.  Any additional cooling means that BOTH the air temperature and the dewpoint temperature must fall together.

But to lower the dewpoint temperature, we must "squeeze" water vapor out of the air -- the easiest way to do this is to allow condensation (water vapor becomes liquid water) . . . the result: dew, and sometimes fog (fog and clouds are composed of microscopic droplets, since water vapor is invisible).

For us, air with high dewpoints is typically air from the Gulf, delivered by south winds.  Air with low dewpoints ("dry" air) comes from the north, often originating in Canada.

On most afternoons and evenings, the air temperature and dewpoint are far enough apart that an extended period of overnight cooling is required before they can become equal.  Hence, dew is most common in the morning, and unusual-to-rare in the afternoon.  But in your instance, the air was so "moist" (high dewpoint) that the evening cooling was sufficient to start the "dew fall" process much earlier.

One other thing adds to the process of dew on your car and some "hard" surfaces.  As you know, the surface of the car heats up during the day much faster than the air -- the metal and glass can be 20-50 degrees hotter than the air in mid-afternoon.  Well, the metal and glass also cool much faster than the air, and can be several degrees lower than the air temperature after the sun goes down.  So the cooler surface acts like a "refrigerator" for the air immediately in contact with it.  So, while the "official" air temperature may be in the 60s, the air in contact with the car, a cold pavement, etc. may be "chilled" to or below the "official" dewpoint by the cooler surface, causing that thin layer of air to condensate and depositing a layer of "dew" on the colder surface.

With such an early start to the condensation process, if the winds remained very light through the night, you might have seen a thickening layer of "saturated" air forming -- the formula for ground fog.

Sorry about the long-winded reply . . . hope this helps!

Jay Grymes
Chief Meteorologist
WAFB Storm Team