What is the dew point, and how does fog form?

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By Scott Sistek

Story Published: Oct 4, 2006 at 1:13 PM PST

Story Updated: Dec 21, 2009 at 8:59 AM PST

There are two main types of fog, and the Northwest gets both varieties. And they both are directly related to the dew point.

The dew point is the temperature at which the air becomes 100% saturated. At that point, the air condenses into water droplets, which we see as fog.

The lower the dew point, the drier the air, and vice versa.

If you have a temperature of 80 degrees and a dew point of 60, that feels quite humid. But if it's 80 degrees and a dew point of 45 degrees, that's much drier and more comfortable.

In the southeast and Midwest summers, dew points can range from 60-70 degrees, which is very humid and feels uncomfortable.

But around western Washington, we tend to be between 45 and 55 degrees in summer, which keeps our summers very comfortable -- even on warm days.

And you've heard the term for the desert that "it's a dry heat." In the Desert Southwest, sometimes dew points are in single digits -- or even negative degrees (We've seen it where it'll be 68 degrees with a -4 dew point in Phoenix.)

So What About Fog?

As we mentioned earlier, fog happens when the temperature and dew point are equal or within a degree and the air becomes saturated.

The fog comes in two varieties:

Ground fog usually begins on clear, calm nights. But, first, during the day, you need the sun to warm the ground.
 

Then, as night falls and the temperature drops, the air cools to where it meets the dew point and becomes totally saturated to where fog forms.

 

This fog is more common in valleys because since cold air sinks, it tends to pool in low-lying areas and makes it easier to reach saturation point.

A breeze will hinder this type of fog development though, as it will mix in the drier air from higher altitudes. Cloud cover also goes against ground fog development, as the clouds act like a blanket that keeps heat from radiating away and keeps it warmer near the ground.

Advection fog is created when a warm air mass moves over a colder area.

This is common here in spring and summer. Out over the Pacific Ocean near the coastline, warm air reacts with cooler air near the ocean water. That cools down the warm air and brings it toward the dew point, where it condenses, creating fog.

It's a near-daily occurrence on the coast, but that fog and low clouds can get carried into the Puget Sound region by an overnight marine push (See: "What Is A Marine Push?"), allowing our days start with fog or a low overcast.

The fog burns off as the sun begins to warm the top of the fog layer, pulling the temperature higher than the dew point and evaporating the fog (thus the term, "burns off.")

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