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Posted 7/3/2003 6:30 PM     Updated 9/26/2005 2:30 PM
Today's Top Weather Stories

Answers archive: Fog formation
Q: Does fog really "burn off"?

A: Although there are no flames involved, the sun does indeed "burn off" fog by warming the top part of the cloud layer. As the air temperature increases, the cloud droplets that we see as fog evaporate and become invisible water vapor. Because the sun "burns off" the fog from the top down, a thick fog will persist longer than a thinner layer.

Check out these USA TODAY resource pages to learn much more about the formation of fog and the different types of fog.

(Answered by Bob Swanson, USA TODAY's assistant weather editor, August 28, 2005)

Q: I am terrified of driving in fog at night. How can I tell before I leave the house if it is going to be foggy out?

A: Some basic knowledge of how fog forms, and paying close attention to wether reports and forecasts should solve your problem most of the time.

Fog forms when the air cools to a point at which water vapor in it begins to condense into tiny water droplets. I won't get into detail about how this works, but you can learn more by going to some USATODAY.com text and graphics listed at the bottom of this page.

The temperature at which water vapor will begin condensing from the air at any particular time is called the "dewpoint." You don't need to worry too much about understanding dewpoint, just realize that when the air cools to this temperature fog could start forming. It doesn't always form then for various reasons that we don't need to go into.

If you go to the USATODAY.com current weather and forecasts for where you live, and look under "Current conditions" at the top of the page, you'll find the dewpoint measured the last time weather observations were made. It's in both Fahrenheit and Celsius degrees.

To find the report for your city, look on the right side of this page where you'll see a box labeled "More weather." In it there's a box that says enter ZIP or city."

Type in the name of your city, or if you are in the USA, your ZIP code, and click "Go." Weather observations aren't taken everywhere, and we use the nearest observations to each location. If you are outside the USA you might have to try a few locations to find the one nearest you. A place with a fairly large airport is most likely to have weather observations.

Now, the secret: If the dewpoint is within five degrees F or 3 C of the reported temperature and the air is getting colder, as it does overnight, fog could form. This is a general rule of thumb pilots use to know when they might have to worry about fog.

As you probably know, fog is more likely at night, and becomes more likely as the night goes on because the air is usually cooling off and sometimes cools to the dew point. The normal lowest temperature of the day is around dawn or a little later. This is also when fog is most likely. Fog is more likely in low places, especially river valleys where a river can add more humidity to the air, than on hill tops.

If the wind is nearly calm and the sky is clear, fog is mroe likely than on a windy night or a cloudy night.

Using the 5-degree F or less temperature-dew point difference isn't a sure way to forecast fog. In fact, it could lead you to expect fog some nights when it doesn't form. But, I assume you'd rather stay home when there is no fog than go out and be caught in fog.

Another time fog can form is where snow covers the ground, or the ground is very cold, and warm, humid air is moving in. In this case, the snow or cold ground cools the arriving warm air to its dew point.

In case you are wondering why fog isn't likely when the temperature-dew point spread is more than 5 degrees F: Large drops in temperature usually happens when colder air moves in to replace relatively warm, humid air. The cold air will be drier.

Some of the USATODAY.com graphics and text with more information are:

Why fog forms on clear, calm nights

Quick warm up can bring fog

Understanding humidity

Understanding clouds and fog

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