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"Fog forms when the air cools to a point at which water vapor in it begins to condense into tiny water droplets. 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 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 more 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.

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.

Information taken from:



This wouldn’t seem to indicate regular fog, but there’s another type of fog that forms when a warm humid front moves in over cold ground, called (if I remember correctly) avective fog. Apparently this doesn’t follow the rule of being within 5 degrees of the dewpoint…?