Where does fog come from?

Fog can not only ruin a day at the beach, but it can also cause dangerous conditions for motorists and keep planes from landing and taking off.

Fog is essentially a low cloud and can either form in place or physically move into an area.

When fog forms in place as air cools when the sun is going down or has set, it is known as radiational fog.

Radiational fog typically forms during the late-night and early-morning hours and is most common during late summer to the middle of autumn. As daytime heat leaves, or radiates, from the ground, the air becomes moist, creating fog.

However, radiational fog can form during any time of the year when winds are light and the air near the ground cools to the saturation point. It can occur in the spring and summer soon after a rain during the late-afternoon or evening hours.

Examples of radiation fog are river valley fog in the Northeast and Tule fog over California's Central Valley.

It is common for radiational fog to form in low-lying areas or in the river valleys first and then to increase in size and reach higher elevations as the night progresses.

The longer the night, the more extensive the radiational fog is likely to become. The lower the sun angle is during the day, the longer the fog is likely to linger after sunrise.

Another common type of fog, advection fog, forms when warm, moist air moves horizontally across a cooler surface. Sometimes fog of this nature moves from one location to the next.

The most common location for advection fog to to form and move into a region is over the ocean and adjacent land areas.

In this case, the ocean or bay water has to be cold enough to cool the warm, moist air to the saturation point.

Fog of this nature occurs often along the California coast and is most common over Southern California during the late spring to midsummer.

However, fog can be pushed around when a light breeze is present. In the case of California, rising warm air over the interior western United States creates a vacuum and draws the moist air in from the Pacific Ocean.

Another example of advection fog occurs during the winter when the ground is cold and may have an extensive amount of snowcover on it. Warm, moist air may be pushed over the cold ground and snow, which causes the moisture in the air to cool and condense.

Storms that bring mild, wet weather following a cold and snowy outbreak can produce thick advection fog. Thick advection fog that forms over cold ground or snowcover can be widespread and very disruptive for travel.

Other types of fog include steam fog, upslope fog and freezing fog.

Steam fog occurs when cold air passes over warm water and causes that air to become moist enough to create a shallow light cloud to form. Another name for this is lake or sea smoke.

Upslope fog occurs when moist air flows up a large hill or mountain and cools and condenses to form a cloud. This is why sometimes the visibility may be good in a valley but poor over the ridges and passes during a rainstorm.

Freezing fog can occur with any of the aforementioned situations and occurs when the temperature is at or below 32 degrees Fahrenheit. A thin layer of ice can form on moist surfaces, which adds to the hazard of poor visibility.

Dense fog in absence of any ice can lead to dangerous conditions for motorists when traveling on the highways. Some of the worst multiple-vehicle accidents have occurred during dense fog outbreaks.

When fog occurs at a major airport, arriving and departing flights must be much more spaced out, which can lead to substantial delays.

Sometimes, the weather may be clear at an aircraft's departure point, but dense fog may be present at the destination airport. This situation may keep that aircraft grounded until the weather clears or is expected to clear at the destination.

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