Anne Costigan, age 10, of Lansing, Mich., oar her question:

What causes fog?

A fog is actually a grounded cloud, sitting on land. or water. It is made of the same misty material which makes a cloud. This gauzy, hazy material is caused by countless tiny droplets of water floating in the air, Fogs and clouds are both formed from water vapor in the air. They form because warm air can hold more vapor than cool air. When warm moist air cools, some of its vapor becomes misty fog or a cloud.

Warm air is always thirsty. It soaks up moisture from the seas and rivers, from lakes and little puddles. On a summers day, a single apple tree pours 150 gallons of moisture into the air. All this moisture in the air is in the form of vapor, which is a gas mingled with the other invisible gases in the air. The hotter the day, the more vapor the air can hold.

Lets measure the vapor in one cubic meter. When the temperature is 86 degrees Fahrenheit the box of air can hold 34 grams of moisture, or a glass full of water. When the temperature is at 60 degrees, the same volume of air can hold only 13 grams of moisture. At freezing point, it can hold a little less than five grams of moisture.

Now lets see what makes warm moist air turn cool. Choose a clear night with gentle winds. The warm air near the surface of the ground is churned up and mixed with cooler air above. This reduces the temperature of the lower air and forces it to give up some of its vapor. The vapor becomes tiny droplets of liquid water floating in the air. This fog tends to drift down slopes and settle in hollows and valleys.

Other conditions can cause a fog to form on a clear night. During the day, the ground has gathered up warmth from the sun. But after sunset, this heat escapes, cooling the air near the ground.

If this air contained its quota of vapor, fog would form as the temperature dropped. This is called a radiation fog.

Fog may also form when warm winds blow over icy seas and frozen lands. Here the cool surface chills the air above and if this air is loaded with vapor, some of it may be turned into misty fog. This is called advection fog. Frontal fogs form at weather fronts where two different air masses meet and clash. The warmer air mass will produce rain which adds moisture to the cooler air mass. Here the temperature does not change but extra moisture is added. Some of this extra moisture may become fog.

Some regions get more than their share of misty fogs. One of these fog bound regions is off the coast of Labrador: Here the warm Gulf Stream clashes with a chilly ocean current from the Arctic. ?4arm and cool air masses above the ocean currents clash to form fogs. Fogs also tend to form around the British Isles and along the coast of California.