Lisa Jean Neal, age 10, of Enid, Oklahoma, for her question:

What causes our fogs to last so long?

Filmy fogs can form almost anywhere, though they are very rare in large, arid deserts. Their density and duration depend upon the whimsical weather. And everywhere the weather is governed by the yearly seasons, by global winds and to some extent by local geography. Most fogs are local events that depart by noon. But certain coastal fogs blanket the land and sea for a week or more. When the weather conditions are just so, even an inland fog can hang around for several days.. However, as a rule it thins out for a while under the noonday sun.

Fogs form from evaporated moisture that adds invisible water vapor to the air. They may disappear when the temperature suddenly rises, especially when a warm strong breeze blows into the local weather picture. When a fog hangs around for a long time, as a rule the air stays cool and calm. In late winter or early spring, such foggy weather conditions can occur in parts of Oklahoma.

The city of Enid lies between the Salt Fork of the Arkansas River to the north and the Cimarron River to the south. These and other big rivers gather numerous streams and flow eastward to join the mighty Mississippi. They drain large areas of the plains and prairies that slope gently down from the west to the great Mississippi

Missouri Valley. The fall and winter months may bring 20 inches of rain and snow to this region, and perhaps extra snow to the Panhandle. However, the mild weather does not freeze the ground in a long, icy grip.

The melted moisture soaks into the soil and runs away with the streams. In late winter and early spring, the mild air evaporates water from the swollen streams and the damp dirt. This moisture becomes invisible water vapor, mixed with the air's other invisible gases. There it must obey the weather regulations that allow warm air to hold more vapor than cooler air. When saturated warm air drops 20 degrees Fahrenheit, it must give up about half of its vapor content. If warm, moist air chills as it rises, its surplus moisture forms clouds. When it chills at ground level, its surplus vapor becomes liquid droplets of misty fog.

In January, the average high temperatures of Oklahoma range from 44 degrees to 56 degrees Fahrenheit. These balmy spells alternate with g,ightly frosts and cool to 34 degrees Fahrenheit. At this time of year, the westerly winds bring moist air from afar and temperatures often swoop down to convert this moisture into filmy fog. When the air stays calm sad cool, a fog may last until everybody is throughly sick of it.

A fog of this sort can happen wherever the geography, the season and the temperature alternates between mild to cool. Usually more masses of moist air seep in to keep it going, though they move too slowly to be called breezes. A misty fog changes everyday scenery into a veiled wonderland of mystery, so let's admire it while we can. It departs when the temperature rises or a stiff breeze blows it away. As a rule, even long, dense fog takes a short break during the lunch hour.