John Murar, age 13, of Campbell, Ohio, for his question:

What causes London fogs?

There was a time when a pea soup fog could bring the great city of London to a standstill. At its worst, the sooty mist blanketed the noonday sun and turned day into blackest midnight. A person on the sidewalk could not see his hand in front of his face, or the street lamp directly over his head. There are still dense London fogs, but much of the midnight blackness is banished because the Londoners, with their customary calm, politely demanded a stringent reduction of their sooty, smoggy, air pollution.

Several different weather systems give birth to fogs. But most fogs occur when masses of cool dryish air tangle with masses of mild moist air. There are certain geographical regions where mild moist air is likely to meet cool drier air. These are the mist prone places where extra dense fogs often occur, either at certain seasons or on and off through most of the year.

One of these foggy zones lies along the coast of England and includes the port city of London. Another foggy belt nestles along the coast of California, where ethereal mists twine filmy scarves around the Golden Gate. One of the world's worst fog belts is off Newfoundland, where it often menaces the North Atlantic shipping lanes.

These famous foggy frontiers lie where masses of mild moist ocean air and masses of cooler drier air frequently meet at ground level. This sort of impact condenses invisible gaseous water vapor into liquid droplets of visible fog. It occurs because the moisture saturation of the air is governed by temperature. When air becomes warmer, it can hold more vapor. When it cools, it has a lower saturation temperature and its surplus vapor is converted into liquid droplets of misty moisture.

On a global scale, air masses sweep along with the prevailing winds, absorbing various amounts of warmth and moisture from the land and sea. Those famous London fogs lie along a weathery boundary, where masses of mild ocean air are likely to brush against stodgey masses of cold air.

In winter, great masses of cold air tend to squat on the continent of Europe. Meantime the mild Gulf Stream is wending its way up from the tropics and across the North Atlantic. Its warmish water warms the air above it and the warmed air absdrbs vaporous moisture from the sea. The warm moist air brushes the coastal region of northwestern Europe. The weathery confrontation cools the mild ocean air and condenses its surplus moisture into cloudy ground fogs.

The same weathery confrontation also befogs numerous villages, small towns and miles of open country. Here there are fewer people to notice and less smoggy smoke to add darkness and density to the basic filmy white fogs. In London, there are reasons why the fogs are more famous. There are more people to notice them. And even though the old pea soup pollution has been reduced, there is bound to be enough city soot to make a London fog something special.