Published Tuesday, September 28, 1999, in the San Jose Mercury News


Special to the Mercury News

Q. How about a review, with the cold season coming up, of wind chill? For example, with an outside temperature of 30 degrees and a wind of 30 mph, what is the wind chill factor? I guess that's how your skin perceives the temperature? Would the wind make your fingers freeze more quickly than at 30 degrees and no wind?  William Dixon -  San Jose

A. The wind chill factor (also called the wind chill index) is based on the estimated rate of heat loss from exposed skin caused by the combined effects of the wind and cold. As anyone who's been outside in inclement weather can attest to, temperature is just one of several factors that determine outdoor comfort. Precipitation, sunlight and wind all play important roles.

The original work on wind chill was done by antarctic explorers in 1941 by measuring the amount of time it took a pan of water to freeze. They found that the rate of heat loss from the container could be determined from the air temperature and wind speed. This methodology is somewhat controversial because humans are considerably more complex than a pan of water and respond differently depending on age, size, health, degree of physical activity, etc. However, the wind chill index can still be a useful rule of thumb for making decisions about outdoor activities (see wind chill chart at right).

These are the possible effects of wind chill:
15 degrees to 30 degrees: cold and unpleasant.
Zero to 15 degrees: very cold, very unpleasant.
Minus 20 degrees to zero: bitter cold with frostbite possible.
Minus 20 to minus 60 degrees: extremely cold; frostbite is
likely and outdoor activity becomes dangerous.
Minus 60 degrees or colder: frigidly cold. Exposed flesh will
freeze within 30 seconds.

Q. During our recent spectacular lightning show I noticed that sometimes there was a greenish tint to the lightning. Can you explain this? Lucy Baldwin - Palo Alto

A. Clouds often take on a greenish hue before severe storms, but this is most often associated with hail. Hail is usually part of a thunderstorm. These tall, dense cumulonimbus clouds often block most of the sunlight, so the greenish tint may be a reflection of Earth's green foliage. However, the cause of the green tint has not been proven conclusively.

Q. What produces turbulence during flights? Can it be anticipated by computer or detected by other equipment? Laura Betina Juarez - San Francisco

A. The turbulence that affects aircraft is most often caused by wind shear in the upper atmosphere. Called clear air turbulence, it results when the wind is blowing at different speeds at adjacent altitudes. These wind speed differences allow pockets of rising or sinking air that can cause a plane to rise or fall hundreds of feet.

Turbulence can also be caused by thermals of warm, rising air, strong winds blowing over a mountain range and in the air
stirred up in the wake of jumbo jets.

Q. What causes ``Indian summers?'' Why is it hotter during an Indian summer than during the normal summer period? What is
the average length of an Indian summer? Catherine Trisno-Hadijanto - San Francisco

A. The term ``Indian summer'' is technically defined as an unseasonably warm period in mid- to late autumn. In New England, where the term probably originated, the warm period must follow a killing frost, and preferably after a substantial cool period. In California, Indian summer is generally any warm spell after the beginning of autumn. Indian summers do not necessarily occur every year. And there can be multiple events in any year.

Q. Is there any relationship between earthquakes and weather? Joe Nardini - Fremont

A. No. Wind, precipitation and temperature affect only the surface and the shallow subsurface of Earth. Earthquakes originate many miles underground, well below the zone affected by weather. Earthquakes occur in all types of weather, in all climate zones, in all seasons and at any time of day.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone and fax them at (510) 657-2246 or e-mail them