Published Tuesday, June 8, 1999, in the San Jose Mercury News


Special to the Mercury News

Dissipating those rumors that contrails cause illness

Q. There is a new mystery on late-night talk radio about contrails left behind by high flying airplanes making people sick. There was a claim that last month there were these types of contrails reported above the central coast. I'm wondering if the contrail mystery is an easy thing to explain in the field of meteorology. I do watch contrails, realizing that they are something left behind jet airliners, but why do they dissipate and make the whole sky a murky whitish haze? T.D. Boulder Creek

A. This contrail controversy has reached almost epidemic proportion on the Internet, where often opinions are taken as fact. I have seen nothing in the scientific literature to support the thesis that contrails are making us sick.

Contrail is a contraction for condensation trail and is nothing more than the water vapor in a plane's exhaust condensing in the upper atmosphere. This usually occurs above 30,000 feet, where air temperatures are 20 to 40 degrees below zero, and the water condenses into ice crystals.

These crystals are the same as those that make up the thin wispy cirrus clouds we see almost daily. When the winds at these altitudes are light, the clouds spread out and dissipate slowly and can spread into the milky sky you mention.

QWhat do the weather people on the radio mean when they say rain turning to showers? Aren't they the same thing? 
Theresa Heinrich Los Altos

A. This is one of those subtleties that meteorologists take for granted and assume everyone knows. From the number of questions I get related to rain or showers, this is not the case.

It is also complicated by the misuse by members of the media. Rain in the generic sense is any liquid precipitation coming from a cloud and having a diameter larger than drizzle (0.02 inches).

When used in the context of a forecast, it means widespread precipitation that is fairly uniform in intensity and is usually associated with a weather front moving through the area. Conversely, showers are much more localized, and their intensity is very changeable. With showers there might be heavy precipitation on one side of town while the sun is out on the other.

Q. Some 95 percent of minimum temperature records for San Jose and San Francisco were recorded mainly between 1900 and 1937, except of course for 1972 and 1990, which claimed a few, and the majority of minimum temperature records seem to have been set in the last 30 years.  San Jose had a maximum temperature twice in July about 80 years ago of only 55 degrees. It seems impossible for that ever to happen again unless we had a deluge falling all day. Is the cause of this all the urban building
around the sites causing heat? It seems at airport locations we would not see this happening. Dennis Horne  San Jose

A. Unfortunately there is a lot of contamination of the temperature records in cities by the effect known as urban heat islands. (NASA has some dramatic images online at

And yes, this would be more pronounced in urban centers as opposed to airport locations. The concrete, asphalt and buildings absorb lots of heat during the day. Some is reradiated to make the days warmer, but more is released at night to keep the minimum readings higher. The difference from this effect alone can be 10 degrees or more.

This process is important to consider when looking at global warming, so that you are measuring the change in climate, not the amount of urbanization.

WEATHER CLASSES: Want to know even more about meteorology? Your Weather Corner columnist will be teaching two introductory classes in the fall semester at San Jose State University and San Francisco State University. The San Jose class will be Meteorology 10 (weather and climate) on Tuesday nights at 7 p.m. At San Francisco State, I will be teaching Meteorology 302 (violent atmosphere and ocean) on Wednesday nights at 7 p.m. Neither class has prerequisites, and both are available through the
respective campuses Open University program.

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