Published Tuesday, June 20, 2000 in the San Jose Mercury News


Extreme heat most deadly weather in United States

Special to the Mercury News

With last week's record hot temperatures still fresh in our minds and summer officially arriving at 6:48 p.m. today, let's examine the issue of heat and its health effects.

Q.  Just how hot was it in San Jose last Wednesday?

A. The reading for San Jose downtown was a toasty 109 degrees. This is the hottest official reading for any day of the year in San Jose's weather records, which go back to 1905. Last Wednesday was only the 10th time the mercury has reached 105 or greater in San Jose.

Q. How hot was it in other cities around the area?

A. Moffett Field set an all-time record with 106 degrees. Downtown San Francisco tied its record of 103. Other readings last Wednesday included King City, 112; Gilroy and Fremont, 108; Hollister and Livermore, 107; and Santa Cruz, 90.

Q. How come my neighbor's thermometer said it was 117 degrees?

A. I'm sure some of the outlying neighborhoods could indeed have been a couple of degrees hotter that the sites noted above, but any variation beyond that is most likely due to measuring errors. Official temperatures are taken over natural surfaces such as grass or dirt, in the shade, and in
well-ventilated areas away from buildings or other objects that would give off heat. Extreme readings are often the result of taking a temperature in the direct sun or in a location that has not been ventilated to capture a representative sample of air.

Q. How dangerous is severe heat?

A. With 144 deaths per year, heat is the most deadly weather phenomenon in the United States, according to the National Weather Service. Flooding comes in a close second, with 143 annual fatalities. For comparison, lightning kills 79 people per year, tornadoes 69, cold 38 and hurricanes 24. From 1936 through 1975 almost 20,000 people died from the heat.

These fatalities are only the direct casualties. There are undoubtedly many more deaths indirectly related to hot weather; in the severe heat wave of 1980, for instance, nearly 1,700 people died from heat-related illness, according to the Centers for Disease Control and Prevention.

Q. What does the heat index tell us about the heat?

A. The screaming message from the heat index, also called the apparent temperature, is that the human body can deal with heat better if the humidity is low. The index is basically a measure of what the weather feels like and how efficiently the body can cool itself. It's the summer equivalent of wind chill.

When the air is humid, sweat cannot evaporate as quickly and the body can't cool itself efficiently, so you feel hotter.

For example, if the temperature is 100 degrees and the relative humidity is 50 percent, the heat index is 120 degrees.

Interestingly, last Wednesday the relative humidities were quite low, with readings around 20 percent, and the heat index was actually a couple of degrees cooler than the air temperature. This indicates that people's bodies would have been very efficient at cooling themselves by perspiring.

Q. How much hotter is it in my car when it's parked in the sun?

A. Studies have shown that readings in an enclosed automobile can rise to almost 50 degrees above the air temperature. In one study from the Louisiana State Medical Society, the temperature in a dark-colored car was 140 degrees while the temperature of the air outside was only 93. Cracking the windows open a couple of inches cools a car only slightly.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o WeatherCorner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone questions at (510) 657-2246, fax them to (510) 315-3015 or e-mail them to