Published Tuesday, July 15, 2003 in the San Jose Mercury News

Weather Corner

Risk of heatstroke climbing with summer temperatures

Summer weather means more than pleasant outside activities. Unfortunately, it can also be deadly.

In the United States, heat is the No. 1 cause of weather-related deaths, with an average of 235 deaths per year. That's far more than the second-place cause of weather-related deaths, floods, which kill 86 people a year.

Heat affects the human body when it can't sufficiently cool itself. This can cause heatstroke and sometimes death due to hyperthermia. This condition is exacerbated in children and the elderly, whose thermal-regulating systems are less efficient.

All too often, children are left in vehicles on warm or hot days. Already this spring and summer, 17 children in the United States have died after being left in hot cars.

Recent studies have shown that even on days when the outside air temperature is as cool as 70 degrees, the interior of an enclosed vehicle left in the sun can reach lethal temperatures of 105 degrees.

The interior of a car or van warms very rapidly on a sunny day. Even cracking the windows is not enough to significantly mitigate temperature increases. As sunlight enters a car, it heats up the interior surfaces, such as the upholstery and the dashboard, to temperatures that can easily exceed 150 degrees. In turn these objects warm the air inside the vehicle, which gets hotter and hotter because the air is trapped inside the vehicle.

In a study of 16 cases last summer on days when the outside air temperatures ranged from 72 to 96 degrees, the interior of a test vehicle reached readings of 112 to 140 degrees after an hour. The average temperature increase inside after just 10 minutes was 19 degrees, then 29 degrees after 20 minutes and 34 degrees after a half hour. After an hour, the temperatures slowly stabilized between 45 and 50 degrees above the outside air temperatures.

The same study found that the effects from either cracking the windows or having a larger lighter-colored vehicle were not significant enough to keep temperatures from reaching deadly levels.

For details on the study, see

Q I hear about hurricanes and tropical storms picking up water from the Gulf of Mexico and the Caribbean, which are both salt water. But then I hear about all of this rain falling that is fresh water. How does it get converted? Joe Dean - Cupertino

A This is not really an issue of the conversion of salt water to fresh water, but of poor communication by meteorologists.

Most of the heat and water vapor that feed a hurricane are the result of the warm tropical water over which they form. This water, which is at least 80 degrees Fahrenheit, warms the overlying air and makes it very humid -- but all of the salt remains in the sea.

Then rising air inside the hurricane cools and condenses, forming massive complexes of cumulonimbus clouds that produce copious amounts of rainfall.

Q What is the reason it can be nearly 100 degrees inland (in Santa Rosa, for example) and yet, just 20 miles west at the coast, it can be some 30 degrees cooler? Why doesn't the cooling from the coast travel inward? Christy Somers - Santa Rosa

A Ah, the joys of the Bay Area's microclimates, which are caused by the juxtaposition of the Pacific Ocean and sharp terrain features.

The cool air does travel inland as the sea breeze, but as it travels over the warm land, it begins to warm as well.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o Weather Corner, 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 or fill out a form online at