Published Tuesday, April 23,  2002 in the San Jose Mercury News


Temperature in closed vehicle rises quickly

Special to the Mercury News
Last year, 32 children who were left unattended in vehicles on hot days died of hyperthermia or heat stroke in the United States.

The toll is only slightly lower than the 39 people who were killed by tornadoes.

When the outside air temperature is 72 degrees, the temperature inside a car can rise to more than 90 degrees in five minutes and more than 115 degrees in 45 minutes. I took those measurements with the thermometer out of direct sunlight. The thermometer reached 154 degrees in direct sunlight.

In general, temperatures inside a vehicle rise by about 30 degrees above the outside temperature after 20 minutes and by 40 degrees after a half hour. Humidity can increase dramatically inside an enclosed vehicle, further exacerbating the ability of the human body to cool itself.

The color of the car and whether the windows are cracked open a couple of inches appear to make little difference.

Heat is the most deadly meteorological killer in the United States, with an average of 206 fatalities a year in the past decade.

During the period 1991-2000, there were an average of 89 flood-related deaths a year. Tornadoes killed an average of 57 people annually, lightning killed an average of 55 people, and hurricanes caused 14 deaths a year.

The students in Genevieve Deppongs' seventh-grade class at St. Raymond School in Menlo Park submitted the following questions.

Q Why does lightning strike trees more than anything else? Spencer D. - Redwood City

A Lightning strikes not only the tallest objects but also those that provide the path of least resistance such as towers and tall buildings. Trees are hit more often because they cover more ground than towers or tall buildings. To see what happens when a tree is hit by lightning, go to

Q Are cars the safest place to be when there is lightning? Jenna S. - Redwood City

A The safest place is inside a wood-frame building, away from windows and electrical appliances, including the telephone.

Q Other planets have clouds. Do they also have lightning? Ashley M. - Redwood City

A Cameras on spacecraft from the National Aeronautics and Space Administration have detected lightning in the gargantuan storms that rage in Jupiter's violent atmosphere. Lightning also has been detected on Venus and Uranus.

Q Why are there different colors of lightning? Elias H. - Redwood City

A The strong electrical discharge of a lightning stroke excites various gases in the atmosphere that glow in different colors.

Q Does lightning strike from top to bottom or bottom to top, or both? Melissa D. - Redwood City

A Lightning travels up and down. The lower portion of a thunderstorm develops a ``stepped leader'' or line of negative charges moving from the cloud toward the ground at 240 miles per second. It draws a streamer or line of positive charges up from the ground.

They meet to complete an electrical connection between the cloud and the ground. Negative charges rush from the cloud toward the ground, causing the air to glow bluish-white.

The bright lightning flash, called the return stroke, moves up from the ground at 61,000 miles per second. This super-heats the air, which expands explosively to create the shock wave we hear as thunder.

Q How many objects can a single bolt of lightning strike? Doug D. - Menlo Park

A The huge electrical discharge can pass through several objects and even jump from one to another. Electricity follows the path of least resistance through objects such as metal that conduct it best.

Q How do charges get into a cloud to cause lightning? Matthew V. - Menlo Park

A Lightning requires a separation of negative and positive charges, a normal condition in cumulonimbus clouds. The base of the cloud is negatively charged; the upper portions are positively charged.

The most popular theory is that this separation is caused by collisions between hailstones and ice crystals within the cloud. Hailstones first collide with liquid water, which freezes onto the hailstone. This releases energy that slightly warms the hailstone's surface. When the hailstone next contacts a colder ice crystal, positive ions move from the stone to the crystal because energy is transferred from warmer to cooler objects.

The hailstones become negatively charged, and the ice crystals become positively charged. Larger liquid water drops also develop negative charges.

The heavier hailstones and water droplets, with negative charges, sink into the lower portions of the cloud. Lighter ice crystals, with the positive charges, migrate to the upper regions.

Q How are airplanes affected by lightning? Brian M. - Redwood City

A Airplanes have special wiring that carries electricity from lightning strikes away from critical parts. If this fails, the most severe effect usually is the destruction of the electronic equipment.

Greater storm hazards are strong winds that can cause an aircraft to crash, large hailstones that can clog an engine or ice that can accumulate on the wings.

For a cool picture of an aircraft being struck by lightning see:

Q If lightning hits a swimming pool, what would happen to the swimmers? Jay K. - Redwood City

A Swimmers in pools and even people in small boats on lakes have been electrocuted. People should get away from the water when thunderstorms approach.

Q Why don't people make houses that resist lightning? Connor F. - Atherton

A We seldom get lightning in the Bay Area. However, in areas where lightning is common, lightning rods direct the electrical discharge safely around the building to avoid damage. This is more practical than totally insulating a house from the ground.

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 send questions via telephone (510-657-2246), fax (510-315-3015) or e-mail