This image, taken by the MODIS sensor on the Aqua satellite, shows the temperatures of California and Nevada during a spring heat wave in May 2004. MODIS (Moderate Resolution Imaging Spectroradiometer) measures the land temperature, which can get much warmer than the air. Notice that the tops of the Sierra Nevada mountains, which were still capped with snow, remain cool, forming a blue line at the California-Nevada border.
Click on image for full size
Courtesy of NASA

Heat Waves

During a heat wave it’s hot outside – much hotter than normal – and the heat lasts for days.

Some heat waves last for a week or more. Other heat waves last only a few days. The temperatures during a heat wave are much hotter in places that are usually warm. A heat wave has lower temperatures in places that are usually cool.

Beware! Heat waves are dangerous. The hot weather can be hazardous to your health. It can cause heat stroke, heat exhaustion, and heat cramps. Stay inside or in the shade during a heat wave and drink lots of water to avoid getting sick. Heat waves are also dangerous to plants. They cause crops to fail and can help start wildfires in dry areas.

How do they form? Sometimes, the jet stream, a flow of air through the mid-latitudes, can bring unusually warm air into an area. If the warm air stays put for a while, it can cause a heat wave. The heat is able to persist when there aren't rain and clouds to cool things off. The heat-trapping ability of cities, known as the urban heat island effect, can make a heat wave even warmer.

There are more heat waves today that there were in the past. The number of heat waves has risen, especially in Europe and Asia, and heat waves are expected to become more common during the this century.

Last modified July 15, 2009 by Lisa Gardiner.

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