NOAA 2002-080
Contact: John Leslie
NOAA News Releases 2002
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Today marks the official start of summer, and NOAA's National Weather Service announces a new technique to warn citizens across the nation of advancing heat waves up to seven days before their onset. The Commerce Department's National Oceanic and Atmospheric Administration developed the Mean Heat Index, which became fully operational in May, to help save lives through better preparations.

The Mean Heat Index is a measure of how hot the temperatures actually feel to a person over the course of the day. It differs from the traditional heat index in that it is an average of the heat index from the hottest and coldest times of each day. It combines factors such as surface and ambient heat with humidity and other environmental factors.

"Heat waves often turn fatal when the nighttime temperature doesn't drop very much from a high daytime temperature," said Jim Hoke, director of NOAA's Hydrometeorological Prediction Center in Camp Springs, Md., where the Mean Heat Index originates. "The Mean Heat Index captures this potentially serious condition by including data from what should be a cooler portion of the day, and factoring that in to give a ‘big picture' of the day's temperatures, not just the day's high."

According to NOAA scientists, a Mean Heat Index above 85 degrees is considered dangerous. NOAA Weather Service statistics show heat-related deaths outpace fatalities in other severe-weather categories. For example, based on a 10-year average from 1992-2001, excessive heat claimed 219 lives each year. By contrast, floods killed 88; tornadoes, 57; lightning, 52; and hurricanes, 15.

"Through improved heat-wave forecasting, plus greater public awareness and education, hopefully these numbers will begin to decline soon," Hoke said. "Heat waves kill with silence. Intense heat can creep up on its victims, because it doesn't have the loud, crash-and-bang of a hurricane or tornado. Its average death toll, however, is much worse."

Today's Mean Heat Index warns of hazardous temperatures in the upper Mississippi Valley and Great Plains beginning Monday and Tuesday.

Alerts and mean heat indices are issued to the public in two ways. One is a daily, graphical representation of the contiguous United States with color-coded overlays indicating the temperatures the Mean Heat Index is expected to reach or exceed in each area or region of the country. Secondly, the agency issues a more localized, text forecast of the Mean Heat Index, available for 90 cities across the country. These products are sent out in NOAA's National Weather Service suite of extended range forecasts.

Last week, the agency released its outlook for summer, calling for above-normal temperatures for states in the West and Southwest through September. The rest of the country has an even chance of feeling above, below or average heat.

Heat waves in America have a deadly legacy. In 1980, a stifling heat wave killed 1,700 people in the East and Midwest; another East/Midwest heat wave killed 454 in 1988; in 1995, a heat wave claimed a total of 716 lives in Chicago, Philadelphia, Milwaukee and St. Louis, and in 1998, a heat wave killed more than 120 in Texas alone.

The Mean Heat Index, Hoke said, gives local health and emergency officials advanced warning when a prolonged period of dangerous heat approaches. "Having more time to warn the public means increasing the chances of saving lives."

NOAA's National Weather Service is the primary source of weather data, forecasts and warnings for the United States and its territories. NOAA Weather Service operates the most advanced weather and flood warning and forecast system in the world, helping to protect lives and property and enhance the national economy.

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