National Weather Service changes heat advisories and warnings

Monday, June 9, 2008 | 10:12 p.m. CDT; updated 7:57 a.m. CDT, Tuesday, July 22, 2008

COLUMBIA — Summer in Mid-Missouri can mean drenching humidity and sometimes, dangerous heat. But how hot is too hot?

This summer, the National Weather Service has new criteria for issuing excessive heat advisories and warnings.

Temperatures exceeding a heat index of 105 or an actual air temperature of 100 degrees are now considered cause for an excessive heat advisory, which indicates temperatures can be dangerous if people don’t take necessary precautions. When the heat index rises to 110 degrees for two or more days, or night heat indices don’t go below 75, an excessive heat warning is issued. Warnings are also issued if advisories remain in effect for four or more days.

The warning is more serious and means that the temperature is an imminent threat to health, said Jim Kramper, warning coordination meteorologist for the National Weather Service in St. Louis.

“We made it easier for us to issue a heat advisory,” Kramper said. The new criteria standardized heat advisories across large areas while staying flexible for localities, he said. He said the old heat guidelines were “piecemeal” and could differ greatly among weather service offices in the Midwest.

While the new criteria pertain to a large area of the United States, they are not the same nationwide. Weather services in urban areas have the option of issuing their own heat warnings. Large populations and increased amounts of concrete and asphalt combine to trap heat and augment temperatures in urban areas.

Heat guidelines are also more stringent in mountain states like Colorado, where much of the population is unaccustomed to the higher summer temperatures.

Steven Zweig, professor and associate chair of MU Family and Community Medicine, said making the heat guidelines more conservative makes sense.

“I think it’s a good idea because we have such high humidity in this part of the country,” Zweig said. Humidity, in addition to heat, is responsible for many summer health problems in Missouri, Zweig said.

Children, the elderly, those with poor housing and the physically active are especially at risk when heat indices rise to advisory and warning levels. These groups may not think of caring for themselves when temperatures rise or have underlying conditions that make them more susceptible to heat-related illness, Zweig said. Those older than 65 are 12 to 13 times more likely to get heat stroke and may take prescription drugs, which prevent the body from cooling itself. Small children may not drink enough water or think to wear light clothing. Those with poor housing may not have access to air conditioning. The physically active may not realize the need for proper hydration and rest.

“Healthy and active adults have much more capacity to deal with heat and often they’ll push themselves past the point that many people would stop. On a hot day during heavy exercise if they don’t drink two to four glasses of something every hour they can get dehydrated and go into shock,” Zweig said. He also cautioned against spending extended periods of time outdoors trying to acclimate to high outdoor temperatures.

“You’re not going to make yourself stronger by spending a lot of time in the heat and humidity,” Zweig said. “You just basically put yourself at higher risk.”

Both Kramper and Zweig agreed that there are many effective ways to stay safe when a heat advisory or warning is issued. Staying in the shade or air conditioning, eating small meals and wearing light clothing can help beat the heat, but proper hydration is the most important precaution to take against hot weather.

“One mechanism of cooling is sweating. If you don’t replace fluids, the body loses volume,” Zweig said. “Water is the most important replenisher.”

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