As if it weren't bad enough to report 100-degree temperatures, thanks to the heat index weathercasters are quick to add that it actually feels even hotter.

Isn't that adding insult to injury?

In a way, yes. But the National Weather Service uses the index — tailored to local conditions in some cases — as a way of warning people that life-threatening conditions exist.

Heat claims an estimated 1,500 lives annually in the United States, making it the top weather-related killer, said Mark Tew, a senior meteorologist at the Weather Service.

"Heat is an underrated killer," Tew said Thursday in a telephone interview. It may not always be listed on the death certificate, but it may have been hot weather that brought on a heart attack, for example, he said.

The Weather Service's heat index uses a formula to calculate the effect of heat and humidity on the human body.

Through normal activity, people are constantly generating heat. Any excess is radiated away from the skin. That works fine up to about 80 degrees Fahrenheit, then sweating may begin, to improve the cooling process by evaporation.

But the higher the humidity, the less efficient the evaporation process. That's why the same temperature in a dry climate such as Phoenix's may not feel as bad as in a humid one like New Orleans'.

People also may react differently depending on how accustomed they are to hot, humid weather.

The Weather Service currently tailors its heat index warnings in about 20 cities and is expanding that to other areas.

Click here for the NWS Heat Index

That's because conditions that are common in Atlanta, with plenty of air conditioning and a population that sees hot weather every year, may be deadly in Syracuse, N.Y., where air conditioning may be less common and people rarely encounter stifling heat.

"Our goal is to implement these systems nationwide for all cities to indicate when heat is going to become life-threatening," Tew said.

Tragedy can occur anywhere, though.

For example, last year there were heat-related deaths in Washington, D.C., among people who had air conditioning but did not turn it on because of the cost, Tew said.

The heat index originated in Australia, where they know something about hot weather. That version was more complex, taking into account layers of clothing, evaporation rates and other variables.

The Weather Service streamlined it and adopted the formula in 1992, Tew said. Originally a heat index of 105 triggered warnings. But the agency began to realize that one size does not fit all and began localizing it to conditions in various cities.