The District of Columbia Office of emergency Preparedness, the area's National Weather Service Office, and the University of Delaware Climate Research Center began using an advanced hot weather forecasting and warning system for the summer of 1997. The new system adds other factors to the apparent temperature, which the National Weather Service and emergency officials generally use as a measure of heat danger.
Apparent temperature combines heat and humidity to rate the danger.
Larry Kalkstein and his colleagues at Delaware's Center for Climatic Research have found that conditions that cause the death rate to rise vary from city to city and also depend on more than the apparent temperature. In Washington, and many other cities, the deadliest weather includes high temperatures and humidity both day and night, mostly sunny days and light winds from the southwest. The dew point of 72 degrees means that the air is very humid. The partly cloudy sky allows the sun to heat buildings, streets and parking lots more than it would on a cloudy day. Also, the deadliest days don't get relief, even if only temporary, from thunderstorms.