A record-breaking heat wave loosened its grip on the South today after a week of 100-degree temperatures and dry heat that caused at least six deaths, four of them in Georgia. As the high-pressure system that spawned the killing heat moved eastward, cooling thunderstorms moved in and a tornado alert was issued for several southern states.

Scores of illnesses and injuries were attributed to the heat, including a woman who burned her feet walking barefoot on hot pavement in Savannah.

The soaring temperatures also hurt the South's poultry industry.

About 1 million chickens died because of the extreme heat in Mississippi, Alabama, Georgia, and northern Florida, said Harold E. Ford, vice president of the Southeastern Poultry and Egg Association, headquartered in Decatur, Ga.

Poultry farmers will continue to see the effects of the heat wave for weeks. "Chickens won't eat when it's this hot," Ford said. "We expect a major loss in pounds to be processed in the next week or 10 days. The chickens are off their feed."

They also lay fewer eggs, and the eggs they lay are less fertile.

"There's an old saying that hens and roosters don't make love in hot weather," Ford said. He said he expects no noticeable increase in poultry prices in the grocery.

Georgia Power Co. in Atlanta easily set an hourly record for electricity consumption, dispensing 13,290 megawatts between 4 and 5 p.m. Thursday, company spokesman Tony Cooper said. A megawatt is the amount of electricity required to power one lightbulb for 1 million hours. The old record was 12,527 megawatts, set Aug. 22, 1983.

No prolonged electrical outages were attributed to the heat, but the overload caused a fire Tuesday in an underground cable in downtown Atlanta, knocking out service to the state capitol and a half-dozen other buildings. Gov. Joe Frank Harris was in Europe, industry hunting.

The heat impinged on President Reagan's visit to Atlanta to promote the reelection of Republican Sen. Mack Mattingly and to visit Northside High School.

Entering the high school gymnasium Thursday morning to talk to students about his tax proposals, Reagan remarked that the commander-in-chief "dictates the uniform of the day," removed his tan jacket and delivered his speech in shirtsleeves.

The 1,400-person audience had overcome the air-conditioning efforts of a 100-ton unit loaned by Delta Airlines and equipment from Dobbins Air Force Base, which were cranked up at dawn in a forlorn effort to cool the gym.

The heat wave was caused by a strong high-pressure system that stalled over the Gulf of Mexico through the week, said Jeff Hardcastle, a forecaster at the National Weather Service in Atlanta. Instead of allowing warm, moist air to flow into the southern states from the Gulf, the high-pressure area forced air clockwise around its center.

Normally, a high-pressure system would not linger so long in one place at this time of year because the jet stream would be a strong enough force to nudge it, said Jerry Justice, professor of geophysical sciences at Georgia Institute of Technology. But this year the jet stream is farther north than usual -- a factor in the wave of tornadoes that struck New York, Ohio and Pennsylvania on May 31.