Undated (AP) _ The big summer heat wave rolled on Tuesday with temperatures boiling past 100 to record highs from the eastern Plains to the Ohio Valley, but the steamy Northeast finally got a break with a surge of cooler, drier air from Canada.

Utilities posted more records for demand for electricity as air conditioners and fans were turned on full blast.

Some school districts in Illinois plan to open schools early and reduce class periods by almost half when school opens next week to avoid the hottest part of the day.

''By noon during summer school, it was very uncomfortable. If it was 90 outside, it was 95 to 100 in the classrooms. That's not a good atmosphere for learning,'' said associate principal Don Tokarski of Rantoul Township (Ill.) High School.

''I think this will be the hottest weather of the summer, taking both temperature and humidity into account,'' said Norm Reitmeyer, a meteorologist with the National Weather Service in Louisville, Ky.

''We've had an oppressively hot and humid summer without a doubt,'' Reitmeyer said. ''But particular days such as this or tomorrow are rare.''

The temperature at Paducah, Ky., reached 102 degrees and Reitmeyer said that and the humidity would push the heat index, a measure of how hot it feels, the opposite of winter's wind chill factor, to 115. He said that is near life-threatening for some people.

Kentucky has had a total of 48 days of above-90 degrees this summer.

Milwaukee reached a record high of 100 degrees, and that made it a record five times the city has hit 100 or higher this year. The old record was four times in 1936.

Some other records included 90 at Duluth, Minn.; 104 at Fargo, N.D.; 103 at Fort Dodge, Iowa; 103 at Huron, S.D.; 102 at Indianapolis; 102 at Madison, Wis.; 101 at Peoria, Ill., and Sioux Falls, S.D.; and 102 at Springfield, Ill., the National Weather Service said.

The 103 degrees at Rockford, Ill., was the hottest reading since July 27, 1955. La Crosse, Wis., marked the 44th day that the temperature achieved the 90 degree mark, another record.

The 102-degree heat wilted about 10,000 competitors wearing heavy uniforms and carrying musical instruments at the Drum Corps International World Championships on the artificial turf at Arrowhead Stadium in Kansas City, Mo.

''You can feel it right through your tennis shoes,'' said Donald MacDougall, a member of the Edmonton Strutters of Canada. ''One of our melophone players almost passed out.''

The 136th Indiana State Fair opened Tuesday with livestock exhibitors taking extra precautions, like putting ice in front of large fans to keep animals cool. Four chickens that were to be entered died when their owners left them in cars with the windows rolled up, fair officials said.

The official high in the 48 contiguous states at 3 p.m. EDT was 105 degrees, not in Texas, Arizona or southern California but at Chamberlin, S.D.

But nature's own air-conditioning - in the form of a cool front from Canada - settled over the Northeast. Unfortunately, forecasts said the heat would soon return.

After 32 days of 90-degree temperatures and swamp-like humidity, including a high of 97 Monday, New York City had a high Tuesday of merely 86 degrees. And the humidity was only around 39 to 42 percent, making it feel almost like early autumn compared to preceding weeks.

''The downturn in temperatures is certainly going to help not only Con Ed but all New York utilities,'' said Bill Murphy, spokesman for the utility that supplies New York City's power.

Con Edison had a record demand Monday of 10,160 megawatts of electricity. The failure of several feeder cables on Staten Island knocked out service to more than 13,000 customers. And earlier Monday, the utility cut off power to 16 large apartment buildings on Manhattan's Upper East Side to prevent a wider blackout after several feeder cables failed, blacking out some 10,000 residents.

The cooler air also brought relief for utilities in Pennsylvania, where Philadelphia Electric Co. was forced to impose a 5 percent voltage reduction Monday. But the clash of the hot and cool air also touched off thunderstorms, which knocked out power to thousands of customers across the state Monday.

Farther south, utilities across Kentucky reported record electrical demand Monday and expected to exceed those amounts Tuesday, but no problems were reported.

''We've been going wild this summer,'' said Dick Lovegrove, a spokesman for Ashland-based Kentucky Power, which had records for power use almost every day during the past week.

Louisville Gas & Electric has set all-time peaks 13 times this summer, said spokesman Calvin Anderson.

The Tennessee Valley Authority, the big federal utility, hit another record peak summer demand Tuesday of 21,304 megawatts. A record Monday of 20,618 megawatts forced the utility to buy about 2,500 megawatts from neighboring utilities, said Robert C. Steffy Jr., senior vice president of power for TVA. TVA's all-time power peak was in the winter of 1983 at 22,478 megawatts.