The first full day of summer Tuesday promised the same hot and dry weather that has buckled highways, sent people without air conditioners to public shelters and added to the woes of drought-plagued farmers.

High temperatures Tuesday were in the 90s across most of the nation, the National Weather Service said. Temperatures didn't have far to go on the Plains, with the overnight low a warm 77 at Bismarck, N.D.Nearly 40 cities in the central and eastern United States set temperature records Monday as the early-season heat continued to intensify, according to the weather service.

Temperatures soared above 100 degrees across much of the northern and central Plains, the upper Mississippi Valley and the desert Southwest. Highs in the 90s were common elsewhere across the nation except for coastal and mountain areas.

Authorities in several areas opened up shelters Monday to residents without air conditioners. In Chicago, where it reached 104, city officials invited the public to use the city's libraries and other air-conditoned municipal buildings during daytime hours to escape the heat. The Chicago temperature reading tied the record for the month of June.

Customers of Chicago's Commonwealth Edison Co. set a single-day record for electricity usage, said spokesman John Hogan, while Consumers Power Co. in Michican set a usage record for the month.

The heat caused discomfort for people with breathing difficulties and heart conditions and prompted authorities in the Chicago area and in Wisconsin to post ozone advisories, cautioning people with those conditions to remain in air-conditioned rooms.

The heat also proved dangerous to animals. Forecasters said the livestock weather safety index entered the emergency zone in several parts of Midwest and Plains. Farmers were advised to keep animals in shade and provide plenty of water.

"Animals react to the heat in the same way humans do," said Michigan Department of Agriculture specialist Edwin Renkie. "It does affect the animals and in some cases can kill them."

Heat distorted railroad tracks in Mobridge, S.D., last weekend, causing five cars of a Burlington Northern train loaded with corn to derail, said spokesman Al Wiegold in St. Paul, Minn.

During the weekend, the heat in the Sioux Falls, S.D., area also caused 25 roads to buckle, about the usual damage for the entire summer, said Delmer Van Ekern of the state Transportation Department.

In Des Moines, searing heat buckled pavement Sunday, slowing traffic on U.S. 6 and I-235. On Monday, Des Moines' high reached 101, breaking the 98-degree reading set for the date in 1933.

Also Monday, Concordia, Kan., reached 106, breaking the 103-degree record of 1952; Sheridan, Wyo., went to 102 degrees, topping the record for the date of 97 also set during the 1933 Dust Bowl drought.

Temperature records also were broken in Michigan, Montana, Vermont, Ohio, Missouri, North and South Dakota, Nebraska, Minnesota and Illinois, the weather service said.

Agricultural experts said the heat is adding to the problems of the drought in the Midwest and the Plains by evaporating what little moisture remains in the soil.

"These real hot days - 95 to 100 degrees - are using more moisture than we can stand to lose," said Harold Hunzicker, agricultural advise in Marion County in southern Illinois. "We're still hanging in there, but that is not to say we couldn't have a disaster."

Tom Vigil, a heavy equipment operator in Denver who spends eight to 12 hours a day laying drainage pipe, had this advice for staying cool: "Think about Santa Claus."