PHILADELPHIA — Americans in much of the country got a slight break from temperatures Sunday that were near or above 100 a day earlier. But for many, the cooler temperatures were not exactly comfortable — they fell only into the 90s.

Cooler air swept southward in the eastern half of the country Sunday, bringing down some temperatures by 15 degrees or more from Saturday’s highs. In St. Louis, the 13-degree drop from Saturday’s high still left residents baking in 93-degree weather — the high Saturday was a record 106.

Temperatures in Philadelphia, Washington, and Indianapolis fell Sunday after crossing the 100 mark on Saturday. For many areas, the cooler temperatures were ushered in by thunderstorms that knocked out power to thousands of electricity customers.

The heat also is blamed for nearly 40 deaths across the country.

In Federalsburg, Md., Sunday, officials said three boys drowned in a creek in which they were trying to cool off. The boys were two 12-year-old cousins and an 8-year-old friend. They were last seen late Saturday afternoon playing basketball in a parking lot.

A 4-month-old girl died and a 16-month-old girl was hospitalized Saturday in separate incidents in suburban Indianapolis when both were found trapped in cars during near-record 105-degree heat.

Residents from Iowa to New Jersey spent the weekend trying to stay cool. They dipped into the water, went to the movies and rode the subway just to be in air conditioning.

If people ventured outside to do anything, they did it early. But still, the heat was stifling.

Officials said the heat caused highways to buckle in Illinois and Wisconsin and caused a train derailment in Maryland last week.