Washingtonians couldn't be sure of anything yesterday -- not even the ground they walked on.

People left footprints in their driveways. A whole row of motorcycles toppled like dominos as the asphalt sagged on a downtown street. Did the heat wave know no bounds?

Apparently not.

The temperature hit a searing 100 degrees for the second time in three days, melting asphalt, pushing the demand for electricity to new highs, tying a 1978 record for the date and leaving area residents panting for a break.

The National Weather Service had bad news: The heat wave should continue at least through Monday night, when a cold front might bring relief wafting down from Canada. Still, forecaster Scott Prosise cautioned, "Don't get your hopes up. It's nothing definite."

Prosise said that a "mountain of air" anchored over the eastern United States has caused Washington's unrelenting heat. The thermometer already has surpassed the 90-degree mark 16 days this month -- unusual for July, which averages 13 days of 90-or-above temperatures, according to Prosise. More of the same is in store for today, with highs in the upper 90s, he said. Some relief is expected at night, with lows near 80 in the city and the mid-70s in the suburbs.

The Weather Service also was predicting a sunny, hot beach weekend along the coast from New Jersey to the Carolinas, with highs of about 90 degrees.

Meanwhile, the heat has taken its toll in the Washington area.

Several area residents have been treated in hospital emergency rooms for heat exhaustion or the more serious heat stroke, according to spokesmen at various hospitals. However, the cases have been sporadic and none of the hospitals reported a large number of heat-related illnesses.

Two men and one woman were taken Wednesday night to the Washington Hospital Center's shock-trauma unit, suffering from heat stroke, according to a spokesman. One patient had a body temperature of 108 degrees, according to spokeswoman Mary Anderson. All three were admitted for treatment, she said.

Dr. Kenneth Latchis, medical director of the hospital's emergency department, warned that the elderly and the very young -- children under 2 years old -- are at highest risk for heat-related injuries because their bodies are most sensitive to temperature extremes. Latchis said the elderly and anyone on medication or suffering from cardiac or respiratory problems should follow some simple rules during the heat wave.

He suggested that they remain in a cool environment, avoid exertion, drink fluids and "most important," watch for signs of heat stroke or exhaustion. Those include elevated temperature, headache, nausea, muscle cramps, rapid pulse, irritability and fainting spells.

The heat wave contributed to problems other than medical.

The District government was forced to shut down its car inspection stations an hour early yesterday when high temperatures and fumes from the cars combined to make the situation intolerable, according to Tara Hamilton, spokeswoman for the Department of Public Works. She suggested that motorists come to the stations early until the heat subsides.

As to the softening streets, Hamilton said that asphalt is affected by heat, but added that it is not a dangerous situation. "If roads were all concrete, they would crack and break," Hamilton said. "That's why we use asphalt on top, because it expands in summer and contracts in winter."

Indeed, Virginia and Maryland highway officials said they were on the lookout for "blowups" on road surfaces, which occur when temperatures hover around 100 degrees and the pavement expands and explodes. Virginia suffered one blowup in an eastbound lane of I-66 Wednesday night, according to a spokesman.

The blowups can be dangerous because there is no way to tell "when or where this is going to happen," said Michael Snyder, district engineer of the Maryland State Highway Administration, who advised motorists to drive cautiously.

The heat also was contributing to what Virginia Power called an unprecedented demand for electricity. The company reported hitting all-time peak hourly demands yesterday for the third consecutive day, as 1.6 million customers consumed 11.2 million kilowatts, according to spokesman James E. McDonald.

McDonald said the company is prepared for another peak today, but added that customers should have no problems.

Two reactors at the Calvert Cliffs nuclear power plant representing almost one-third of the generating capacity of the Baltimore Gas & Electric Co. shut down automatically yesterday, raising at least for a time the possibility of blackouts or brownouts because of the heavy demand for electricity. The company reported that there was no danger of radioactive release.

A company spokesman said last night that the cause of the shutdown had been determined and the generators were being restarted. One should be operating this morning and the other by early afternoon, spokesman John Metzger said.

In the meantime, Metzger said, the utility repeated its recommendation that customers voluntarily curtail electricity use. Special correspondents Eric Charles May and Amy Worden contributed to this report.