Balmy weather, a charitable cause and the pleasures of free flight beckoned about 30 hot-air balloonists back into the sky yesterday morning, a day after a teen-ager was killed and her father seriously burned when their balloon hit a power line and exploded during a benefit exposition near Gaithersburg.

"Why shouldn't we go up again?" said Wallace Henderson, one of the balloonists. "People get killed every day in automobiles, but people continue to drive . . . . Life goes on."

A 75-foot-tall multicolored balloon piloted by Robert Van Newkirk of Pennsylvania struck a 69,000-volt transmission wire in a clearing off Goshen Road about 7:20 a.m. Saturday, sending the propane-laden craft plummeting to the ground in flames. Van Newkirk, 39, was dumped out of the balloon's basket, but his 16-year-old daughter Kristine died after being trapped in the fire.

Van Newkirk, an elementary school teacher from North Hills, Pa., remained in serious condition yesterday in the Washington Hospital Center's intensive-care unit with second-degree burns and broken bones.

Officials said the incident is being investigated by the Montgomery County Fire Marshal's Office and the Federal Aviation Administration.

"There are all kinds of assumptions from balloonists that there might have been a wind shear," a sudden downshift in air current that could have caused the balloon to hit the power line, according to Capt. Ray Mulhall of the Montgomery County Fire Department.

Also, authorities and family members have speculated that Van Newkirk's view of the power line may have been obstructed by trees.

About 50 colorful balloons were launched Saturday from the grounds of Comsat Laboratories off I-270 in Clarksburg to raise money for Kids Inc., a local group that tries to grant wishes to terminally ill children.

After the accident, the balloon pilots and organization officials decided to continue the event and donate money raised through concessions, sponsors and entrance fees to the Van Newkirk family. A launch scheduled for Saturday evening was canceled, however, because of brisk winds, and some balloonists decided not to return yesterday, a spokeswoman said.

The Maryland incident was one of three balloon accidents reported around the country Saturday that resulted from collisions with power lines. A newlywed was killed but his wife was uninjured in a Charlotte, N.C., accident, and three passengers were injured in a balloon crash in Conestoga, Pa.

Balloonists yesterday characterized them as "freak happenings" that belie the safety of a graceful and beautiful sport. The accidents coincided with the start of peak ballooning season in late spring and early summer, experts said, when the winds are typically calm and low.

Deke Sonnichsen, chairman of the California-based Sport Balloon Society of America, said power lines are an accepted danger even in remote areas and are often difficult to detect from the air.

"It's a hazard everyone realizes is there, but you would have to go into the middle of the desert to avoid power lines," he said in a telephone interview. The organization has lobbied for underground power lines in new construction, but pilot training and heightened caution are most important, he said.

Balloon pilots and their aircraft must be certified by the FAA, Sonnichsen said. About 10,000 balloonists are certified, he said.

A local organization, the Chesapeake Balloon Association, has about 100 members from an area that stretches roughly from Baltimore to Richmond, according to a spokesman.

The National Transportation Safety Board says that balloon fatalities are rare. There were no reported fatalities in 1984, three in 1983 and seven in 1982.

Navigation of hot-air balloons depends on fuel heaters that blow air into the balloon, lifting the craft. For the descent, the hot air is vented and a pilot steers the craft by changing altitude to move into wind currents of different directions, according to experts.

Sonnichsen said the event that led to Kristine Van Newkirk's death, a "hare and hound" race in which balloonists try to drop a marker or land near where the lead or "hare" balloon has landed, requires less skill than many ballooning events.

Yesterday's launch was a "key grab" competition in which balloonists tried to take a key off a 20-foot pole without touching ground.