National Park Service ranger Nicholas Karim spent most of yesterday morning on the National Mall giving directions to lost visitors. As the afternoon sun beat down, however, he improvised a way to cool off, grasping the yellow nozzle of a hose and squeezing a gentle mist onto grateful passersby.

"I've sprayed at least 50, including myself," said Karim, 23, of Vienna. "It's not an official job. I just decided to do it."

It was that kind of holiday as sweltering officials and residents alike figured out ways to beat the 99-degree heat that baked the region. Today and tomorrow are supposed to be even worse, with weather forecasters predicting dangerously high temperatures between 100 degrees and 105 degrees. Washington's all-time high is 106 degrees.

As Karim squirted, Aaron Todd, 9, danced in delight and opened his mouth to catch the spray. His father, Rob Todd, 36, said the family traveled from New Boston, N.H., to enjoy the Mall but was surprised by the heat wave. The solution? "A camel back full of water," Rob Todd said, referring to a water-filled backpack, the kind that bicyclists use.

Elsewhere, thousands of area residents flocked to beaches, neighborhood pools and fountains. The Water Mine splash park at Lake Fairfax Park opened at 8 a.m. and was at its 800-person capacity two hours later.

"It can't get any more crowded," said Kevin Sonnhalter, 21, of Herndon, one of 30 lifeguards at the three-year-old water hole. "There are places you can't see everything that's going on in the water."

On the Mall, Marni Sandler, 22, of New York, came prepared with an ice chest filled with ice cubes and water bottles as she and a friend waited for a concert. Tony Brown dished out huge wedges of watermelon for $1.50 outside the busy Smithsonian Metro stop.

"Come on brother, come on, let's move it," Brown said as long lines of wilting Mall walkers lined up for the refreshing fruit, devouring it on the spot and going back for more.

"Oh, this is sweet and cold, cold, cold," said Anja Bleecken, of Richmond, who was carrying gallon jugs of ice water with her.

Many holiday revelers simply stayed inside. In Howard County, the suffocating heat kept many people away from the daytime festivities at Lake Kittamaqundi in Columbia. Local folk singer Elizabeth Perez played to just a few families.

Law enforcement and hospital officials reported relatively few heat-related emergencies. Authorities said 42 people were taken to hospitals for heat-related symptoms.

Weather forecasters said the area's first extended heat wave of the summer will continue at least through Friday.

The National Weather Service issued an extreme heat warning yesterday, saying the heat index -- the combined effect of heat and humidity that more closely reflects the temperature your body feels -- is projected to be 115 today. "It's a very dangerous situation, especially for the very young and the very old," said AccuWeather forecaster Laura Hannon.

Slight relief is expected Wednesday, forecasters said, when a cold front is predicted to pass through the region, reducing forecasted highs to a still toasty 90 degrees, rising again Thursday and Friday into the mid-90s. The average high in July is 88 degrees.

Some entrepreneurs saw opportunity in the heat yesterday.

Cabdriver Surinder Singh, 40, of Herndon, said he had a brisk business shuttling visitors near the Mall in his air-conditioned taxicab. Singh, of Jalandhar, India, said yesterday's heat was slightly less than that on a typical July day in Punjab, but the humidity was definitely higher.

"I was born in hot weather," he said. "I have no problem" working.

Shaved-ice vendor Richard Louck, 52, of Annapolis, also seemed happy about the weather.

"We want the heat; it's great for snowballs," Louck said. Roasting in the sun, sweaty and red-faced, Louck covered his head and neck with a wet towel -- a secret, he said, to staying cool.

Staff writers Hannah Allam, Marcella Bombardieri, Daniel Grech, Tomoko Hosaka, Mary Louise Schumacher and Jamie Stockwell contributed to this report.

CAPTION: Paramedic Andy Gonzalez, center standing, of the Sterling Park Rescue Squad, assists parade marchers suffering from heat-related symptoms.

CAPTION: Heather Rudzenski, left, and Jessie Hall rejoice in the spray from a fire hydrant on Constitution Avenue during the Independence Day parade. Today and tomorrow may top the 100-degree mark. Washington's all-time high is 106.

CAPTION: A Hot Old Time: The scaffolded Washington Monument looms behind a group of Fourth of July revelers waiting in the sweltering heat for the fireworks on the Mall. Under the flaglike sun shade are, from left, Tom Moore, Rachel Zargo, Samantha Bowman and Adam Nordstrom. More on the holiday festivities is in Metro, Page B1.

CAPTION: Fantastic: Fireworks light up the sky over the scaffolded Washington Monument and the U.S. Capitol. Although the searing heat kept some people away from the Mall during the day, the crowd grew after nightfall as revelers came out to witness the annual explosion of color. More on the holiday festivities in Metro, Page B1.