THE ENDLESS SUMMER has gotten longer -- and colder. The snowboard, an abbreviated surfboard designed for use in snow, has caught on big with surfers who previously found their quest for the year-round wave hampered by inclement weather or exorbitant travel bills.

What began 10 years ago as an off-season substitute for surfing is now a bona fide winter sport to more than 100,000 fans. Described as faster than surfboards and more manageable than skis, snowboards have begun a challenge for space on the nation's slopes.

"You can surf rolling hills just like a wave," says Blair Rhodes, a confirmed snowboarder and manager of the Sunshine House in Bethesda. "You can do everything you could on a surfboard, just faster."

But skiers, upon whose money and slopes snowboard manufacturers must rely for mainstream success, have been slow to warm up to what many fear is a dangerous fad.

"Snowboarding is not the same as skiing," says Eric Flynn, mountain manager for the Roundtop resort in Lewisberry, Pa. "It conflicts with skiers and is a poor risk for us from an insurance standpoint. I can't think of any resort in this region which will allow them on their slopes."

Most mid-Atlantic resorts say they don't allow snowboards because the bindings strap both feet to the board, making them hazardous on ski-lifts. And resort operators feel that the rudder and steel edges on some models damage their groomed slopes.

In addition, snowboards are not recognized by the National Ski Area Association, leaving them uncovered by standard resort insurance policies.

Most ski shops disdain snowboards, forcing manufacturers to rely upon surf shops as their main outlets. Nevertheless, snowboard sales have doubled annually for the past six years. "We have refined snowboards to such a point that we feel they are actually safer than skis," says Paul Sundman, vice president of the Burton Corporation in Vermont, the nation's leading maker of snowboards. "It's just a matter of gaining acceptance, and that's what we are trying to work constructively towards."

"I'd say within two years they'll be legal everywhere," says Rhodes. "The sport is growing too fast to be stopped."

The snowboard lobby has had some success. Snowboards are allowed at 65 resorts in the United States, mostly at areas large enough to absorb the potential liability, such as Stratton (Vermont), Aspen (Colorado) and Jackson Hole (Wyoming), and at mountains in Canada, Japan and Austria as well. A professional circuit has evolved, including a $5,000 purse at this year's U.S. Open at Stratton in March. And James Bond even used one to elude his enemies in "A View to a Kill."

Snowboarders describe their sport as a cross between surfing and skateboarding. Although downhill speeds in excess of 60 miles per hour are common, enthusiasts feel the boards are safer and more manageable than skis.

While it's too early to have any reliable statistics, at least one sports medicine expert believes snowboarding should cause fewer lower body injuries than skiing. Judy Hayes, who used to test ski equipment and is now a spokesman for the Sports Medicine Center in Chevy Chase, says snowboards have three safety advantages over skis: greater stability in the legs; lower center of gravity; and no poles or long skis to increase the chance of injury in a fall.

"I always tell skiers if they don't have a surfing or skateboarding background, they will find snowboarding very difficult," says Paul Salitsky, manager of Potomac Ski and Sail. "But all you really have to do is keep the tip up."

The economics of snowboarding can also be attractive. "You can get a good snowboard for the cost of skis without having to spend more money on stuff like boots, poles and bindings." says Don Giese, owner of Caravan Surfboards in College Park.

Burton began making boards eight years ago and now leads with annual sales of approximately 20,000, but quality boards are also produced by Sims, Ski Tech, Winterstick and Ski Board. Prices range from $125 for elementary, wooden boards to $280 for the steel-edged, fiberglass models used in competitions.

Since nearby downhill resorts haven't cooperated, enthusiasts have resorted to the rolling hills of golf courses and public parks in this area.

"But that doesn't last too long," says Rhodes. "Once you've had a small slope, you always want something bigger."

BUYING THEM -- Here are some of the places snowboards can be found locally:

CARAVAN SURFBOARDS -- 4938 Edgewood Road, College Park (441-2020).

POTOMAC SKI AND SAIL -- 3610 University Boulevard, West Kensington (949-6800).

SKI CENTER -- Massachusetts Avenue and 49th Street NW (966-4474).

SUNSHINE HOUSE SURF SHOP -- 4851 Cordell Avenue, Bethesda (652-8900).

TRYING THEM -- Generally it's up to you to just find a good-sized hill in your neighborhood if you want to snowboard around here, but Battery Kemble Park, Chain Bridge Road and Nebraska Avenue NW, is one good spot.

In addition, here are two of the closest out- of-town resorts that allow snowboarding:

MOUNT TONE SKI AREA -- Lake Como, Pa. 717/798- 2707.

ALPINE SKI AREA -- Analomink, Pa. 717/722-9400.