SCHOOL KIDS and skiers know the number of snow days is fast dwindling. But the ski crowd can at least buy some more time on the slopes with a little night skiing.

Most resorts within driving distance of Washington have lighted slopes and it's not unusual for them to do 30 to 40 percent of their business after sunset.

Besides squeezing in some extra hours of skiing, all those skiers queuing up in the semidarkness know that lift lines are shorter, costs are lower and they're in for a different experience.

Some night skiers report an illusion of greater speed. Others, like Wintergreen ski manager Uel Gardner, say it's no illusion: "People tend to ski faster at night. No question about it. And I'm not the only one who has noticed it."

Part of the reason may be that night visibility is aided by greater contrast. On a cloudy day, when light is "flat," it's difficult to see changes in the snow as you move down a trail. On a sunny day, glare can be a skier's worst enemy.

"At night, with lights so bright, you can see moguls and hills and changes in the condition of the snow," says Joan Montgomery, a spokesman at Camelback.

Night skiing is cheaper if you're looking at the bottom line and not the hourly rate. At Ski Liberty, for example, a day lift ticket is $24 on weekends; a night lift ticket, which puts you on the slopes from 5 p.m. to 10 p.m., is $14 on weekends.

But what about the potential drawbacks?

Temperatures are certainly lower, "but you're not out there as long," said Steve Showalter, the ski area manager at Massanutten. "The cold doesn't seem to be a problem. People expect it to be a little colder, and everyone seems to handle it pretty well."

Ice, another foe of Eastern skiiers, generally isn't a problem either, according to resort spokesmen, since many areas, such as Massanutten, groom trails between day and night sessions.

The obvious obstacle to night skiing -- darkness -- is negligible, but don't expect to find huge banks of lights lining each side of a ski run.

Massanutten, for example, places two lights at hundred-yard intervals on each side of the mountain. One light faces across the mountain, the other faces down to minimize shadows. Massanutten uses mercury vapor lights, the same lights used in many major league ballparks.

Wintergreen, on the other hand, uses metal halide lights, which, Gardner boasts, also are guaranteed to make the true colors of your ski parka recognizable.

What also varies from resort to resort is the percentage of lifts and runs available at night. At, say, Canaan Valley in Davis, W. Va., nine of the 21 slopes and trails are lit and two of four lifts are operating. Massanutten has 70 percent of its trails lit for night skiing. At Wintergreen (Va.), which has been trying to promote its night skiing program, five of 10 slopes are lit and four of five lifts are operating at night.

Of course, skiing without the sun does expose you to the occasional blackout. Massanutten lost its lighting for ten minutes recently when one of its lift attendants, driving on the other side of the mountain, collided with a power line. No one in the car or on the slope was hurt. NIGHT SLOPES: Here's a sampling of resorts within two to six hours of Washington that offer night skiing.

ALPINE LAKE -- Terra Alta, W. Va. 304/789-2481. Saturday nights only ($8 for adult, $4 for children); three lifts and three trails operating. BIG BOULDER -- Lake Harmony, Pa. 717/722-0101. Every night from 4 to 10 ($13 for adults and children); seven lifts and 11 slopes operating. BLUE KNOB -- Claysburg, Pa. 814/239- 5111. Every night from 6 p.m. to 10 p.m. ($11 on weekends, $8 on weekdays); five slopes and four lifts operating. BRYCE MOUNTAIN -- Basye, Va. 703/856- 2121. Every night from 6 to 10, except Sunday ($10 per person); six slopes and two lifts operating. CAMELBACK -- Tannersville, Pa. 717/629- 1661. Every night from 5 to 10 ($14 per person); 13-17 slopes and trails and three to five lifts operating. CANAAN VALLEY -- Davis, W. Va. 304/866- 4121. Every night, 4:30 to 10, except Sunday and Monday ($10 for adults, $8 for children midweek, $12 for adults and $9 for children weekends); nine slopes and two lifts operating. DOE MOUNTAIN -- Macungie, Pa. 215/682- 7109. Every night 5 to 10 except Sunday ($12 adult, children half price with adult ticket); 10 slopes and seven lifts operating. ELK MOUNTAIN -- Uniondale, Pa. 717/679- 2611 or 800/233-4131. Every night 4:30 to 10 ($10 midweek, $12 weekends); six trails and four lifts operating. HIDDEN VALLEY -- Somerset, Pa. 814/443- 6454 or 800/458-0174. Every night 4:30 to 11 (reduced rate until end of season: 50 for adult midweek, 50 on weekends; $4 for children midweek, $5 on weekends); 10 slopes and five lifts operating. LAUREL MOUNTAIN -- Boswell, Pa. 412/238-6688. Every night 4:30 to 10:30 ($13 midweek, $15 weekends; $6 for children midweek, $8 for children weekends); five slopes and two lifts operating. MASSANUTTEN -- Harrisonburg, Va. 703/289-9441 or 800/334-6086. Every night 6 to 10 ($12 for adults, $9 for children); five trails and three lifts operating. MONTAGE -- Scranton, Pa. 717/969-7669 or 800/VIP-SNOW. Every night 5 to 10 ($10 per person); nine trails and four lifts operating. SEVEN SPRINGS -- Champion, Pa. 814/352- 7777. Every night 4:30 to 11 ($15 for adults, $13 for children under 12); 18 slopes and five lifts operating. SHAWNEE MOUNTAIN -- Shawnee on Delaware, Pa. 717/421-7231. Every night 5 to 10 ($13 Tuesday-Saturday, $11 on Monday); 13 slopes, four to five lifts operating. SKI LIBERTY -- Fairfield, Pa. 717-642-8282. Every night from 5 to 10 ($8 midweek, $9 on weekend); 14 slopes and five lifts operating. SKI ROUNDTOP -- Lewisberry, Pa. 717/432-9631. Every night from 5 to 10 ($12 per person); 11 slopes and eight lifts operating. WINTERGREEN -- Wintergreen, Va. 800/325-2200, 804/325-2200. Every night from 7 to 11 (Friday and Saturday only after March 9: $12 midweek for adult, $11 for children; $15 weekend for adult, $13 for children); five slopes and five lifts operating. WINTERPLACE -- Ghent, W.Va. 304/787- 3221. Every night from 5 to 10 ($13 for adults midweek, $7 for children six and under; $16 for adults on weekends, $7 for children six and under); 15 trails and four lifts operating.