A FEW YEARS AGO, a New England skier recently arrived in the Nation's Capital grinned in disbelief at a new friend who suggested they take a day off to go skiing at a local slope.

It was March, and in these southern climes spring already had arrived bringing sunny and mild days. Not a flake of snow had fallen for weeks.

"We'd be smarter," said the skier, "to take our hiking boots."

The prospects seemed little brighter on the 120-mile drive to a Virginia resort. Only a mile to go and still no trace of snow on the brown and already partially greening hillsides.

Then, around a curve, the mountain came into view. Down through the trees snaked three glistening paths of white -- the product not of nature but machines.

Even to a regular at our "Banana Belt" slopes, the sight of these clearly defined ski runs traversing an otherwise snow-barren landscape can be startling.

On this day, the hard-packed snow measured from 1 to 5 feet deep. Since only the trails were covered, the effect was to put the crowd of skiers on a raised platform of snow. A slip could send them skidding over the edge into the mud.

In the past, when the snow didn't fall you didn't ski. But recent technology has changed all that with the introduction of a high-powered snowmaking capability.

Air and water are combined by hose at high pressure to spray snow on the slopes from huge nozzles -- much as you would water a lawn. When dozens of nozzles are going full blast, you've got a blizzard.

All that the resorts need are below-freezing temperatures to blanket their trails in white. But, complain ski resort operators, this fact hasn't fully registered in the minds of many of the area's skiers.

"We need natural snow for people to get thinking about skiing," says Dick Whitney, assistant manager of nearby Ski Liberty in Fairfield, Pa. When the city is snow-free, "people can't believe we're open. We get only the advanced skiers and the diehards who know." That was the case before Christmas this season, he says, following Liberty's Nov. 20 opening.

For the Washington area, snow-making has meant the difference between having 20 resorts to choose from within a 5-to 6-hours' drive, or perhaps only a few or none.

Snowmaking is "absolutely essential," says Lars Skylling, ski area director at Seven Springs Mountain Resort in Champion, Pa. Even with Washington's somewhat balmy pre-Christmas temperatures, his resort also was able to open in November.

"It's their insurance policy," says Cal Conniff, executive director of the National Ski Areas Association. "It takes a substantial element of risk out of being in the ski business." In areas of marginal snowfall, such as Washington, "they couldn't afford to put in chairlifts and large base facilities just hoping it would snow."

Adds James A. Moore, president of Camelback in Tannersville, Pa.: "Putting a snowmaking plant in a Banana Belt ski area is a little bit like J. P. Morgan and his yacht -- if you have to worry about the cost, you'd better not open the ski area. Of course this is an exaggeration, but it is true that snowmaking is our lifeblood."

About 50 percent of the nation's 692 ski resorts, says Conniff, have installed at least some snowmaking equipment -- even areas of normally heavy falls such as New England and the Rocky Mountains. They see snowmaking as a protection against occasional dry years.

"A vivid demonstration," he says, was the 1980 Olympics at Lake Placid. "Prior to the games, they didn't have enough snow there to make a snowball. The games came off totally on machine-made snow."

Last year Ski Liberty got only 14 inches of natural snow, says Whitney, but with 106 days of skiing "we had our longest season ever. We don't need natural snow."

Conniff's association points out that ski areas from Pennsylvania south saw a 10.7 percent increase in business last year over the previous season, in large part because of "plenty of cold weather for snowmaking."

While local resorts have been able in recent years to cover most of their runs, they are continually upgrading their capability to produce more snow faster to extend their season. This year is no exception.

Seven Springs president Herman Dupre says the resort has "nearly doubled" its snowmaking. Now, when the temperature drops to at least 26 to 28 degrees, Seven Springs can -- "starting from bare ground" -- blanket all its runs "in 24 hours" and operate all eight lifts.

Wintergreen, Massanutten and Bryce in Virginia; Canaan Valley in West Virginia; Wisp in Maryland; and Elk Mountain, Laurel, Camelback, Blue Knob, Hidden Valley and Ski Liberty in Pennsylvania -- all report new investments in snowmaking and grooming equipment.

"We can make more snow at slightly warmer temperatures," says Ski Liberty's Whitney, "and a lot more at colder temperatures."

Though the resort opened before Christmas with machine-made snow, warm weather and rain shut down the lifts several days until the Christmas week cold snap provided the weather area resorts needed to make snow at full power.

Liberty, explains Whitney, has more than 200 "snow guns" operated by two five-member crews. They work 12-hour shifts around the clock as long as the temperature stays cold.If it warms up during the day, they can still make snow at night.

"Any time the temperature drops below freezing -- especially this time of year -- we make snow." He notes the thermometer at Ski Liberty generally reads 10 degrees lower than in the city, giving them a head start on winter.

Whitney estimates the cost of operating the snow guns at about $400 an hour, a major budget item. "We spend about a half-million a year on snowmaking."

You might think sun -- or rain -- are a resort's biggest threats. But, says Whitney, it's "warm fog. A day of rain, or even steady rain when we've got a couple of feet of snow, doesn't really affect us. A night that stays humid and foggy -- that's when the snow melts."

As for those who consider machine-made snow as something of a fake, Whitney argues: "There's really no difference."

Machine-made snow, he says, falls more as "little ice cyrstals" than as natural flakes. "It's a little more abrasive. It's going to feel a little slower. But once it's packed and skied on first thing in the morning, there's no difference at all."

The resort, he says, tries to lay down a wetter snow because drier snow "will blow away or skiers will push it away." With a good base, "toward morning we can limit the water to make a softer, drier, fluffier snow for the surface."

Marc Hodler, president of the International Ski Federation, calls machine-made snow "superior" to natural snow. "We plan to recommend to other countries that they install snowmaking equipment because it clearly produces better surfaces for skiing . . ."

Seven Springs' Skylling would agree. For crowded ski resorts, he believes, the manufactured product "holds up better than natural snow." Still, he says, the crystals spewing from those big nozzles lack one special quality of the real stuff:

"Natural snow gives you powder -- which is very, very beautiful."