WHENEVER there's enough snow in the Washington area -- usually once or twice a year -- out come the cross-country skis. You can see the narrow parallel tracks left on the roads, sidewalks and backyards, as cooped-up skiers jump at the chance to get outside into a freshly white landscape.

In Alexandria, for instance, the tracks often lead to the bicycle paths along the river, north or south of the city, or perhaps to the paths along Four Mile Run and the Washington and Old Dominion Railroad Park that stretches westward nearly to the Blue Ridge.

For folks eager to cross-country ski more than once or twice during the season, the mountains immediately to our west -- no more than a four-hour trip by car -- offer the opportunity almost all winter long.

Which helps explain why the sport has become so popular hereabouts. Not to mention the fact that you don't need ski lifts or expensive equipment, just some skinny skis and bindings, low boots, poles and snow.

Cross-country skiing is a throwback to what skiing originally was all about: getting efficiently and economically from one place to another when there's snow on the ground.

It can be glorious, too, kicking and gliding quietly along a forest track after a snowfall, looking for the evidence left by other creatures that have passed that way -- the five-clawed prints of a squirrel or the widely spaced prints of a hurrying rabbit.

And cross-country -- or x-c for short -- gives you plenty of exercise as well. Your legs, arms, chest and lungs all get a workout. It's a lot more energetic than standing in a downhill skier's lift line to get a ride to the top of the mountain; cross- country comes about as close as any sport can to exercising your entire body.

Most of all, though, there is a sense of simplicity. Getting out and doing it, for all the pleasure it gives, is no big deal.

Several ski shops in the Washington area will sell you a beginning x-c package with skis, mounted bindings, boots and poles for a little more than $100.

The skis are longer, narrower and more flexible than the downhillers, and are bowed upward in the middle so that the skier glides on the forward and rear portions. In the middle is the "kick zone" that comes into contact with the snow when you put your weight onto it and push yourself forward.

Once x-c skiers had to choose the proper wax to put on the kick zone to provide the necessary purchase on the snow -- hard waxes for very cold days and new snow; soft, messy klister waxes for old snow and warmer days. Many skiers still prefer that approach, but most x-c skiers now use waxless skis that have a pattern of groves or "fish scales" cut into the kick zone. When the skis are pushed downward and backward, the pattern digs into the snow.

The heavier you are and the taller, the stiffer and longer the ski you need. The trick is to find one whose kick zone is off the snow when you are standing with your weight evenly distributed on both skis but which comes down when you shift your weight to one ski.

How can you be sure you have the right skis? Put an index card or dollar bill under a ski; if it moves when your weight is on both skis but can't easily be pulled out when you stand on one ski alone, you've got the right skis.

In general, the wider the ski, the more stable it will be. Touring skis, usually about 52 to 55 mm wide, are good for beginners and for making your own trails -- which is the case with most x-c opportunities near Washington. Racing and light-touring skis are narrower and faster but less stable.

Compared to downhill ski boots, most x-c boots are small and light. Most are made to attach to bindings only at the toe. However, some also have a groove in the heel to fit onto a plate on the ski to give better lateral control.

The boots usually are made with a wide projecting toe piece with three holes that fit pins in the binding -- the so-called 75 mm Nordic norm. Others use a similar but narrower binding, the 50 mm norm. And then there are various "systems" -- super light and narrow and more expensive.

Make sure your boots fit well with enough socks on to keep your toes from freezing!

Poles should fit under you armpits snugly. They may be made of bamboo, fiberglass or other material. Because of its flexibility, bamboo, the cheapest, may survive a hard fall without breaking.

If you expect to go out in loose powder snow, you'll also want to get some gaiters to keep the snow out of your boots. They'll help keep you warm, too.

Once you're outfitted, where to go? Well, just about anywhere if there's snow.

The bicycle trails along the Potomac, the C&O Canal Towpath, Rock Creek Park and Anacostia River Park are all close and pleasant. Farther afield are places such as Cedarville State Forest in southern Prince George's County, Burke LakePark in Fairfax County and Lake Needwood and Lake Frank parks in Montgomery County.

The rolling countryside of Manassas National Battlefield Park just off I-66 in Prince William County is an ideal place to ski. A detailed trail map is available at the visitor center.

Farther west is Shenandoah National Park with its extensive network of trails, many of which can be skiied, particularly the wide fire roads. When there's enough snow to close Skyline Drive, that, too, becomes a superb trail.

When there isn't any snow close to Washington, you can go west, about 175 to 200 miles to a series of state parks in the Allegheny Mountains: Canaan Valley and Blackwater Falls in West Virginia, New Germany and Herrington Manor in Maryland and Laurel Hill and its neighbors in Pennsylvania. All have x-c trails.

Next to Canaan Valley lies the privately operated White Grass Ski Touring Center. It has about 30 miles of trails, about seven of which are groomed -- that is, they have two parallel double tracks set by machine to help guide your skis. Both Canaan Valley and White Grass offer x-c lessons and equipment rental, as do a few other downhill areas such as the Homestead in Hot Springs, Virginia.

Once you get comfortable on your x-c skis, there are all sorts of other possibilities for adventure.

Telemark and mountaineering skis and boots are a sort of cross between downhill and x-c equipment that allow skiers to tackle truly rough country. Or there's x-c racing, with a series of races scheduled for this winter at the state parks in the Alleghenies.

In other parts of the country there are ski touring centers, such as Jackson, New Hampshire, where the Jackson Ski Touring Foundation and its 100 miles of trails have revitalized an entire village.

Another exciting possibility is to tour through the Rocky Mountains on a trail that runs from Aspen to Vail in Colorado, with overnight huts along the way. Or there's great x-c skiing in Yellowstone and Grand Tetonational Parks, or . . . go out your front door and down your sidewalk.

WHERE TO GO & HOW TO GET THERE

Here are some places that offer good cross-country opportunities. Caution: All depend on natural snowfall, so it's always best to call ahead.

WASHINGTON AND NEARBY

Two books by Alan Fisher, "Country Walks Near Washington" and "More Country Walks Near Washington," include maps and detailed instructions for reaching areas suitable for x-c skiing. They're available in local bookstores. A sampling of local areas includes Theodore Roosevelt Island, Rock Creek Regional Park, Northwest Branch Park, Greenbelt Park, Anacostia River Park, Burke Lake Park, Cedarville State Forest, Riverbend Park, Great Falls Park, Little Bennett Regional Park, Washington and Old Dominion Railroad Regional Park, C&O Canal Towpath and the bicycle paths along the Potomac River.

MANASSAS NATIONAL BATTLEFIELD PARK -- 18 miles, about 25 minutes from the Beltway: west on I-66, north on Va. 234 to visitor center on right. Extensive trail network. Map. Call 703/754-7107.

PRINCE WILLIAM FOREST PARK -- 20 miles, about 25 minutes, south on I-95 from the Beltway, follow signs to park. Map. Call 703/221-7181.

MARYLAND

CATOCTIN MOUNTAIN NATIONAL PARK -- 50 miles north from the Beltway via I-270 to U.S. 15 north at Frederick. At Thurmont take Md. 77 three miles west to visitor center, open daily 8 to 5. Map. About 15 miles of hiking trails and closed roads suitable for x-c skiing. Call 301/663-9330.

NEW GERMANY STATE PARK -- 145 miles, about 3 hours, west from the Beltway via I-270, I-70 and U.S. 40 and 48. Take marked exit west of Frostburg, Md., and follow signs south to park. Map. 10 miles of trails, heated building. Trail fee $1 on weekends and holidays only. Call 301/895-5453.

HERRINGTON MANOR-SWALLOW FALLS STATE PARK -- 25 miles west of New Germany, about 33/4 hours from the Beltway. Continue on U.S. 48 to U.S. 219 south to Oakland, Md. Turn right one block past traffic light. 12 miles of trails; five are packed and six are packed and groomed. Cottages available. Call 301/334-9180, Monday-Thursday 8 to 4, Friday 8 to 4:30.

VIRGINIA

SHENANDOAH NATIONAL PARK -- 75 miles west from the Beltway on I-66 and U.S. 211 to Skyline Drive at Panorama. Maps available at area camping supply stores. Call 703/999-2229 or 999-2243.

PENNSYLVANIA

HIDDEN VALLEY SKI AREA -- 190 miles north and west via I-270 and I-70 to Somerset exit of Pennsylvania Turnpike and Pa. Rte. 31 10 miles west. Map. 30 miles of trails, all groomed. $5 fee. Equipment rental and lessons, x-c and telemark. Downhill skiing and lodging. Call 800/458-0175 and ask for the x-c barn, or 814/443-1900.

KOOSER STATE PARK -- Adjacent to Hidden Valley, with 1.5 miles of groomed trail connected to that area's network. Office open to skiers. Lodging operated by a concessionaire December 24-March 31. Call 814/445-6814 for lodging. Call 814/445-8673 for the park.

LAUREL RIDGE STATE PARK -- South of Hidden Valley. From Somerset go south on Pa. 281 for 10 miles and west on Pa. 653 about 8 miles. 12 miles of loop trails, more being built. Campfires allowed. Call 412/455-3744.

LINN RUN STATE PARK -- 195 miles, about 4 hours, north and west, also in Pennsylvania. From the Somerset exit follow signs to U.S. 219 north and U.S. 30 west. About 9 miles west, turn left on Summit Road at top of the mountain. Park at the Gate House for Laurel Mountain Ski Area. Map at Ski Patrol building. 50 miles of trails. Equipment rental at Laurel Mountain, which has mountaineering trail beginning at the top of lift. Regular lift ticket required. Telemark and x-c equipment rental also at Ligonier Mountain Outfitters in Laughlintown, 5 miles west of Summit Road on U.S. 30. Limited lodging. Call Laurel Mountain, 412/238-6688, or Linn Run State Park, 412/238-6623.

WEST VIRGINIA

CANAAN VALLEY STATE PARK -- 200 miles west from the Beltway. Take I-66 to I-81 south to U.S. 11. In Strasburg, Va., take Va. 55 west to Petersburg. Take W. Va. 28 west and south to Seneca Rocks, go west on U.S. 33 to Harmon and north on W. Va. 32 to the park. Map; 18 miles of trails. No fee for x-c trails. One slope for telemark skiing and lessons. No x-c equipment rental. Also downhill skiing and accommodations. Call 800/624-8632 outside W. Va. for lodging and skier info, and 800/624-9110 for snow report. In state, call 800/642-9058.

BLACKWATER FALLS STATE PARK -- 9 miles north of Canaan Valley. Map; 8 miles of trails with additional trails in adjacent Monongahela National Forest. Lodging. Canaan Valley shuttle bus for park guests only. Call 304/259-5216, or for lodging reservations only, 800/624-8632 Monday-Friday, 8:30-4:30.

WHITE GRASS SKI TOURING CENTER -- Adjacent to Canaan Valley. Map; 30 miles of trails, 7 miles groomed. Telemark slope with a 400-foot drop. Accessible from Canaan Valley ski lift and by trail from Timberline Ski Area. $4 fee weekends, $3 weekdays. Equipment rental, lessons. Call 304/866-4114.