A DIP in the icy waters of the Atlantic Ocean can seem mild in comparison to a winter outing on the Mount Vernon bike trail. But these chill days don't have to be a total loss for the bicycle enthusiast.

Instead of slogging along the bike path wondering if your hands and feet will thaw out by spring, head off the pavement and try riding on a dirt trail. Within a few minutes that infernal chill will be replaced by a warm glow because you have to work so much harder.

Bicycles are no strangers to dirt trails. At the turn of the century, cyclists wheeled their machines over the same rugged tracks traveled by horses and carriages. Some of the first races were held on a grass-covered circuit, and over the years three distinctive styles of cross-country cycling have evolved.

Many European professionals ride in cyclo-cross competitions during the winter months, riding their lightweight bicycles over tough hilly circuits to maintain condition for the summer road season. BMX, or bicycle moto-cross, has been popular with children for years. A group of kids tackling a homemade obstacle course is a common sight in many suburbs.

The mountain bike, or all-terrain bicycle, is a grown-up version of the BMX bike. Like touring and racing bikes, mountain bikes come in a variety of sizes and are equipped with multiple gears. But these sturdy machines are more versatile than either the BMX or cyclo-cross bikes. The only limitations on where a mountain bike can travel are those imposed by the rider.

You don't necessarily have to run out and buy a new bike to enjoy this sport. You can adapt a regular 10-speed racer by putting on narrow gauge tires with knobby tread to grip on rough and uneven surfaces, and by replacing drop bars with a set of straight ones to improve handling.

But you'd be better off with a mountain bie or a city bike, which is basically a mountain bike frame with different gears and smoother tires. A quick change of both will make your city bike trail-worthy.

If the ground is dry and fairly smooth, any bike will do. However if you plan to ride on rough trails, only a mountain bike will stand up to the stress.

The all-terrain bike frames are designed to take a pounding. Pedals are set high for easy clearance over large obstacles, while brakes and gear levers are mounted within easy reach on wide straight handlebars. The range on the 15 gears is much lower than those used for racing or touring.

Dirt roads are perfect for beginning off-roaders. The surfaces can be rough but not too frightening for riders used to smooth pavement. But don't expect to go too fast at first. Part of the fun on a mountain bike ride is enjoying the scenery.

Trail riding requires a little practice, too. Once you've mastered ascending and descending, test your skill on an easy horse trail. Try lifting up the front wheel and pedaling over logs or any other obstacle along the path. If you don't make it, heave the bike over your shoulder until the obstacle is cleared and then carry on.

On rough descents, lower the seat a few inches and keep your weight over the back wheel. Use the front brakes and one foot, if necessary, to slow down. Shift into the lowest possible gear when climbing -- trails and dirt roads are rarely graded and are steeper than paved ones.

Any off-road cyclist should be aware of the impact a bike can have on the environment. Riding cross-country can damage fragile ecosystems and can aggravate erosion, so it's important to stay on the trails at all times. One advantage to cycling off- road when temperatures are below freezing is that the ground is harder and less vulnerable to damage. (On the other hand, it's important to take special care in areas where snow has softened the ground.) Another rule of the trail: If you encounter horses or hikers, pull off to the side and wait for them to pass.

Before you head out to the nearest wooded area, take a few precautions. Most undeveloped land is owned either privately or by the government. Plan your route carefully and obtain permission before inadvertently trespassing on someone's property. Topographical maps show trails and dirt roads, and county tax records will indicate who owns the land.

Many of the National Parks permit bicyclists on fire roads and horse trails. However, wilderness areas are off limits. Check with the ranger station at the park you wish to explore beforehand for information on accessible areas. Ordinances vary in state and county parks.

If you're heading out for a long excursion, be sure to tell someone where you intend to go and approximately what time you expect to return. Check the weather forecast and wear enough clothing to insulate against any changes in temperature. Add one extra layer every time the thermometer drops ten degrees, and take along a knapsack so you can discard or add layers throughout the day.

Bring snacks, water and rain gear. A thermos filled with hot cocoa or coffee is a blessing on a cold day. Wear a sturdy pair of shoes, preferably with high tops, that will stay dry in case you accidentally lose your footing and land in a half-frozen puddle of water. There's nothing worse than cold, wet feet. And don't forget a pair of heavy gloves to protect your hands from road shock and the cold metal handle bars.

Happy trails!

OFF THE BEATEN BIKE PATH

For more information about racing and touring off-road, contact the National Off-Road Bicycle Association (NORBA), 2175 Holly Lane, Solvang, CA 93463, 805/688-2325. The following local areas are open to off-road cyclists:

*C&O CANAL -- The unpaved towpath extends for 185 miles between Georgetown and Cumberland, Maryland. There are hiker/biker campgrounds every five miles for those who want to ride the entire distance. Parking facilities at various locations make the canal perfect for day trips of any length. Sections of the canal are closed right now because of flood damage. Phone 399-3613 for information on trail conditions. Maps and further information about the canal are available from the National Park Service. Rated Easy.

*GREAT FALLS NATIONAL PARK -- (Virginia side) There's an extensive network of horse trails open to cyclists. Check at the Visitors Center for information and maps. Rated Moderate to Difficult.

*PRINCE WILLIAM FOREST PARK -- Quantico, Va. It has many hiking trails and fire roads park that are perfect for cycling. Check at the Visitors Center for trail information and maps. Rated: Easy to Moderate.

TRAIL GUIDES

If you want to leave the planning to others, a number of bike shops organize winter trail rides. Among them are The Bicycle Exchange in Falls Church (538-5000), College Park Bikes (588-6160), The Bicycle Place in Silver Spring (864-2241), and Twinbrook Cycles in Rockville (340-0050). In many cases, however, their rides may run as long as four hours, and may be too challenging for a beginner.

One place that does cater to beginners is Adam's Bicycles in Gaithersburg.

ADAM'S BICYCLES -- 240 N. Frederick Avenue, Gaithersburg Plaza, Gaithersburg. 840-0484. Daily rental of mountain bikes. In addition, it will provide information on places to ride in the area. A group trail ride leaves from the shop every Sunday at 11. Phone for details.