If you want watermelon this year, you'd better get it soon; the extra-hot weather has shortened the season in the Maryland-Delaware fields. When those crops go, it's the last watermelon of the year for the whole country. This area is as far north as they can be grown for the market.

Maryland and Delaware produce more melons for the market per year than any other state because of their climate and soil. A 15 square-mile area around Salisbury on the Eastern Shore produces most of the local crop. Watermelons need the good fertile sandy soil that area provides. A farmer can get from 1,500 to 2,000 melons from an acre there in a good year with irrigation and spraying. But this year Maryland farmers fear they may lose as much as 40 per cent of their yield because the heat has shortened the season.

A ripe melon has a hollow sound when patted. An immature one will hardly have any color on the bottom; when it's ripe the bottom is just starting to turn yellow.

Look for melons that are firm, symmetrical, fresh and attractive and of good color. The color may vary from a deep solid green to a gray, depending on variety. Sweetness largely depends on variety and on ripening on the vine, not on appearance.

If you buy melon by the slice in supermarkets, look for a fresh firm texture and bright color. Dry mealy flesh or flesh that is watery or stringy usually means an overripe melon. The flesh should be bright red and the seeds black.

Larger melons have more edible flesh in proportion to weight than smaller ones. But quality depends on how fast you use a ripe melon. If you can't eat it all within about 48 hours, you may be better off to buy two smaller ones than a large one. Melon starts to evaporate when it's cut. If you see it drying, it's losing nutritional value, quality and flavor.

If you cut the melon out of the rind and store it in a glass container covered tightly with plastic wrap it keeps better than if left in the rind.

Another tip: A quicker way to chill a melon than to try to chill it whole is to slice it and then put it in the refrigerator.

Experts recommend the Crimson Sweet variey as best for pickling and eating. The light green Charleston Gray variety is also recommended for pickles.

Watermelon is surprisingly high in some nutritive values. A 2-pound wedge supplies half the daily recommended intake of both vitamins A and C. It contains more iron per serving than most other common fresh fruits. Since about 92 per cent of the melon is water the same amount contains only about 120 calories.

Can you grow melons in the backyard? A vine is likely to occupy a space about eight by five feet. Small varieties, "Bush Babies" and "Sugar Bush," occupy somewhat less space but should only be grown on the same location every 8 years. The clay soil found in many local back yards does not produce good watermelon.

Watermelon, incidentally, is classified as a vegetable by the USDA, although most people think of it as a fruit. But whichever you call it, watermelon won't be around much longer this summer. WATERMELON SNOW (6 servings) 2 cups cubed watermelon, seeds removed 1 cup orange juice Juice of 1 lemon 1/2 cup sugar 2 egg whites

Puree melon in blender. Add the juices and sugar and blend well. Freeze in ice-cube tray about 1 1/2 hours, or until partially firm. In a separate bowl beat egg whites until stiff. Beat partially frozen mixture briefly until light. Fold in the egg whites by hand. Freeze but serve before it gets hard.