Using 'The Old Farmer's Almanac' to outsmart Jack Frost

It's getting late in the season and frost isn't on the punkin yet - not in my patch anyway. That's nice because some late starters among the African white pumpkins (actually great-tasting squash) are continuing to put on weight by the day.

But, I have to be on the lookout for frost - and that's where the The Old Farmer's Almanac for 1984 comes in. There's a chapter in there by Calvin Simonds (from Rodale Press's ''The Weather-Wise Gardener'') that tells how to predict a frost in your own backyard. It's just one of the very practical pieces of information which the almost-200-year-old almanac gives each year to those of us who work the soil.

The almanac (''calendar of the heavens'' is a rough translation of this Arabic word) is informative, interesting, a little screwball here and there, and always makes good reading. I wouldn't be without one.

It's not the long-range weather forecasts that grab me, for even the editors at Yankee Publishing admit it is ''impossible to predict weather with anything resembling total accuracy.'' Nor is it the interesting inserts that tell me, for instance, that Dec. 17 is Wright Brothers Day or that Iowa has 5.3 pigs for every human being. Did you know that Wyoming (first) and Rhode Island (last) are at opposite ends of the energy-consumption-per-capita scale?

As for me, I value the almanac most for the monthly ''Farmer's Calendar'' (each essay a lesson, in delightful writing) and for the practical advice that pervades the booklet.

The '84 almanac, for instance, tells us how to find true north (or south) with a watch. That can be important when siting a new garden. There are planting tables and average dates for first and last frosts, and even plans for making a stovetop oven. Then there is the Simonds piece.

Climates vary so much from place to place that the local weather forecast on TV may not come close to telling you when to expect your first frost. These, then, are the things to watch in your own home, says Simonds:

* Temperature. In moist climates, such as the Northeast, any time the day temperature reaches 75 degrees F. you can be pretty sure no frost will occur; in dry climates, make that 80 to 85 degrees F.

* Wind. On still nights the air tends to settle in layers, with the coldest layers at the bottom. Wind tends to mix up these layers so that what might be a light fall frost on a still night won't occur at all when there's a breeze. But, cautions Simonds, a breeze at dusk doesn't mean one will stay around all night. The local weather forecast should be able to help in this respect.

* Clouds. Even high, thin clouds can hold in the heat and keep temperatures from falling. Again, be cautious. If clouds are thinning as the sun sets, don't count on them. If ''the sun sets through an increasingly dense network of vapor trails and mare's tails, then I don't worry,'' Simonds says.

* Ground conditions. Warm, moist soil offers somewhat more protection against falling temperatures than dry soil. Soil temperatures are generally more helpful in the fall than in the spring.

* Night length. Remember, the longer the night, the more hours there are for the earth to lose its heat.

* Phases of the moon. The moon controls atmospheric tides just as it does the tides of the sea. The highest tides come at a full or new moon which, added to a conventional high-pressure situation, can lead to frost.

* Dew point. Whenever the dew point is 45 degrees F. or higher (the temperature at which the air will give up moisture in the form of dew and release some heat at the same time), a frost is highly unlikely. Dewpoints are generally given on TV weather forecasts.