As air is heated it expands and will hold more moisture. Cooler air, on the other hand, holds less moisture. If the moisture content of the air is unchanged by weather changes, the air becomes "heavier" or more moisture?saturated as it cools, because it is capable of holding less and less as it cools. At some temperature it will not be able to hold any more. Humidity is then 100%. That temperature is called the "dew point."

If the dew point is 35 degrees the humidity will be 100% at 35 degrees. If the temperature drops much lower, the result will be fog. Is not fog, like clouds, tiny droplets of water? The more moisture in the air, the more heat will be retained, trapped. The dryer the air, the greater the heat loss. The kiss of death is a night that stars are shining real bright ? no humidity or clouds to block the heat ? and no wind. The higher the dew point the less chances of frost. The lower the dew point, the greater the chances. Seldom will you get frost with a dew point of 45 or higher. When the dew point is down in the 20's you are in for a frost unless daytime high was above 70 degrees.

Why is there frost on the grass before the tomato plants were killed? When the earth is experiencing rapid radiational cooling, the heat is rising very fast. Therefore, it is warmer 40 feet off the ground than at ground level, and the bottom few feet are really dramatic. Just barely above the ground is the coldest. A thermometer can read 33?34 degrees at 4 feet high, yet there can be frost on the grass. If one is placed at grass level and another in between at four feet the temperature can be 33?34 degrees. At two feet it can be 31 degrees, and at grass level it will be 29?30 degrees. There can actually be a degree drop per foot.

Because the temperature can differ at various heights off the ground, the National Weather Service places its recording instruments at exactly four feet high. We would do well to do the same so as to have a common frame of reference. Just remember that on a dry, cold night, the ground can be as much as 4 to 5 degrees colder (or as little as 0 or 1 degree) depending on cloud cover, dew point and wind..

One of the keys to predicting the first frost is the moon. Before you laugh this idea off as lunacy consider the following.

Because at the time of a new or a full moon both sun and moon are lined up to bring about a maximum gravitational pull giving rise to the highest tides, these periods have frequently been presumed to have a particular influence on what to plant and when to plant it. Root and tuberous crops are supposed to perform more satisfactorily when planted on a waning moon, diminishing light; while flowering, fruiting and foliage crops are presumed to produce higher yields when planted under a waning moon, increasing light. Although no experimental proof of these benefits has been recorded, even if there were some such influence, so many different considerations are involved in planting that such a schedule of planting would be impossible to follow without considerable difficulty in practice.

However, the moon does have at least one valid, well?documented effect on the weather effect on the weather which might be considered in relation to the planting and culture of vegetables. Considerable heat is reflected by the moon from the sun to the earth's upper atmosphere during the nights when the moon is full. Most of this heat is dissipated and serves to evaporate a haze or to thin heavy cloud formations which at times hold heat near the earth's surface. Clearing the sky, as a full moon frequently tends to do, allows heat to be lost from the earth by radiation. Late spring and early fall frosts are therefore of more frequent occurrence, on any given date when the moon is full.

Dryer times are generally immediately preceding or during a full moon or just before a new moon. Perhaps the tides have something to do with the quantity of water that is evaporated into the air during a new or full moon and subsequently precipitated. Dryer times mean frosty times. Once moisture comes in, only if all it does is to increase dew points, further critical temperatures will usually not occur until there is some new mechanism to bring drier air back in again. Critical minimum temperatures is not only a function of how cold the air is, but how dry the air is and to a lesser effect how moist the soils are. I will stand by the November 22 prediction. So don't laugh at the moon. Perhaps if planting in relation to the signs of the zodiac and the astrology were forgotten, and possible associations between the various phases of the low night temperatures and rainfall were observed for a period of time, the moon information on the calendar might become of considerable value in certain sections of the country. Even though you may no longer be romantically inclined, it may be worthwhile to keep up with the changes in spring and fall moons.