The new air world

CHAPTER VII
FROST

There is nothing in the study of the atmosphere that so intimately concerns the horticulturist and the gardener as knowledge of the conditions under which frost forms, and the methods that may be pursued to gain immunity from its disastrous effects, or to lessen the loss.

Frost does not necessarily form from air that has fallen to the freezing point, as many suppose. On the contrary, the air ten feet or less above the vegetation may be several degrees above freezing when there is a heavy and destructive frost upon vegetation. The fact is that vegetation radiates heat towards a clear sky faster than does the air and may fall to the freezing point or below; while the air, except the molecules actually in contact with the vegetation, is considerably warmer. Frost is not frozen dew. The water vapor is precipitated, or rather congealed, upon the vegetation without passing through the liquid state at all. Frost is spoken of as light, heavy, and killing. Tomato plants are killed by only a light touch of frost, while fruit blossoms will stand several degrees of cold below freezing. Therefore the tomato grower would consider as killing a frost that to the fruit grower would only appear as light.

The radiation of heat from the earth is continuous both day and night when there are no clouds to obstruct the passage of the heat rays. The amount received from the sun during the day is greater than the loss by radiation from the earth and the temperature of the air rises. After the setting of the sun the radiation of the earth goes on but there is no incoming heat from the sun to offset the loss and the temperature of the air falls. As previously stated, the soil and vegetation radiate faster than the air and the air in immediate contact with the soil is cooled by conduction to it. Thus over a level plain on a clear calm night there is found a relatively thin layer of cold air near the ground, which increases in temperature up to two hundred or three hundred feet, or which may be only five or ten feet deep. Over sloping ground the force of gravity tends to cause this thin surface layer of cold air to move down the slope and to gather in depressions in somewhat the same manner as water would move. Such movement is called Air Drainage. Of course this air is slowly gaining heat by compression as it passes to lower levels, but it is hugging closely to the cold earth and losing by conduction much or all that it thus gains by compression.

After a study of the contour of the region with respect to air drainage the writer purchased a considerable tract of land near Rockville, Montgomery County, Maryland, and planted extensive orchards thereon, with the result of harvesting nine successful crops of fruit in a period of ten years after the trees became large enough to bear. With the composition and the surface covering of the soil the same, the low places in a field are always the ones that suffer most when frost is possible. Figure 8 shows a minimum temperature of 25° to have occurred at the base of a steep hillside when on the higher ground at an elevation of but fifty feet the lowest temperature was 44°, and at two hundred and twenty-five feet up the mountainside the minimum was 52°.

In selecting a location for an orchard it is not so much a problem of elevation above sea level as elevation above the surrounding region. The direction in which the slope faces makes little difference. The prime consideration is to get sufficient air drainage to gain the greatest protection against frost without selecting land with such a steep slope as to furnish excessive soil drainage and which would be difficult to cultivate and move about upon in the spraying of trees and in the picking of fruit. In the Maryland orchard the elevation was only five hundred feet above sea level and only about two hundred feet above the surrounding region, and the slope was so gradual as almost to be imperceptible to one passing over it.

After nightfall the air on mountain peaks and on hills and ridges soon becomes cooler than the air at the same elevation out over the open valley, due to contact with the elevated earth, which radiates heat and cools faster than the air.

Water vapor has a great capacity for heat. It is the most effective of the various gases present in the atmosphere in obstructing radiation of heat from the earth, as well as in absorbing incoming radiation from the sun. The night temperature, therefore, falls more slowly when the relative humidity is high than when it is low, that is to say, when the air is nearer saturation, or nearer its dew point. Drops of water that collect on the outside of a pitcher of ice water on a warm day are formed through the chilling of the air in contact with the pitcher; they begin to form as soon as the temperature of the pitcher reaches the dew point of the air, which temperature varies in accordance with the amount of water vapor present in the air at the time. After sundown the temperature of exposed objects falls, of some faster than others, depending on their capacities for radiation. Vegetation radiates freely and often falls to the dew point of the air, at which time dew begins to form on it and continues to be deposited as long as the temperature remains above freezing. Now, here carefully note that if the dew point is above 32° the condensation of water vapor in the form of dew liberates latent heat, which usually will be sufficient to check the fall of temperature and prevent the formation of frost. If the dew point of the air is 32° or lower frost forms. If the dew point is very low the temperature may fall low enough to cause much damage without the formation of any frost. As an example, if the dew point be 20° and the temperature falls to 24° much damage might be done to growing crops and no frost appear. This phenomenon is called black frost; it seldom occurs. From the foregoing it might be assumed that the possibilities of frost might safely be forecast from an observation to determine the relative humidity taken early in the evening, but unfortunately experience has shown that reliance cannot be placed in such method of forecasting, as the humid air of early evening may be displaced by much drier air before the hour of minimum temperature the next morning.

One of the best locations to gain immunity from frost at the critical period of plant growth is immediately to the leeward of a considerable body of water. Wind blowing from a large body of water is always heavily laden with moisture, which decreases the rate of radiation both day and night, but especially during the period of cold in the early morning when frost is liable to occur. Such winds, largely affected by the temperature of the water over which they have passed, modify the temperatures of both day and night.

The all-important condition for the formation of frost is an atmosphere already cool, with a gentle northwest wind and a clear sky, which condition, with more or less coolness, always accompanies the high barometric areas that follow the low-pressure areas of warmth, cloudiness, and moisture.

At an expense of two millions of dollars per annum the Government maintains some two hundred observation stations of the Weather Bureau, and twice daily telegraphs observations to all the large cities of the nation, but unfortunately in many cases these are not published for the benefit of the people who could make valuable use of them. The Bureau’s own deductions from these observations, in the form of forecasts and warnings, are extremely valuable, but an even greater service could be rendered the public by neatly lithographing an evening weather map and mailing it from all large cities each night, so that every intelligent person whose business is affected by the weather could, through a study of the chapter on Forecasting in this book, judge for himself as to the effect that the coming weather may have on his particular interests. One could then watch the movements of the high barometric areas and the low areas and become weatherwise himself, and he who studied these charts the most diligently would have an advantage over less progressive competitors.

Evaporation goes on at all temperatures, even below freezing and from solid ice, its rate, of course, being diminished by low temperatures. At times, in spring or fall, the temperature of the air over rivers, when there is little wind, falls so far below the temperature of the water that the water vapor rising from the river by evaporation is quickly condensed in the form of fog, which may cover a part or all of the low contiguous land, checking radiation and preventing a further fall in temperature.

In valleys near the ocean, fog sometimes drifts in from the water when frost is imminent and prevents its formation. On nights with fog, contrary to the usual condition, the hillsides are always colder than the lowlands, unless the fog extends high enough to cover them.

In 1891-1894 the writer, in studying the conditions under which frost forms on the cranberry bogs of Wisconsin, was impressed with the fact that the occurrence of frost on a given field depended as much on the character of the surface and its covering as it did on the temperature of the air a few feet above, one place receiving an injurious frost, another a light frost, and still another none at all, while each had the same conditions as to temperature, wind velocity and direction, and all were at the same elevation, so that the differences could not be accounted for by air drainage.

In one case the marsh was cleanly cultivated and covered with sand, in another there was clean cultivation but no sand, and in still another case there was a thick growth of vegetation. As the result of a long series of observations conducted by Professor H. J. Cox, working under the directions of the writer, minimum thermometers were placed among the vines over newly sanded surfaces in two marshes, one at Cranmoor and one at Mather, Wisconsin. The locations selected for this inquiry represented the best results that could be secured from sanding, draining, and cultivating. Comparison was made at each marsh between the readings taken close to the vines of the clean part of the marsh and those taken close to the surface over the unsanded peat bog. The average lowest night temperature over the sand for the four months was 5.9° higher than over the peat at Cranmoor, and 4.2° at Mather. On one night the minimum over the surface at Cranmoor was 12° higher than over the peat, while at Mather a difference of nine degrees was recorded on another night.

Through cultivation the marsh may be kept free from weeds, moss, or other rank growth, thus permitting the sun’s rays to reach the soil and increase its temperature during the day, while a growth of thick vegetation screens the soil from the sun’s rays, and there is consequently less heat in the latter soil to be given out during the hours of low temperature at night. Drainage lowers the specific heat of the soil and decreases the cooling effect of evaporation. Therefore, under sunshine, the dry soil becomes warmer than the wet and, whether or not it has a greater quantity of heat to give off at night, it has a higher temperature and therefore radiates more freely to the air above. A covering of sand likewise lowers the specific heat of the surface and thereby causes it to gain a higher temperature during the day than an unsanded surface receiving the same solar rays. It therefore radiates more rapidly at the critical time when heat is needed to prevent the temperature of vegetation from falling to the freezing point and gaining a deposit of frost.

In many orchards in the Rocky Mountain States, where fruit growing is highly profitable and the injury from frost more than probable every year, an extensive use is made of oil and other fuel-burning heaters between the rows of trees. Those who wish further information with regard to this matter should send to the Weather Bureau, Washington, D. C., for Farmers’ Bulletin No. 1096. At first thought it would seem that heat so applied would be blown away or instantly escape upward. But on frosty nights there is not much wind; if there is, there is little danger from frost. And then, as previously stated, on such nights there is what is called temperature inversion, and the temperature actually rises with the first few feet of ascent, and the heated air soon reaches air of its own temperature, when no further ascent occurs. When the air forty feet from the ground is ten degrees warmer than it is around and in contact with vegetation, as often occurs on frosty nights, the heat from the fires is nearly all expended in raising the temperature of the air within this forty feet. Figure 9 furnishes the result of an experiment illustrating the correctness of the foregoing theory.

Figures 10 and 11 show the average dates of the last killing frost in spring, and of the first killing frost in fall.