[Illustration: FIG. 8.—Continuous records of the temperature from 4 P.M. to 9 A.M. at the base and at different heights above the base of a steep hillside, showing the great differences in temperature that sometimes develop on a clear, still night. Although the temperature at the base was low enough to cause considerable damage to fruit, the lowest temperature 225 feet above on the slope was only 51°. Note that the duration of the lowest temperature was much shorter on the hillside than at the base.—_Weather Bureau._] 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.