1284 



FROST 



FROST 



The warnings are distributed by mail, telegraph and 

 by telephone. They are telegraphed at Government 

 expense to many telephone companies. 



Forecasting frost from local observations. 



It is not possible to forecast frost twenty-four or 

 thirty-six hours in advance without the aid of the 

 weather map; but, by observing the local conditions 

 during the late afternoon and early evening, it is pos- 

 sible often to determine whether a frost will occur 

 before morning. Assuming that it is the frost season, 

 the conditions to be considered are: (1) the character 

 of the preceding weather, (2) the state of the sky, 

 whether cloudy or clear, (3) the direction and force of 

 the wind, (4) the trend of the temperature, and (5) 

 the atmospheric pressure. 



Preceding weather. — Since the weather comes in 

 waves, an abnormally warm period is likely to be fol- 

 lowed by the opposite extreme. 



State of the sky. — A clear sky increases loss of heat 

 by radiation, as explained already. 



Direction and force of wind. — A southwest to west 

 wind indicates that the cyclone is passing (except per- 

 haps on the Pacific coast, where other conditions may 

 modify the directions), and that the anti-cyclone is 

 approaching, while a northwest to north wind indicates 

 that the anti-cyclone is near. If the wind dies away it 

 will become colder near the ground. 



Trend of the temperalure. — If the temperature falls 

 steadily during the late afternoon, reaching 40° by 6 or 

 8 P. M., with a clear sky and a hght wind, frost ia 

 indicated before morning. 



Atmospheric pressure. — The actual stage of the 

 barometer is not important, except if the pressure has 

 been very low during the day it indicates the passage 

 of a deep depression which is likely to be followed by 

 a high crest. If the barometer rises rapidly during the 

 late afternoon or early evening it indicates the rapid 

 approach of the anti-cyclone. 



The influence of local conditions on frost. 



Everyone who has lived in the open country is 

 familiar with the fact that some places are more sub- 

 ject to frost than other places. Crops in one part of a 

 field may be destroyed by frost, and in another part of 

 the same field remain uninjured. The explanation for 

 this seeming discrimination is found in the influence 

 of local conditions. 



There are five factors that determine the frost risk 

 at any place: (1) location, city or country, (2) ele- 

 vation and topography, (3) proximity to bodies of 

 water, (4) exposure to the sun, (5) soil and soil cover- 

 ings. 



Location. — Frosts are much more hkely to occur in 

 the open country than in cities. Many investigations 

 confirm this. The higher night temperatures of cities 

 is attributed to the heat given off from buildings and 

 pavements, and to the smoke from the many city 

 fires that collects over cities on quiet nights. 



Elevation and topography. — The average tempera- 

 ture decreases with elevation above sea-level at a rate 

 of 1° for each 300 feet of ascent. From this it might 

 be expected that hilltops would be more frosty than 

 adjacent lower lands. Such is not the case. On clear 

 quiet nights the colder air that settles to the surface 

 drains away from the hilltops and hillsides and accumu- 

 lates over the low lands, decreasing the temperature of 

 the vegetation and sometimes causing frost. If the val- 

 ley into which the cold air drains is closed, so that it 

 cannot flow away as rapidly as it accumulates, a "frost 

 pocket" results. Some farms have many such "frost 

 pockets." A walk over a farm on a clear quiet night 

 often will reveal their location and extent. Low hill- 

 tops and hillsides, but not too near the valley floor, 

 should be chosen for fruits or other crops that are 

 liable to be injured by frost. 



Proximity to bodies of water. — Under similar condi- 

 tions, land warms and cools about five times as rapidly 

 as water. During the season of spring frosts, the water 

 is relatively cool, although considerably above the freez- 

 ing-point. Therefore, it tends to cool the air over 

 adjacent lands during the day and to warm it at night. 

 The influence of a body of water on nearby vegetation 

 is twofold: by lowering the day temperature it retards 

 growth, and by increasing the night temperature wards 

 off frost. The fruit-belts along the various lakes are 

 examples of this twofold influence. The cool air from 

 the lake by day retards the blooming period, and also 

 gives immunity from frost at night. The influence of a 

 body of water is more marked in the fall than in the 

 spring, because of the heat stored up by the water dur- 

 ing the summer. It also increases the length of the 

 growing season over adjacent lands by warding off 

 frost both in spring and fall. The distance to which 

 the influence of a body of water will extend inland 

 depends upon the volume of water, its temperature 

 relative to that of the land, the area of its free surface, 

 the slope of its shores, and the prevailing winds. The 

 prevailing winds in the middle latitudes are from the 

 west. Thus, the influence of a lake extends farthest 

 on its east side, and farthest also when the land slopes 

 gently away from the water. The influence of Lake 

 Michigan, mainly because of the gentle slope of its 

 eastern shore, extends nearly half way across the state 

 of Michigan, while the influence of Lake Erie, because 

 of the abrupt rise of its eastern shore, extends inland 

 only a few miles. 



Expo,sure. — Hillsides exposed toward the south are 

 warmest; next come those facing east, then west, and 

 finally north. Frost liability follows in the reverse 

 order, being greatest on the north side. In the eastern 

 states many fruit-growers prefer the northern slope for 

 an orchard site, notwithstanding its greater liability 

 to frost. This preference is based partly on the opinion 

 that the colder soil and air of northern slopes tends to 

 retard the blooming time until the period of spring 

 frosts is passed. 



Soil and soil coverings. — Dark-colored, sandy soils, 

 because good absorbers of heat, are least liable to 

 frost. Many cranberry-growers cover the surface of 

 the bogs with an inch or two of sand as a means of 

 protecting from frost. The sand stores up heat by day 

 with which to combat frost at night. Well-drained 

 soils are less frosty than poorly drained soils, because, 

 when the soil is wet, the heat from the sun is expended 

 in evaporating the water, and not in warming the soil. 

 Good tillage reduces the frost risk, because a loose, 

 porous soil absorbs more heat than a hard, compact 

 soil. WiLFORD M. Wilson. 



The protection of orchards against frost-injury. 



Although much interest has been manifested in the 

 prevention of frost-injury to orchards in recent years, 

 it is well known that the protection of plants and 

 fruits from such injury dates back more than 2,000 

 years. PUny the Elder, one of the most noted of Roman 

 writers, who lived from 23 to 79 A.D., states that the 

 Romans practised heating and smudging as a protec- 

 tion against frost-injury. We have no doubt that the 

 practice was successful, since it was recommended by 

 Phny whose one surviving work, his "Natural Histor.y," 

 is considered a storehouse of facts. Smudging was also 

 recommended by Olivier de Serres, a French agricul- 

 turist, in the sixteenth century. He recommended the 

 use of wet straw and half-rotten manures so as to jjro- 

 duce a heavy smoke. In the latter part of the eighteenth 

 century, the practice of smudging was compulsory 

 in parts of Germany, and failure to comply with certain 

 regulations resulted in prosecution before an officer 

 of the law. According to Boussingault, the celebrated 



