These however, are average values. In individual cases the difference between surface winds and stratus clouds may be considerably greater. It may be as much as 180 degrees; that is, the cloud may move directly opposite to the wind. In general there will be a difference of 10 to 20 degrees. WIND AND RAIN The law of wind direction, approximate cooling and rain. When the lower clouds are moving from the north or northwest, without sharply defined edges, the LOW is east or northeast of the observer; and rain or snow is not likely unless there is a rapidly falling temperature. [Illustration: TURNING OF WIND WITH ALTITUDE, BLUE HILL FIG. 13. TURNING OF WIND WITH ALTITUDE] When a stream of warm air with a high absolute humidity flows north on the east side of a LOW, and a cold northwest wind follows quickly after the LOW, rain or snow may be expected. Any rapid chilling of warm, moist air produces cloudiness and rain or snow; but a cold stream blowing into a warm area will not produce as much rain as a warm stream blowing into a cold area. DURATION OF WIND The average duration of wind from various directions is as follows: From the north about 16 hours each week; from the northeast, the same; from the east, 11 hours; from the southeast, 10 hours; from the south, 24 hours; from the southwest, 27 hours; from the west, 33 hours; and from the northwest 31 hours. During an individual disturbance lasting about 36 hours, we may have 8 hours of southwest wind; 4 hours of west wind, backing during the next 4 hours to south; 2 hours of south wind; 2 hours of southeast wind; 2 hours of east wind; 8 hours northeast wind and 4 hours north wind, 2 hours northwest, when it may be considered that a new pressure distribution prevails. The above values hold only for a storm moving with normal velocity. LOWS are often blocked by slow moving HIGHS in advance. In such cases the duration of east winds is greater. THE WINDS OF A YEAR The following table shows the marked increase in the prevalence of northwest and west winds during winter months, the decrease in north winds during July, the increase in northeast winds in May, also in east winds; the increase of south and southwest winds in July; and the falling off of southeast winds in December. See Table, page 72. In cities near the Atlantic Coast, a continuance of northeast wind, especially in the fall and winter months, results in frequent altho not necessarily heavy rains. On the other hand a period of continued northwest and west wind is a dry period. In summer, southeast and east winds bring fog and cooler weather; while southwest winds are favorable for the development of thunderstorms. WINDS OF A YEAR TABLE I.--Number of Hours the Wind Blows from Different Directions. -------------------------------------------------------------------- Jan. Mar. May July Sept. Nov. Year Feb. Apr. June Aug. Oct. Dec.