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.