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.

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.