“North wind doth blow and we shall have snow”. You may recall that phrase from nursery rhymes. But have you figured out whether a north wind really does bring snow? In Kansas, at least, this is true only some of the time. For snow to fall, two things must be present: cold temperatures and sufficient moisture for the condensation rate to exceed the evaporation rate. North winds usually bring cold, dry Arctic air – cold enough to allow the moisture to condense into a frozen state. It’s the second part of the equation that frequently is missing. The condensation isn’t faster than the evaporation. The results: You get brisk north winds; cold temperatures; and clear, sunny skies. South of the Great Lakes, however, is another story. There, the north wind moving across the unfrozen lakes can pick up a lot of moisture. This can turn to snow as it moves over land, resulting in lake-effect snows that can last for days and fall at the rate of 3 inches an hour.
Mary Knapp, Weather Data Library
mknapp@ksu.edu