Snow and topography

Frontal disturbances with a low cloud base (i.e. high surface humidity) dump most of their precipitation on the upwind side of a high mountain ridge. Snow amounts are usually highest on the mountain ridge, or just downwind of the ridge. In fact large amounts of snow are sometimes found far in the lee of a mountain ridge, for several reasons.

• Firstly, snow has a fall speed of about 1 m/s, whereas rain falls at about 5 m/s (depending on the drop diameter), so snow tends to be transported further downwind from the mountain crest than does rain.

• Secondly, wave motion occurs in the troposphere in a stable environment with increasing winds with height (as is typical in winter storms). Rapid ascending motion occurs in the lee at heights well above the mountain crest, just behind the foehn gap. In this case, much of the precipitation formation occurs in the lee, especially if the cloud base remains above ridge level, and the cloud becomes glaciated as it is cooled during ascent in the gravity wave. Bands of light precipitation may also occur much further downstream of the mountain ridge, in areas were downwind gravity waves cause ascent (1).

• Finally, thunderstorms may form as air is lifted over the mountain crest in a potentially unstable atmosphere, even in winter storms. During their typically short lifetime such thunderstorms may be most productive in the lee.

There are notable snowstorms downwind (i.e. to the east) of the Great Lakes of North America, even though the downwind shores are essentially flat. This lake-effect snowfall occurs mostly at times when the skies are clear upwind.

(1) Klimowski, B.A., R. Becker, E.A. Betterton, R. Bruintjes, T.L. Clark, W.D. Hall, B.W. Orr, R.A. Kropfli, P. Piironen, R. Reinking, D. Sundie, and T. Uttal, 1998. The 1995 Arizona Program: toward a better understanding of winter storm precipitation development in mountainous terrain. Bull. Amer. Meteor. Soc., 79, 799-813.