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As should be clear from the previous pages, it takes upward motion (and fairly deep upward motion at that) to produce precipitation. Those of us who live in the middle of a barren plain near the Huntsville State Penitentiary (Wall Street Journal, Oct. 1997) experience two basic types of upward motion. The first is what meteorologists call synoptic-scale vertical motion, the vertical motion associated with fronts, cyclones, troughs, and other such synoptic-scale weather systems. The module on upper air maps discussed ways of diagnosing synoptic-scale vertical motion when it was talking about vorticity advection, temperature advection, and the like. The second is convection, the vigorous upward motion produced by air which is unstable: moving upward and remaining warmer than its surroundings.
Two other forms of vertical motion take place elsewhere in the United States. The one I'm discussing on this page is called upslope.
The principle is simple: if air is going over a topographic barrier such as a mountain range, it has to go up. And if the air has to go up so much that the water vapor starts condensing into cloud droplets and eventually precipitation particles, the upslope flow will be directly responsible for precipitation.
A nice example is the Olympic Mountains of Washington. These coastal mountains are in a location that would get lots of rain and snow anyway, what with the frequent pounding of coastal storms. But the 6000 ft+ mountains force air a considerable distance upward, cooling the air and causing lots of moisture to condense out and fall as rain and snow. The effect is so strong that the western slopes of the Olympic Mountains are home to the only rain forest in the lower 48 states.
Sometimes it's not necessary for the topography to do all the work. If the air is convectively unstable, it may not be necessary for the upslope flow to do much more than force the initial condensation before the instability takes over and produces convective rain or snow. This mechanism is common over the Texas Hill Country, where the first convection during the afternoon is almost always located over some of the highest topography.
Here's a question to think about. I'm not sure whether most meteorologists would get it right:
Which of the following situations ought to produce the most rain?
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