Rain Shadows - The Orographic Effect

Why is there a rain shadow there, and not near Norfolk? If you remember high school physics, warm air can hold more moisture than cool air. When you take a shower, the warm air cools on the mirror in the bathroom, where it condenses and fogs up the mirror. The same thing happens on a larger scale outdoors. When the sun goes down in the evening, the air in your neighborhood cools. In Virginia during the summer, the air may cool enough to reach dew point and leave a film of moisture on windshields. In the winter, frost may form on your car windshield when it's close to 32 degrees Farenheight.

There's an "orographic effect" when air masses are pushed up by the Appalachians. The air cools as it rises, at an "adiabatic lapse rate of 10 degrees Celsius per 1000 meters rise in elevation for dry air. (The rate is 6 degrees Celsius per 1000 meters for moist air.) The mountains essentially squeeze the clouds the way we might squeeze a sponge, extracting the moisture before it can fall on the Shenandoah Valley. Since our air masses typically come from the west, extra rain and snow falls in West Virginia.

When the relatively-dry air crosses the Allegheny Front at North Mountain, on the western border of Shenandoah and Frederick counties, it drops into the Shenandoah Valley. Just as it cooled at the adiabatic lapse rate, it also warms when it drops in elevation. The effect of the "rain shadow" is magnified by the warm air, drying the soil east of the Appalachians.

The orographic effect is obvious in the average temperatures as well as the average precipitation statistics. In the days before air conditioning, wealthy Virginians would travel to the resorts in the Blue Ridge and the mountains west of the Shenandoah Valley. The hot springs and mineral springs were thought to have a therapeutic effect, but the cooler temperatures and less-humid air were clearly more comfortable than the hot, muggy coastal plain.

The summer mosquitoes would also spread yellow fever from sailors infected in other ports, so a season in the mountains was healthier for those who could afford it.

Why didn't the rich Virginians vacation at Virginia Beach in the summer before the Civil War? There was no way to get there from here, until a resort developer finally built a railroad from Norfolk to the Atlantic Ocean in the 1880's. Only after World War II, once air conditioning allowed tourists to beat the heat indoors, did the beach became a major destination resort.

One astute student in the 2001 class said:

I think a way to think about rain shadows is to imagine washing your car with a sponge. If the windshield is a mountain, your roof is the summit, and the rear windshield is the backside of the mountain, then the water soaked sponge can represent a cloud saturated with water.

When you start washing the windshield, water is squeezed out of the sponge like water is squeezed out of the cloud as it moves up the mountian, increasing elevation(and thus decreasing temperature, which causes the water vapor to condense) By the time you reach the roof of the car, or top of the mountain, much of the water you started with has already been squeezed out of the sponge, but what is left is used to wash the roof.

When you reach the rear windshield the sponge has used up all of the water that you started with, just like the cloud has squeezed all of its moisture out on the mountain.