average normal rainfall, 1971-1990
The greatest rainfall and warmest temperatures are in the southeast corner of the state, where the Chesappeake Bay and the Atlantic Ocean provide warmth and moisture. There is also a rain shadow in the lower Shenandoah Valley, which receives on the average 10 inches less per year than the Dismal Swamp in the cities of Suffolk and Chesapeake. Mountains in West Virginia near Canaan Valley force the winter storms higher, where the air cools to the "dew point" (where moisture condenses and clouds form). Ski resorts are concentrated in that area, taking advantage of the extra snowfall.
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" or mountain effect when air masses are pushed up by the Appalachians. The air cools as it rises, at an adiabatic lapse rate of approximately 3 degrees per 1,000 feet in Virginia. (Technically, the rate is 10 degrees Celsius per 1000 meters rise in elevation for dry air, and 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 rain shadow works in both directions. When Hurricane Fran crossed from North Carolina through the Virginias into Ohio, the mountains squeezed the clouds on the eastern side of the Blue Ridge and Allegheny Front. That generated both massive amounts of rain in Virginia and a very reduced amount of rain in the Ohio River Valley, until the storm regained strength and drenched central Ohio.
As mentioned in Virginia's
Climate:
"Along the Appalachian and Blue Ridge Mountains of western Virginia, this
airflow is sometimes from the west and sometimes from the east. When the flow
is from the west, the New River and Shenandoah River valleys are in the rain
shadow of the Appalachian Mountains; when the airflow is from the east, they
are in the shadow of the Blue Ridge Mountains. As a result, both the New River
and the Shenandoah River valleys are the driest portions of the state."
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, in the days before modern medical cures. However, the cooler temperatures and less-humid air were clearly more comfortable than the hot, muggy coastal plain, even for those who did not get in the maturally-heated water.
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.
