Climate of Virginia
Climate is defined as the average weather patterns experienced by a given region in a normal year. The two main components of climate are temperature and precipitation. Virginia's climate is characterized by hot summers and mild wild winters and a fairly uniform distribution of preciptation throughout the year. This type of climate is known as Humid Subtopical (Cfa in the Koeppen climatic classification system) and is typically found on the east coasts of continents between 25 and 45 degrees latitude.
Annual temperature patterns are largely a function of latitude. The actual temperatures experienced at a given latitude are further influenced by elevation and nearness to the sea. Land near the coast, or peninsulas surrounded on three sides by the sea, or small islands completely surrounded by the sea will have a lower annual range of temperatures that sites located inland on a continent: maximum temperatures in summer may not get as high; and, more importantly, winter temperatures are mider than farther inland. This moderating affect of the sea is known as maritime influence. The greater annual range of temperatures that occurs in the interior of continents is called continentality.
One way that temperature differences across the state are evident is in varying lengths of the growing season. A growing season is defined as the number of days between the last killing frost in Spring and the first killing frost in Autumn. This is the period of time available for growing crops or other annual plants. The longest recorded growing season in Virginia is 259 days at Cape Henry. The shortest recorded growing season is 135 days in Burkes Garden. (Indications are that in recent years the growing seasons across the US are lengthening in response to global climate changes.)
Precipitation requires that a body of moist air be brought over an area and then forced to rise. The rising causes the air mass to cool and release some of its moisture. (Cooler air can hold less water vapor than warmer air.) The source of much of the moist air reaching Virginia is the Atlantic Ocean despite the fact that the state lies in the latitudes of prevailing westerly winds. A semi-permanent cell of high pressure, commonly referred to as the Bermuda High, lies over the Atlantic more or less centered at 30 degrees north latitude. Winds spiral out from the Bermuda High in a clockwise direction and force air masses onto the east coast of North America. As the air masses pass over the sea they draw up moisture thorough the process of evaporation thereby carrying moisture to the continent.
Because of the unique shape of the North American continent, moist air masses blowing from the Bermuda High reach Virginia from two directions. Some comes directly of the Atlantic from the southeast. Some comes from the west, having been forced over the Gulf of Mexico and into the Mississippi River Valley, where it is intercepted by the prevailing westerlies driven eastward. In terms of total amounts of precipitation received in Virginia, about half comes from the southeast and about half from the west. However, in terms of number of storms and amount of rainfall (or snow) per storm, the patterns are distinctly different. Storms coming from the west are more frequent in Virginia but generally release small amounts of precipitation during any given weather event. In contrast, storms coming from the southeast are fewer in number, but the storms tend to be more intense and release larger amounts of rain per event. Hurricanes are an extreme examples of storms entering from the southeast.
On average, Virginia gets about 45 inches of precipitation a year. Spatially, the distribution pattern is not uniform however. Physiography influences the amount any given reigon receives by forcing air masses to rise or fall as they pass over the state. Where air is forced to rise, most dramatically on the windward side of the mountain ranges, greater than average amounts of rainfall are received. Where air flows downhill on the leeward side of the mountains, areas receive less than average amounts. The southeastern and southwestern corners of the state can expect about 50 inches of precipitation a year, whereas the Shenandoah and New River valleys get 34 to 36 inches in a normal year.
Created by SLW, 3/14/00.