Climate

Mean Annual Temperature

The climate of the Southern Appalachians varies considerably with location and elevation. Not only are higher elevations colder,they experience greater cloud cover, relative humidity, and precipitation. The Southern Appalachian climate is produced largely by continental arctic air masses from the north during the winter and maritime tropical air masses from the Gulf of Mexico and Atlantic Ocean during the summer. Based on the Koppen-Geiger system of climate classification (Aguado and Burt, 2000), the Southern Appalachianclimate is classified as Moist Subtropical with a symbol of Cfa (C stands for warm temperate climate, f for sufficient precipitation in all months, and a for warmest monthly mean temperature over 71^o F). The northern area is classified as Moist Continental climate with a symbol of Dfa (D stands for snow climate.) Here, dry continental polar air masses bring cold and dry winters.

Temperature

Temperature varies both with latitude and altitude over the region. The lowest temperatures are on the Allegheny Mountains (AM), and the warmest in the Coastal Plain (CP). The figure shows mean annual temperature for the physiographic provinces of the Southern Appalachians and the surrounding region. The overall trend, of course, is for colder temperatures to the north. However, cold temperatures extend farthest south in the Southern Blue Ridge province, owing to the high elevation of this province. The 39 weather stations in this province have a mean elevation of 698 m, somewhat higher than the mean altitudes for the other provinces. The lowest mean minimum temperature (45.9^o F) occurs in this province. Warmer temperatures extend farther north in the Piedmont province than in the western parts of the Appalachians, owing to lower elevation of the Piedmont.

Precipitation

Mean Annual Precipitation

Many areas in the Southern Appalachians receive large amounts of precipitation. The figure below shows that there is a general increase in precipitation from north to south, owing to the incursion of tropical air masses more frequently into the southern part of the region. In the southern part of the region, precipitation decreases from west to east.The Eastern Piedmont (P) has a modest rain-shadow effect. Aguado and Burt (2000) note that in the southwestern part of the region, moist maritime air masses from the Gulf move ashore, contributing to the development of heavy but brief showers in summer. As the air masses move northward, they gradually lose moisture by precipitation, so that by the time they reach the northern provinces the amount of precipitation is substantially lower than in the southern part of the region.

Superimposed on these general trends are small areas of high precipitation, particularly in the Southern Blue Ridge (SBR). The highest amounts of precipitation, nearly 125 inches per year, occur in the region near the border of Georgia and the Carolinas. This small area is so rainy that some have called it the only temperate rainforest in the U.S. east of the Mississippi (McCrone et al. 1982). Cloud cover and fog frequently occur throughout the Southern Appalachians, because the humidity is high. Heavy snowfalls occur in higher elevations; Mt. Mitchell received over 100 inches of snowfall one year (Redington 1978). Three weather stations in the Southern Blue Ridge Province have mean annual precipitation values greater than 80 inches. Two other high-precipitation areas occur along the western edge of the Cumberland Plateau (CPT) and may represent an orographic effect produced when Gulf air masses encounter the modest (about 300 m) but abrupt rise in altitude along the western Cumberland Escarpment.