Climate of the Southeastern United States

Present climate of the southeastern U.S.

The location of the Southeast and its direct relationship to the Gulf of Mexico and Atlantic Ocean strongly influence the area's weather. Since it encompasses locations along the coast as well as areas farther inland, the Southeast experiences nearly every variety of extreme weather. Heat waves and cold snaps, droughts, floods, blizzards, tornados, and hurricanes all affect the Southeast.

Although much of the Southeast falls within the category of a warm temperate zone, using a single label to describe the Southeast's climate obscures its diversity. The main features that influence the Southeast's climate are latitude, the presence of the Atlantic Ocean and the Gulf of Mexico, and regional topography. For example, the Florida peninsula has a distinct summer rainy season, while other inland areas receive uniform precipitation throughout the year. The highest elevations in North Carolina and Tennessee can receive as much snow as parts of New England.

Temperature

Average temperatures in the Southeast tend to decrease northward, which is simply the influence of latitude; lower latitudes receive more heat from the sun over the course of a year. The warmest temperatures are found in Florida, Georgia, and Mississippi, while the coolest are found in West Virginia and Virginia. The Southeast's overall average high temperature of 22°C (72°F) and average low of 9°C (48°F) reflect, on the whole, a more uniform climate than that found in most other parts of the United States. By comparison, the average high and low temperatures for the entire United States are 17°C (63°F) and 5°C (41°F), respectively.

Another factor besides latitude that influences temperature in the Southeast is proximity to the ocean, which has a moderating influence. Air masses that have passed the Gulf of Mexico rarely get either extremely hot or extremely cold, and the Gulf Stream current that travels northward past the Atlantic seaboard carries warm tropical water with it, influencing temperatures on land. Thus, the most extreme temperatures in the Southeast are found toward the center of the continent.

The record for highest temperature in the Southeast is held by Mississippi, which has experienced a high of 46°C (115°F). The record low temperature of −38°C (−37°F) is shared by Kentucky and West Virginia. Of course, major temperature fluctuations can occur in every state. In July, average daily maximum temperatures range from 35°C (95°F) in southern Georgia and Florida to 24°C (75°F) in mountainous parts of West Virginia. Wintertime has a broader range of temperatures, with average daily minimums in January varying from around −7°C (20°F) in northern Kentucky to 16°C (60°F) in South Florida. Although the Southeast's climate is subtropical, it can get cold. Subfreezing temperatures are sometimes a concern for Florida orange growers, since oranges are particularly sensitive to damage in freezing conditions.

Average Annual Temperatures in the Southeastern United States

Precipitation

The average amount of precipitation for the United States is 85.6 centimeters (33.7 inches). In the Southeast, however, average annual precipitation typically ranges from about 100 to 125 centimeters (40 to 50 inches) inland to over 150 centimeters (60 inches) along the Gulf Coast of Mississippi, Alabama, and the Florida Panhandle. High annual precipitation in near the Gulf Coast demonstrates the impact of moisture carried inland from the adjacent Gulf of Mexico. Some pockets of high precipitation also occur in the Appalachian Mountains along the Eastern Continental Divide, a topographical high point where air is forced upward from both sides of the mountain range. High precipitation also occurs along the Atlantic coast.

Severe Weather

Hurricanes

In the summer and fall tropical cyclones (hurricanes) often bring heavy rains to the Gulf and Atlantic coasts. Some of these cyclones, such as Hurricane Andrew in 1992, are extremely powerful and have devastated communities in the Southeast. Thousand-year weather events, referring to the 1-in-1000 chance of intense events happening in a given year, have increased in frequency in recent years, and climate models predict a continuation of that increase. One such event occurred in the fall of 2015 when heavy rains associated with Hurricane Joaquin (but not actually part of the hurricane) brought over 50 centimeters (20 inches) of rain to parts of South Carolina, causing over one billion dollars in damage.

Thunderstorms and tornados

The geography and climate of the Southeast are nearly ideal for their formation of thunderstorms and tornados, especially in the summer.

Storms form when there is strong convection in the atmosphere. Convection occurs when buoyant warm air rises (moves up) while denser cool air sinks (moves down). Convective mixing forces the moisture in warm air to condense as it comes into contact with cool air, forming vapor (clouds) and precipitation (for example, rain or hail). A strong difference in air temperature at different heights creates instability; the warmer the air near the surface is relative to the air above it, the more potential (stored) energy the warm air has to move up, and the more potential for a storm. The final ingredient is wind. Wind moves the air, promoting mixing. The formation of precipitation also causes electrical charging of particles in the atmosphere, which in turn produces lightning.

Typically, a storm blows itself out once the warm air has moved up and the cool air has moved down. Convective mixing stops because the vertical column of air has turned over so that the cool air is at the bottom and the warm air is at the top. The directions of air currents in the Southeast, however, promote mixing. The Southeast receives warm, moist air from the Gulf of Mexico, and cold, dry air from the Rocky Mountains and the northern US. The warmer, lower air from the Gulf is moving north, constantly adding more heat and moisture. The colder, higher air is moving west. Where these air masses meet, vigorous mixing causes storms. The inputs of warm, dry air from the Gulf of Mexico allow these storms to persist and strengthen.

This movement in different directions is also the reason for the Southeast's unusually high incidence of powerful tornados. Only Kansas has more tornados per square mile than Florida, and several other southeastern states rank in the top ten for tornado frequency. Tornados often accompany severe thunderstorms, so where thunderstorms occur, tornados are likely.

Regional climate variation

In much of the lower Southeast, the climate is typical of a humid subtropical zone, characterized by temperate winters, long, hot summers, and rainfall that is fairly evenly distributed through the year. This blanket classification is somewhat misleading however, masking the variation in climate and weather that occur throughout the Southeast.

Alabama, Georgia, and Mississippi

In Georgia, elevations range from sea level at the Atlantic coast to the summit of Brasstown Bald at 1458 meters (4784 feet). The climate varies considerably, but snow and prolonged freezing temperatures are uncommon throughout the state. In Mississippi and Alabama, thunderstorms occur throughout the year; they are most commonly in the summer, but most destructive in the fall and spring. These states are also commonly in the path of tropical storms and hurricanes moving northward off of the Gulf. Freezing temperatures reach to the Gulf Coast in most winters, but they rarely persist.

Florida

Florida is commonly in the path of hurricanes and tropical storms that move across the Mid-Atlantic and the Gulf of Mexico. The climate of the state's north and central parts is humid subtropical, while southern Florida has a tropical savanna climate. There is a defined rainy season from May through October, when thunderstorms built in the heat of day drop heavy but brief summer rainfall.

Kentucky

Kentucky experiences four distinct seasons, with substantial variations in the severity of summer and winter, but extreme cold is rare. Precipitation does not vary greatly throughout the year. Fall is normally the state's driest season, while the spring season is typically the wettest. Thunderstorms are responsible for much of Kentucky's summer rainfall, and they often bring intense rain that may be highly localized. Generally, Kentucky experiences relatively humid, rainy, and warm summers; winters are moderately cold and snow is not especially uncommon, although occasional extreme snowfalls occur. Snow cover seldom persists for more than a week in the south or for more than two weeks in the north.

North Carolina

In North Carolina, the climate varies from the Atlantic coast in the east to the Appalachian Mountains in the west. The Appalachians help to block cold air from the interior of the United States and storms from the Midwest from entering the Piedmont of North Carolina. Most of the state has a humid subtropical climate, except in the higher elevations of the Appalachian Mountains, which have a subtropical highland climate.

South Carolina

South Carolina's humid subtropical climate arises from the combination of the state's relatively low latitude, its generally low elevation, the proximity of the warm Gulf Stream, and the Appalachian Mountains, which in winter help to block cold air from the interior of the United States. Most precipitation falls in spring and summer. While cold weather occurs each winter, its duration tends to be brief; snow is uncommon and generally occurs only in the mountains and upper Piedmont.

Tennessee

Tennessee has a temperate climate, with warm summers and mild winters, although the state's varied topography leads to a wide range of climatic conditions. Most of the state has a humid subtropical climate, with the exception of some of the higher elevations in the Appalachian Mountains; these high-elevation regions are classified as having a mountain temperate climate or a humid continental climate. Severe storms occur infrequently. The state has hot summers and mild to cool winters with generous precipitation throughout the year. The highest average monthly precipitation generally occurs in the winter and spring months, between December and April. Snowfall varies and is more prevalent in eastern Tennessee than in the western section. Nashville gets about 25 centimeters (10 inches) a year, Memphis only 13 centimeters (5 inches).

Virginia

The climate of Virginia is considered mild for the United States. The southern part of the Shenandoah Valley, the Roanoke Valley, and most of Virginia east of the Blue Ridge Mountains have a humid subtropical climate, although the range of topography yields a range of climate types. Some locations, like Charlottesville, have long growing seasons and infrequent subzero temperature minimums. In contrast, winters on the northern Blue Ridge frequently produce bitterly cold temperatures like those of Chicago. Similarly, annual rainfall totals can vary from a fairly dry 84 centimeters (33 inches) typical of the Shenandoah Valley to more than 150 centimeters (60 inches) in the mountains of southwestern Virginia. Severe weather, in the form of tornados, tropical storms, and winter storms, impacts the state on a regular basis. Areas of Virginia have seen substantial snowstorms in recent years.

West Virginia

West Virginia has the lowest annual average temperature in the Southeast, reflecting its inland position and higher latitude, as well as the mountainous topography in the eastern part of the state. The lowest-elevation regions of West Virginia have a humid subtropical climate, whereas the highest elevations have a humid continental climate. As in Virginia, annual rainfall is variable. Parts of the eastern panhandle are fairly dry, which about 89 cm (35 inches) of precipitation per year. The mountains, in contrast, receive much more precipitation, including an average of over 254 centimeters (100 inches) of snow each year.

Past climate of the southeastern U.S.

This section covers the climate of the southeastern U.S. through the Phanerozoic, from about 541 million years ago to the recent. For the climate on early Earth, see the Intro to Climate section.

Paleozoic (541 to 252 million years ago)

Cambrian to Devonian (541 to 359 million years ago)

With the start of the Paleozoic era, climates across the world were warm, and North America was located in the low and warmer latitudes of the Southern Hemisphere. As the Cambrian progressed, North America moved northward; by about 480 million years ago, what would become much of the southeastern U.S. was located near the Tropic of Capricorn.

Fossils of archaeocyathids, trilobites, brachiopods, and mollusks reveal that most of the area was probably covered by warm, shallow seas from the Cambrian into the Ordovician. In the late Ordovician (about 460 to 430 million years ago), the Earth fell into a brief but intense ice age. Although sea level dropped during this event, North America's position near the equator kept its climate relatively warm.

Limestones throughout Tennessee and Kentucky contain Silurian and Devonian fossils like trilobites, brachiopods, corals, and crinoids. These fossils indicate that the Southeast still contained warm, shallow seas through the late Devonian. These shallow seas were especially productive. In fact, plankton productivity grew so high that it depleted all the oxygen from the seafloor and sediments. The lack of oxygen allowed organic matter to accumulate instead of decay, leading to the deposition of black, carbon-rich shale. This shale occurs throughout the subsurface in east-central Kentucky and central West Virginia and is one of the richest sources of petroleum in those states.

From 430 to 345 million years ago, North America moved north across the equator, and the cycle of warming and cooling was repeated yet again. During this time time, most of the southeastern states were still submerged. As fragments of the Avalon microcontinent collided with North America, the Acadian Orogeny formed a huge chain of mountains in what is now eastern North America. The remains of these high mountains include the northern Appalachians and the igneous and metamorphic rocks of the Piedmont. As these mountains rose, they blocked the equatorial easterly winds, forming a large rain shadow.

Carboniferous to Permian (359 to 252 million years ago)

In the early Carboniferous (Mississippian), ice capped the South Pole and began to expand northward. Although the Earth's temperature fell during this time and glaciers growing far to the south caused sea levels to drop, the western Southeast was covered by a warm, shallow sea. Limestone accumulated in this sea and it supported abundant marine life, including heavily armored bony fish, sharks, bryozoans, corals, and echinoderms.

Farther east, the uplift and erosion associated with mountain building caused the landscape to transition to a swampy coastal environment. The extensive forests contained many different kinds of plants including lycophytes, sphenopsids (plants related to modern horsetails), tree ferns, and seed ferns (seed plants with fern-like leaves) as well as insects, amphibians, and early reptiles.

By the late Carboniferous (Pennsylvanian), North America had collided with Gondwana, advancing the formation of Pangaea. Pangaea was so large that it created a strong monsoonal climate, much as Asia has today. A monsoonal climate is a warm climate with wet and dry seasons. The monsoonal climate counteracted the effect to the rain shadow produced by the Central Pangaean Mountains (in North America, the Appalachian and Ouachita mountains) by bringing in moisture from the west. 

Although global temperatures again declined as glaciers formed near the South Pole, temperatures in present-day eastern and southeastern North America remained warm. Late Carboniferous fossils from the Inland Basin provide evidence of a warm climate with scale trees and other swamp plants. The large coal swamp forests that eventually formed the rich coal beds of Tennessee, Kentucky, and West Virginia grew along broad floodplains.

The region experienced the repeated rising and falling of sea levels as the glaciers waxed and waned. This ice age lasted well into the Permian, ending about 260 million years ago, during which time the swamps dried out permanently.

Mesozoic (252 to 66 million years ago)

Triassic to Jurassic (252 to 145 million years ago)

The Earth remained warm and ice-free at the poles through much of the Mesozoic era. After reaching its greatest size during the Triassic period, Pangaea began to rift apart, and the climate gradually shifted, becoming wetter and warmer. As the Triassic period began, the Southeast moved north from the equator. Triassic rocks are known only from the rift basins of Virginia and North Carolina, where they contain a rich terrestrial and lake fauna of fishes, amphibians, dinosaurs, and other reptiles.

The breakup of Pangaea caused the Gulf of Mexico to open, flooding it with seawater. Because the climate was still relatively warm and dry, evaporation rates were high, and extremely thick deposits of salt accumulated there. These salt deposits have played a key role in trapping petroleum along the Gulf Coast.

Cretaceous (145 to 66 million years ago)

Throughout the Cretaceous, sea level was as much as 100 meters (330 feet) higher than it is today. Shallow seaways spread over many of the continents. Global temperatures during the Cretaceous were very warm, as much as 10°C (18°F) above present. There was likely little or no glacial ice anywhere on Earth, and temperatures were highest in lower latitudes.

By the start of the Late Cretaceous, North America was divided in two by an inland sea known as the Western Interior Seaway, and the Southeast experienced a hot and humid tropical climate. A rich record of this warm, shallow sea is found in Mississippi, Alabama, and Georgia, where marine fossils range from tiny coccolithophores (a type of marine alga that makes up Cretaceous chalk deposits), mollusks, and crabs to sharks and large marine reptiles.

At the very end of the Cretaceous, the Gulf Coast experienced an enormous disruption when an asteroid or comet collided with Earth in what is now the northern Yucatán Peninsula in Mexico, just a few hundred miles away from the southeastern U.S. The impact vaporized both water and rock, blocking out sunlight for weeks to years, leading to a collapse of photosynthesis and food webs on land and in the oceans. After this event, the climate may have cooled briefly, but it soon rebounded to a warmer state.

Cenozoic (66 million years ago to present)

Paleogene (66 to 23 million years ago)

As the Cenozoic era began, global temperatures rose even further into the early Eocene. At the boundary between the Paleocene and Eocene epochs (around 56 million years ago), temperatures spiked upward in what geologists call the Paleocene-Eocene Thermal Maximum (PETM). During this event, which lasted perhaps only around 10,000 years, the atmosphere and ocean warmed by as much as 8°C (14°F) in as little as 4000 years. This greenhouse effect persisted for 100,000 years. This warm interval reflected in the types of land plants and diversity of marine life—especially clams, snails, and echinoderms—represented by fossils in the Southeast Coastal Plain.

In the late Eocene, the Earth began cooling, and global temperatures fell sharply at the boundary between the Eocene and Oligocene epochs (around 35 million years ago). The rocks and fossils of the Southeast show that the area did not cool as much as areas farther north did. The oldest rocks in Florida are coral-rich Eocene limestones that clearly formed in warm seas.

Neogene (23 to 2.6 million years ago)

Climates warmed slightly during the Miocene, a change reflected in the diverse marine and terrestrial fossils of the Atlantic Coastal Plain from Maryland to Florida. Global temperatures again began to fall after a slight peak in the middle Miocene. These changes are also visible in the Southeast's fossil record, with species that had lived farther north retreating southward over time.

Around 3.5 million years ago, glacial ice began to form over the Arctic Ocean and on the northern parts of North America and Eurasia. A major contributing factor to this event was a geological change the emergence of the Isthmus of Panama, which was formed by undersea volcanoes. The new dry-land isthmus blocked the warm ocean currents that had been flowing east-to-west from the Atlantic to the Pacific for more than 100 million years. These currents were diverted into the Gulf of Mexico and ultimately into the western Atlantic Gulf Stream.

The reefs that make up the bulk of Florida's carbonate platform formed in the warm tropical sea that resulted from this change in ocean circulation. The changes in ocean circulation throughout the Caribbean and Gulf of Mexico also affected nutrient supplies in the coastal ocean, which may have contributed to an increase in the extinction of marine animals—including everything from mollusks and corals to whales and dugongs—during the late Pliocene.

Quaternary (2.6 million years ago to present)

Eventually, a sheet of sea ice formed over the Arctic, and ice sheets spread over northern Asia, Europe, and North America, signaling the start of the most recent ice age. Since 800,000 years ago, an equilibrium has been reached between warming and cooling, with the ice caps growing and retreating primarily due to the influence of astronomical forces (i.e., the combined gravitational effects of the Earth, Sun, moon, and planets). The ice sheets in the Northern Hemisphere did not extend into the Southeast, even at their largest.

The Southeast's climate was, however, affected by the presence of ice to the north. Studies of pollen and plant fossils from South Carolina's coastal plain indicate that between 19,000 and 12,800 years ago, the climate was suited to trees typical of much colder climates today. During glacial intervals, the area was also somewhat wetter than it is today, with wetlands and forests covering much of what would later become grassland. Pleistocene deposits in Tennessee, West Virginia, North Carolina, Florida, and Alabama contain abundant fossils of terrestrial vertebrates such as horses, camels, bison, mastodons, mammoths, and ground sloths. Some of these are animals that we associate with cold climates.