Climate of the South-central United States

Present climate of the south-central U.S.

The location of the south-central U.S. and its direct relationship to the Gulf of Mexico strongly influence the area’s weather. Since it encompasses locations along the coast as well as areas farther inland, the south-central U.S. experiences nearly every variety of extreme weather. Heat waves and cold snaps, droughts, floods, blizzards, tornados, and hurricanes are all considerations for the residents of the south-central states. Average temperatures in the south-central U.S. 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.

Temperature

The warmest temperatures are found in Louisiana and Texas, and the coolest found in Missouri and Kansas. The south-central states' overall average high temperature of 20°C (68°F) and average low of 9°C (49°F) are indicative, on the whole, of a more uniform climate than that found in most other regions of the United States. By comparison, the average high and low temperatures for the entire United States are 17ºC (63ºF) and 5°C (41°F), respectively.

Another factor besides latitude that influences temperature in the south-central U.S. 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. Thus, the most extreme temperatures in the south-central U.S. are found toward the center of the continent. Record high and low temperatures are both held by Kansas, which has experienced a high of 49ºC (121°F) and a low of -40°C (-40°F).

Average Annual Temperatures in the South-central United States

Precipitation

The average amount of precipitation for the United States is 85.6 centimeters (33.7 inches). In the south-central states, however, average precipitation ranges from 146.3 centimeters (57.6 inches) in Louisiana to 74.4 centimeters (29.3 inches) in Kansas, demonstrating the impact of moisture carried inland from the adjacent Gulf of Mexico.

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 Katrina in 2005, are extremely powerful and have devastated communities in the south-central states, particularly Louisiana and Texas. Five-hundred-year or thousand-year weather events, referring to a 1-in-500 or 1-in-1000 chance of an event of a given intensity happening in a given year, have increased in frequency in recent years; climate models predict a continuation of that increase. One such event occurred in August 2017, when the heavy rains of Hurricane Harvey brought nearly 61 centimeters (about 24 inches) of rain to Houston, Texas, over the course of two days, causing widespread flooding.

Thunderstorms and tornados

The geography and climate of the south-central U.S. 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 south-central U.S., however, promote mixing. The south-central U.S. receives warm, moist air from the Gulf of Mexico, and cold, dry air from the Rocky Mountains and the northern U.S. 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 south-central states' unusually high incidence of powerful tornados. A large swath of the south-central U.S. is part of an area with frequent tornados known as "Tornado Alley."

Regional climate variation

Today, the south-central U.S. lies at the intersection of several distinct climate zones, with much of the region characterized as warm temperate. Northern Missouri and northern Kansas are characterized as continental, and the western parts of Kansas and Texas are semi-arid.

Arkansas

Arkansas’ climate is influenced by its topography as well as its relative proximity to the Gulf of Mexico. The state’s winters are short, while its summers are hot and humid with heavy precipitation. Arkansas is often subject to heavy rainfall from the remnants of tropical storms that arrive from the Gulf. The state is known for its extreme weather and abundant storms, including thunderstorms and tornados.

The Ouachita Mountains, running northeast through southern Arkansas, are high enough to influence the state’s climate as well. Due to a minor rain shadow effect, the land north of the mountain chain is drier than that to the south, as the mountains block northward-moving precipitation. Snow does fall in the winters, but it is primarily restricted to the northwestern section of the state.

Kansas and Oklahoma

In Oklahoma, summers are long and warm, and winters are shorter than in adjacent states. Because of moist warm air moving northward from the Gulf, rainfall increases dramatically toward the state’s eastern portion, with an average of 43 centimeters (17 inches) in the west and 142 centimeters (56 inches) in the far southeast. In the winter, snowfall follows the reverse pattern, with more snow in the west than in the east, due to the state’s elevation gradient. This same pattern is also present in Kansas, where the annual average precipitation ranges from 107 centimeters (42 inches) in the southeast to 51 centimeters (20 inches) in the west, and the annual average snowfall ranges from 38 to 102 centimeters (15 to 40 inches) along the reverse gradient. With its low topographic relief, Kansas is also commonly home to tornados and dust storms.

Louisiana

During the summer months, rainfall increases in southeastern Louisiana, where moist tropical air arriving from the Gulf of Mexico results in almost daily showers. The state is also commonly in the path of tropical storms and hurricanes moving northward off of the Gulf. Louisiana is south of the path of many winter storm centers, which travel from the northwest, but the northern parts of the state are susceptible. For this reason, Louisiana’s winter precipitation pattern is reversed from the summer, with the heaviest precipitation found in the state’s north.

Missouri

Missouri’s location in the U.S. interior, and the absence of nearby large bodies of water or mountain ranges that would moderate the state’s climate, means that it is subject to major temperature extremes. The warm, moist air of the Gulf influences summer precipitation, while Arctic air from the north affects the winters. Missouri experiences a temperature fluctuation of approximately 17 to 22 degrees Celsius (30 to 40 degrees Fahrenheit) in any 24-hour period.

Texas

Covering nearly 700,000 square kilometers (270,000 square miles), Texas is the second largest U.S. state and encompasses a wide variety of climate regions. The rugged terrain of western Texas receives little rainfall and ranges from desert to semi-arid climate, although the area’s highest peaks do receive significant snowfall in the winter.

The central and eastern areas of Texas possess significantly less complicated topography, with the terrain descending from northwest to southeast. In areas where the terrain drops abruptly, such as in the Caprock Escarpment, topography affects the local climate, enhancing precipitation and promoting the formation of thunderstorms. Overall, precipitation along Texas’ topographic gradient ranges from near-desert conditions in the west to annual accumulations close to 152 centimeters (60 inches) along the coast thanks to moisture from the Gulf. Although the humid air amplifies summer heat, the Gulf’s waters cool during the winter, moderating coastal temperatures during the spring. Texas’ coastal area is prone to severe thunderstorms and tornados. It is also vulnerable to the occasional hurricane.

Past climate of the south-central U.S.

This section covers the climate of the south-central U.S. through the Phanerozoic, from about 541 million years ago to the recent. For the climate on early Earth prior to 541 million years ago, see the Introduction 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 the south-central U.S. was located just below the equator.

In Texas, the presence of Cambrian sandstone indicates that sediments were carried to the sea from the land to the northwest. East of the sandstone, a rich and diverse marine fauna, including trilobites, brachiopods, bivalves, sponges, and other invertebrates, is contained within dolomite and limestone units. Most of the south-central was probably home to warm, shallow seas that persisted into the Ordovician. 

At the end of the Ordovician, from 460 to 430 million years ago, the Earth fell into another ice age. Silurian and Devonian fossils in Oklahoma (including trilobites and brachiopods) indicate, however, that the south-central U.S. still contained warm, shallow seas through the early Devonian.

After an episode of uplift and erosion, the environment changed dramatically. The seas became deeper, and 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 decaying, leading to the deposition of black, carbon-rich shale. Drill cores show us that this rock occurs throughout the subsurface in the western two-thirds of Oklahoma and northern Texas; it is one of the richest sources of petroleum in those states.

From 430 to 300 million years ago, ancient North America moved north across the equator, and the cycle of warming and cooling was repeated yet again. Glaciation in the southern hemisphere occurred during the late Devonian, while the supercontinent Gondwana was located over the South Pole. While the south-central states were still submerged, the oceans between Gondwana and North America began to close.

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 northern part of the south-central U.S. returned to a warm, shallow sea . Limestone accumulated in this region, which supported abundant marine life, including brachiopods, corals, and echinoderms. Meanwhile, the southern part of the sea accumulated thick deposits of sandstone and shale.

By the late Carboniferous (Pennsylvanian), North America had collided with Gondwana, advancing the formation of Pangaea. Pangaea was a supercontinent composed of nearly all the landmass on Earth. The Ouachita Mountains, remnants of what was once a chain of mountains that may have been as high and broad as the Tibetan Plateau, bear witness to this event.

Sedimentary rocks indicate that most of the south-central U.S. was covered by rivers and plains. Oklahoma, Kansas, and Arkansas had flourishing coastal swamps filled with vegetation that has since been transformed into rich coal beds by heat and pressure. Many Carboniferous rocks in the south-central U.S., especially in Kansas, are cyclic, showing repeated episodes of sea level fall and rise as the ice cap in the Southern Hemisphere advanced and retreated.

During the Permian, sea levels dropped. Shallow marine waters gave way to lowland coastal areas, and most of the south-central U.S. became terrestrial. The Permian Basin in western Texas remained marine, however, and an enormous barrier reef formed around its rim. Part of this reef can be seen at El Capitan in Guadalupe Mountains National Park.

The Permian climate was drier than that of the Carboniferous, and extensive salt and gypsum deposits indicate that evaporation rates were high. A shift in plant type—from water-loving ferns and horsetails to plants better adapted for drier conditions—is further evidence of arid conditions during this time. By the end of the Permian, the southern ice sheets had disappeared, and desert conditions existed in the core of the supercontinent (as indicated, for example, by what look like sand dunes preserved in sedimentary rocks).

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, until worldwide temperatures began to dip again around 150 million years ago. After reaching its greatest size during the Triassic period, Pangaea began to break apart into continents that would drift toward their modern-day positions.

Around 220 million years ago, the south-central U.S. moved north from the equator. The climate of the south-central U.S. gradually shifted, becoming wetter. Triassic rocks are known only from far western Oklahoma and Texas, where they contain a rich terrestrial and lake fauna of fishes, amphibians, and reptiles.

Jurassic outcrops in western Oklahoma are terrestrial and contain petrified wood, dinosaurs, and other reptiles indicative of lake environments, revealing that the environment there did not change much from the Triassic (except to become a bit wetter). Farther south, however, 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. At the same time, the portion of the south-central U.S. that bordered the coastline began to subside, and thick deposits of coastal and marine sediments began to accumulate, a process that continues to this day.

Cretaceous (145 to 66 million years ago)

The Earth warmed near the beginning of the Cretaceous, and sea level rose. Throughout the Cretaceous, sea level was as much as 100 meters (330 feet) higher than it is today, largely as a result of water displacement by continental rifting and rapid sea-floor spreading. 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. This sea flooded most of the south-central U.S., covering older rocks and creating a wide belt of rock that is Cretaceous and younger in age. This belt forms the Coastal Plain region and extends many hundreds of miles up the Mississippi River and covers all of Louisiana, about half of Texas, and parts of Arkansas, Oklahoma, and Missouri. 

At the close of the Cretaceous, 66 million years ago, global climates (though still much warmer than those of today) were cooler than at the era’s start. 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 south-central 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. It continued to warm into the Eocene.

Cenozoic (66 million years ago to present)

Paleogene to Neogene (66 to 2.6 million years ago)

By the early Cenozoic, the continents had approached their modern configuration, and India began to collide with Eurasia to form the Himalayas. The formation of the Himalayas had a significant impact on global climate, as the newly exposed rock began to serve as a sink to take up atmospheric CO₂. With the reduction of this greenhouse gas, global temperatures cooled. Antarctica moved south, and by 30 million years ago, temperatures were low enough that glaciers began to grow on its mountains.

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.

During this time, the south-central U.S. continued to accumulate sediment brought in by myriad rivers, including the antecedents to the Mississippi River. Sea level dropped, and the continued withdrawal of the sea is reflected in almost-parallel belts of progressively younger rocks that extend toward the Gulf Coast.

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 reached northern Missouri and northeastern Kansas during its maximum extent, while the rest of the south-central U.S. merely experienced a cooler climate than it does at present. The area was also somewhat wetter than it is today, with wetlands and forests covering much of what would later become grassland. Rocks of this age contain fossils of terrestrial vertebrates such as horses, camels, bison, mastodons, and mammoths. Much of the Mississippi River’s great delta and alluvial fan was deposited when the glacial ice melted, creating rivers that eroded older rocks and carried sediments previously scoured by the glaciers.