Present Climate of the South Central

The location of the South Central 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 experiences nearly every variety of extreme weather. Heat and cold waves, droughts, floods, blizzards, tornados, and hurricanes are all considerations for the residents of the South Central.

The Köppen Climate Map

Wladimir Köppen developed a commonly used system of climate categorization based on the kinds of vegetation areas sustain. He defined 12 climate types, many of which are familiar: rainforest, monsoon, tropical savanna, humid subtropical, humid continental, oceanic, Mediterranean, steppe, subarctic, tundra, polar ice cap, and desert. Updated by Rudolf Geiger, it has been refined to five groups, each with two to four subgroups.

Today, the South Central lies at the intersection of several distinct climate zones, with much of the region characterized as warm temperate (represented by “C” in the Köppen system). Northern Missouri and northern Kansas are characterized as continental (represented by “D”), and the eastern parts of Kansas and Texas are arid (represented by “B”).

See Chapter 10: Earth Hazards for more information on extreme weather in the South Central.

Average temperatures in the South Central 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 Louisiana and Texas, and the coolest found in Missouri and Kansas (Figure 9.8). The South Central’s overall average high temperature of 20ºC (68°F) and average low of 9ºC (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 5ºC (41ºF), respectively. Another factor besides latitude that influences temperature in the South Central 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 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 (121ºF) and a low of -40°C (-40°F).

The average amount of precipitation for the United States is 85.6 centimeters (33.7 inches). In the South Central, however, average precipitation ranges from 146.3 centimeters (57.6 inches) in Louisiana to 74.4 centimeters (29.3 inches) in Kansas (Figure 9.9), demonstrating the impact of moisture carried inland from the adjacent Gulf of Mexico.

The geography and climate of the South Central are nearly ideal for the formation of thunderstorms. Storms occur when there is strong convection in the atmosphere. Because warm air can hold more moisture than cool air can, convective mixing with cool air forces moisture to condense out of warm air, as vapor (clouds) and precipitation. It is hypothesized that the formation of precipitation causes the electrical charging that produces lightning. Of course, air cannot mix without moving, and that movement is caused by the wind.

See Chapter 10: Earth Hazards to learn more about tornados and hurricanes.

A strong temperature difference at different heights creates instability— the warmer the air near the surface is relative to the air above it, the more potential energy it has to move up. The South Central receives warm, moist air moving north from the Gulf of Mexico, and cold, dry air moving in from the Rocky Mountains and the northern US. Where these air masses meet, vigorous mixing causes storms. Typically, a storm blows itself out once the warm air has moved up and the cool air down—a vertical column turning over as a unit. But because the lower air from the Gulf is moving north while air higher up is moving west, more heat and moisture is constantly added to the system, allowing the storm to persist and strengthen. This movement in different directions is also the reason for the South Central’s unusually high incidence of powerful tornados.

Figure 9.8: Mean annual temperature for the South Central States.

Figure 9.8: Mean annual temperature for the South Central States.

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.

Figure 9.9: Mean annual precipitation for the South Central States.

Figure 9.9: Mean annual precipitation for the South Central States.

Arkansas’ climate is influenced by its topography as well as its relative proximity to the Gulf of Mexico. The state’s winters, like those of Louisiana, 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 (Figure 9.10), 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 northwest section of the state.

In Oklahoma, summers are long and warm, and winters are shorter than in other states of the Great Plains. 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 same gradient. With its low topographic relief, Kansas is also commonly home to tornados and dust storms.

Figure 9.10: The rain shadow effect.

Figure 9.10: The rain shadow effect occurs when moisture-laden air rises up the windward side of a mountain, only to release this moisture as precipitation due to cooling and condensation. Once the air reaches the leeward side, it warms and expands, promoting evaporation (and a lack of precipitation).

Missouri’s location in the US 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.

See Chapter 4: Topography for more information about escarpments.

Covering nearly 700,000 square kilometers (270,000 square miles), Texas is the second largest 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 conditions, although the area’s highest peaks do receive significant snowfall in the winter. Texas’ central and eastern areas 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 has a greater effect on 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, and it is also vulnerable to the occasional hurricane.