Weather Hazards
Weather is the measure of short-term atmospheric conditions such as temperature, wind speed, and humidity. The South Central is among the most active locations on Earth for two very different kinds of high-energy atmospheric events: tornados and hurricanes. It also experiences a variety of other weather hazards, including high temperatures and drought.
Storms, Tornadoes, and Derechos
Rainstorms occur where colder air from higher latitudes abruptly meets warmer air. This often happens in the mid-latitudes (particularly in the South Central US) where air may warm up as it passes over flat open spaces or when warm, moist air is delivered off the Gulf of Mexico. At the boundary between warmer and cooler air, buoyant warm air rises, and then cools because air pressure decreases with increasing height in the atmosphere. As the air cools, it becomes saturated with water vapor; condensation occurs and clouds begin to form. Because liquid water droplets in the clouds must be very small to remain suspended in the air, a significant amount of condensation causes small water droplets to come together, eventually becoming too large to remain suspended. Sufficient moisture and energy can lead to dramatic rainstorms. Because warm air has a lower pressure relative to cold air, and the movement of air from areas of high pressure to areas of low pressure generates wind, the significant difference in air pressure associated with these boundaries and rainstorms also generates strong winds. Flat regions, such as the Great Plains, allow winds to move unimpeded by topography, and are often subject to severe thunderstorms.
While severe thunderstorms are common in some parts of the South Central, two less common storm hazards have the potential to cause serious property damage and endanger lives: derechos and tornados. Both of these storm events are associated with wind shear, which occurs when the wind’s speed or direction changes with increasing height in the atmosphere. Wind shear can happen when a cold front moves rapidly into an area with very warm air. There, the condensing water droplets mix with the cooler, drier air in the upper atmosphere to cause a downdraft. When these downdrafts are very powerful, they can cause a derecho, or a set of powerful straight-line winds that exceed 94 kilometers per hour (kph) (58 miles per hour [mph]) and can often approach 160 kph (100 mph). These powerful windstorms can travel over 400 kilometers (250 miles) and cause substantial wind damage, knocking down trees and causing widespread power outages. The lightning associated with these intense storms can cause both forest fires and house fires. Approximately one derecho every year or two will occur in Arkansas, eastern Oklahoma, and southern Missouri (Figure 10.25), but they do occur with decreasing frequency through most of the remaining parts of the South Central US.
The differences between tornadoes and derechos are indicated in their names: derecho is the Spanish word for straight ahead, while the word tornado has its roots in the Spanish word tonar, which means to turn. Both types of storm events can be associated with the same major cold front boundary because they require similar conditions to get started. However, tornado formation is more complicated. At the frontal boundary, warm, moist air rapidly rises as cooler, dry air descends; in the meantime, the pressure differences between the warm and cold air masses cause strong winds. Clouds with a visible horizontal rotation can appear, appearing to roll like waves crashing on the shore of a beach. This horizontal motion can tilt, lifting the rotating cloud vertically, and the rolling cloud will form a tornado. Most tornados will last a few seconds to several minutes. During that time, many tornado-prone areas will use tornado sirens to alert residents of the danger. A smaller tornado might generate flying debris that can cause injury or damage to buildings, while larger tornados can cause buildings and houses to be completely broken apart. Tornados are classified by their ranking on the Enhanced Fujita scale, or EF scale. These classifications are estimates of wind speeds based on the type of damage that is observed following the storm.
Figure 10.25: Derecho frequency in the continental US.
Measuring Tornado Intensity
Tornado intensity is measured on the Fujita scale, or simply F-scale, based on the amount of damage that a tornado can cause. The scale ranges from F0 to F5. The scale was modified recently to more accurately reflect specific wind speeds; this newer scale is known as the “Enhanced Fujita scale” and is labeled EF0 to EF5.
“Tornado Alley” is the nickname for an area, extending from Texas to Minnesota, that experiences a high number of exceptionally strong tornados due to its flatter topography and high incidence of severe thunderstorms. Most of the South Central resides within Tornado Alley, leading to more tornados in this part of the United States than in any other (Figure 10.26). From 1991 to 2010, for example, an annual average of 115, 62, and 96 tornados occurred in Texas, Oklahoma, and Kansas, respectively. To the east of Tornado Alley, far fewer tornado strikes occur, with an annual average of 37, 39, and 45 striking Louisiana, Arkansas, and Missouri, respectively. The boundaries of Tornado Alley vary in application, depending on whether the frequency, intensity, or number of events per location are used to determine the area’s borders. Some people apply the name “Dixie Alley” to the adjacent tornado-prone area from Louisiana and Arkansas east to Florida.
Figure 10.26: Annual tornado reports per 29,500 square kilometers (10,000 square miles) in the continental US, between 1950 and 1995.
Although specific tornado paths are not predictable, the conditions that produce them are used to alert people so that they can seek shelter. The National Weather Service issues a watch, if the conditions are right for a type of storm event, or a warning, if the conditions are occurring or imminent for the storm event. The National Weather Service is part of the National Oceanographic and Atmospheric Administration, which maintains a US map of all current watches and warnings. Since the atmospheric conditions can change very quickly, an important factor in preventing loss of human life is getting the public to act upon the severe weather alerts. One recent attempt to improve public response to warnings is through a tornado alert index that helps people evaluate the risk of a local tornado. The Tor:Con index used by the Weather Channel provides a number from 1 to 10 that represents the probability of a tornado occurring. Meteorologists evaluate the atmospheric conditions associated with a storm and assign a score. For example, a 4 on the Tor:Con index would indicate a 40%, or moderate, chance of a tornado forming in a particular area.
Hurricanes
Hurricanes occur when a warm, moist, low-pressure air mass forms over the Atlantic Ocean south and east of Florida. These storms gather strength as warm surface ocean water evaporates in the summer, yielding humid, low-pressure air that rises; the moisture condenses into water droplets that form clouds, releasing latent heat, and thereby providing energy for even greater evaporation of warm ocean water. This positive feedback cycle continues until the low-pressure center moves over land. These storms are considered tropical depressions when wind speeds are below 63 kph (39 mph). As the storm grows, it develops a more organized structure, with warm air rising in the center and somewhat discrete bands of rain being formed. It becomes known as a tropical storm when its wind speeds reach the 63 - 117 kph (39 - 73 mph) range, and it is called a hurricane once winds have reached 119 kph (74 mph). The western Atlantic, Caribbean, and Gulf of Mexico area is one of the world’s most active for hurricanes, though they also occur in areas of the western Pacific, where they are known as typhoons, and in the South Pacific to Indian Ocean, where they are called cyclones.
Measuring Hurricane Intensity
Hurricanes are ranked in the Saffir-Simpson scale from category 1 to 5, with 5 being the highest, based on wind speed. Category 5 hurricanes occur on average only about once every three years in the Atlantic and Gulf of Mexico.
In an average year, about a dozen hurricanes travel through the western Atlantic and sometimes the Gulf of Mexico. Of these, roughly one a year hits the Texas and/or Louisiana coast, though these occurrences vary considerably. The peak month is September, followed by August and October. More rarely, hurricanes may hit the coast in June, July, or November. The 2005 hurricane season was the most active in recorded history, with a record number of 15 hurricanes, 7 of which strengthened into major (category 3 or greater) hurricanes (Figure 10.27). Two of these—Katrina and Rita—were category 5 hurricanes that did substantial damage to the Gulf Coast. Katrina (Figure 10.28) destroyed large parts of New Orleans and other areas along eastern Louisiana, while Rita did substantial damage in southwest Louisiana and went ashore at Sabine Pass, Texas. More recently, Category 4 Ike (2008) caused damage along the Louisiana coast and made landfall at Galveston, Texas.
Figure 10.27: Tracks of all Atlantic hurricanes during the 2005 season. Warmer colors indicate higher maximum sustained wind speeds.
Figure 10.28: Satellite image of Hurricane Katrina as it approached the Louisiana coastline.
Once hurricanes reach land, they lose energy rapidly, though they typically continue to deliver substantial precipitation and somewhat high winds for hundreds of miles onshore. Hurricane tracks over eastern Texas and Louisiana generally veer north to northeast, heading across eastern Texas and Louisiana to eastern Oklahoma, Arkansas, and southeast Missouri.
Extreme Temperature and Drought
Extreme temperatures can create dangerous conditions for people and may lead to property damage. Heat waves are periods of excessively hot weather that may also accompany high humidity. Temperatures of just 3°C (6°F) to 6°C (11°F) above normal are enough to reclassify a warm period as a heat wave. Under these conditions, the mechanism of sweating does little to cool people down because the humidity prevents sweat from evaporating and cooling off the skin. Heat waves have different impacts on rural and urban settings. In rural settings, agriculture and livestock can be greatly affected. Heat stress recommendations are issued to help farmers protect their animals, particularly pigs and poultry, which, unlike cattle, do not have sweat glands.
The impacts of heat waves on urban settings include a combination of the natural conditions of excessive heat and the social conditions of living in a densely populated space. Cities contain a considerable amount of pavement, which absorbs and gives off more heat than vegetation-covered land does. Air conditioning units that cool down the inside of buildings produce heat that is released outside. Pollution from cars and industry also serve to elevate the outdoor temperatures in cities. This phenomenon, in which cities experience higher temperatures than surrounding rural communities, is known as the heat island effect. Other social conditions can increase the hazards associated with heat waves in urban areas. People who are in poor health, live in apartment buildings with no air conditioning, or are unable to leave their houses are at greatest risk of death during heat waves. In 2011, the South Central experienced the nation’s hottest summer heat wave in 75 years, with temperatures reaching upwards of 55°C (131°F) during a period of four months (Figure 10.29). Texas, Oklahoma, and Arkansas took the brunt of the extreme heat, which contributed to severe drought, amplified heat-based health emergencies, and caused a heavy spike in electricity usage (related to increased air conditioning use) that generated a record-breaking demand on the power grid and led to increased energy prices.
While high temperatures can be directly dangerous, a larger scale hazard arises when these temperatures are coupled with lack of precipitation in an extended drought period. Many significant droughts have occurred in the South Central states. Most famously, high temperature and drought in the 1930s, combined with deep plowing that removed moisture-trapping grasses, led to the Dust Bowl— dust storms that carried vast clouds of black dust across the Midwest and eastern US, greatly damaging both the ecology and agriculture across that portion of the country (Figure 10.30). The Dust Bowl, which was most intense in the panhandles of Texas and Oklahoma and also affected adjoining parts of Kansas, New Mexico, and Colorado, displaced 3.5 million people.
Texas experienced a seven-year record drought in the 1950s, and the lowest average statewide rainfall record was set in Texas as recently as 2011. That year, nearly the entire state was categorized as experiencing “exceptional drought,” the highest of the five drought levels recognized by NOAA’s US Drought Monitor. Today, much of the South Central is still experiencing moderate to extreme drought, with exceptional drought still occurring in some areas of Texas and Oklahoma (Figure 10.31).
Figure 10.29: Number of days with temperatures reaching above 100°F during the year 2011.
Figure 10.30: A dust storm approaching the town of Stratford, Texas during the Dust Bowl in 1935.
Figure 10.31: Drought severity in the South Central, as of March 2015.
