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The 2020 Hurricane Season

The year 2020 was the most active hurricane season ever with 30 named storms and 13 major hurricanes. Twelve of these affected the continental United States with a 1–2 punch in Louisiana from Hurricanes Marco and Laura. These two storms were only separated by about 48 hours, which complicated response operations. While Marco was not as bad as anticipated, Laura produced 77 deaths and $19 billion in losses. Other storms, including Hurricane Sally, had major impacts on Alabama due to strong winds, storm surge flooding, and at least 20 inches of rain. The total amount of damage from the 2020 Hurricane season is estimated to be over $60 billion.

A thunderstorm is another atmospheric hazard. Thunderstorms are produced when warm, moist air rises through convection (otherwise known as thermal uplift). These storms also occur along cold and warm fronts where different air masses collide or when clouds traverse mountain chains (i.e., orographic lifting). When a thunderstorm cell forms (with cumulus and cumulonimbus clouds), air rises and then descends quickly leading to rain, sometimes in copious amounts. This precipitation can lead to severe flooding and mudslides (which will be discussed further in sections 1.2.3 and 1.2.4).

Depending on weather conditions and temperatures, the vertical movement of air also freezes water droplets that fall to the earth as hail. Most hail is small (e.g., pea size), but it can be larger at times (e.g., baseball or even grapefruit size). Hail can damage the roofs of buildings, destroy car windshields, and even kill those that are struck by it. Hailstorms can be costly natural disasters such as the one that hit Fort Worth, Texas, during a 1995 Mayfest celebration. The event resulted in at least $1 billion in losses. Over 100 people had to be taken to area hospitals after being struck by softball‐sized hailstones.

Thunderstorms also result in downdrafts and straight‐line winds (which travel down to the ground and then move horizontally along the earth’s surface). Such winds move quickly and can slam airplanes to the ground as was the case of USAir Flight 1016 on approach to the Charlotte‐Douglas International Airport in 1994. This crash resulted in 37 fatalities and injury to another 20 passengers. Downdrafts and straight‐line winds may also severely impact homes, barns, fences, etc. On August 10–11, 2020, a major and widespread windstorm (called a “derecho”) impacted several states including Wisconsin, Iowa, Nebraska, Illinois, and Indiana. Winds, ranging from 70 to 140 mph, created utility disruptions, decimated corn and soybean crops, and produced losses upwards of $11 billion dollars.

Severe thunderstorms are not just common to the mid‐western portions of the United States. There are over 16,000 thunderstorms per year in all locations excluding the North and South Poles. Such storms also generate lightning, which is the emission of electrical bolts from clouds as a result of the interaction of positively and negatively charged fields. Approximately 6,000 lightning strikes occur every minute around the world. Lightning often hits buildings, trees and the ground. Homes and forests may be ignited with fire due to lightning. Because the temperature of the bolt is extremely hot (perhaps up to 50,000 degrees Celsius), people can be killed as well. Burns, respiratory failure, and cardiac arrest result from lightning strikes.

Tornadoes are another type of atmospheric hazard. Tornadoes are closely associated with thunderstorms. In fact, the name “tornado” stems from the Spanish name for such storms. As warm, moist air collides against cool, dry air, winds may move in a circular or rotating direction. One portion of the rotating air shaft drops, while the other portion moves upward in a vertical manner. If this air shaft touches down on lakes or oceans, it is known as a waterspout. When the resulting funnel reaches the ground, it becomes known as a tornado. The velocity of winds is the factor used to describe the strength of tornadoes under the Enhanced Fujita Scale. The Enhanced Fujita Scale is a scale used to categorize the size of a tornado, including the affiliated wind speed (see Table 1‐3). Small tornadoes (e.g., F0 or F1) are very common and possess slower wind speeds (e.g., 65 mph). Large tornadoes (e.g., F4 or F5) are infrequent, but their wind speed may reach over 200 mph. At such high speeds, windows are broken, roofs are ripped from walls, and even foundations can be sucked from their moorings. Glass, brick, two‐by‐fours, and even cars become projectiles and may penetrate other structures. The destruction from tornadoes can be extremely severe. For instance, the large F5 tornado that struck Joplin, Missouri in May 2011 became the costliest tornado in U.S. history. Damages in this disaster amounted to over $2.5 billion. St. John’s Regional Medical Center was one of the larger building structures that received extensive damage. Over 150 people were killed due to dangerous winds associated with this massive tornado.

Table 1‐3 Enhanced Fujita Scale for Tornado Damage.

Source: National Oceanic and Atmospheric Administration, implemented in the U.S. on 1 February 2007. http://www.spc.noaa.gov/faq/tornado/ef‐scale.html.

	Fujita scale	Derived EF scale	Operational EF scale
F number	Fastest 1/4 mile (mph)	3 second gust (mph)	EF number	3 second gust (mph)	EF number	3 second gust (mph)
0	40–72	45–78	0	65–85	0	65–85
1	73–112	79–117	1	86–109	1	86–110
2	113–157	118–161	2	110–137	2	111–135
3	158–207	162–209	3	138–167	3	136–165
4	208–260	210–261	4	168–199	4	166–200
5	261–318	262–317	5	200–234	5	Over 200

Important note about enhanced F‐scale winds: The enhanced F scale still is a set of wind estimates (not measurements) based on damage. It uses 3 second gusts estimated at the point of damage and wind estimates vary with height and exposure. The 3 second gust is not the same wind as in standard surface observations. Standard measurements are taken by weather stations in open exposures.

Tornadoes are very common to the Midwest portion of the United States due to the movement of the jet stream and the collision of air from Canada and the Gulf of Mexico. In fact, 90% of the world’s tornadoes take place in the United States. Although a typical year sees roughly 1,000 tornadoes, in 2011 there were 1,691 twisters, which killed 700 people and produced $25 million in damages.

Oklahoma is one of the states that is frequently impacted by tornadoes, as was the case on May 3–5, 1999. Fifty‐nine tornadoes were reported in central Oklahoma during this period, and many of them lasted several minutes and traveled great distances. At least 40 people were killed during the outbreak and 675 people were injured. Over 10,000 homes were also damaged or destroyed. Losses were estimated at $1.2 billion. Throughout history Oklahoma has experienced numerous tornadoes, including three that traveled virtually the same paths in the City of Moore. For this reason, Oklahoma and many other midwestern states are considered part of “tornado alley” ‐ the central portion of the country that is frequently prone to these types of storms. However, it should be pointed out that tornadoes have occurred in many locations around the United States including in California, Florida, Utah and many others.

Winter storms are atmospheric hazards that occur mainly in December, January and February in the United States. Such seasonal storms include snow, sleet and ice, and are associated with extremely cold temperatures (Figure 1‐3). Snowstorms include fluffy flakes of water that have frozen as they fall to the ground. While snow is often described as a “winter wonderland,” excessive snow can be a problem. On January 28, 1977, Buffalo, New York, received 93 inches of snow. This is an amount greater than the average for that area during the entire year!

When accompanied by high winds, these events can turn into blizzards. Such storms can leave several inches or feet of snow on the ground, making transportation difficult as was the case in Denver in October 1997. Up to 31 inches of snow fell in some locations and drifts were as high as 10 feet on the plains to the east of this city. Hundreds of cars had to be abandoned, and countless roads were shut down for a few days before plows could clear them.

When snow falls on steep slopes, the potential for avalanche may result. Avalanches are quick and violent movements of snow down the mountainside. They occur around the world and result in the loss of 150 lives each year. In one case, a major avalanche in Central Italy hit the 4‐story Rigopiano Hotel on the afternoon of January 18, 2017. The snow slide, which may have resulted from a series of earthquakes, moved the hotel more than 33 feet and killed 29 people. In the United States, avalanches are common in Alaska, California, Washington and states located in the Rocky Mountain region. The characteristics of snow, changing temperatures, wind, and presence of skiers and snowmobiles can trigger avalanches. In 2021, nine individuals died from avalanches while participating in winter sports in the mountains of Utah, Colorado, and New Hampshire.

Although snow and avalanches create several challenges including property destruction and fatalities, the cold temperatures associated with winter storms may also produce hypothermia in individuals who are exposed to such weather. In some cases, the heating or lighting of homes with open flames during winter storms may lead to fires that also cause death and destruction.

Figure 1‐3 The 2021 Valentine Storm resulted in snow and ice on a beach near the San Louis Hotel in Galveston, Texas (National Weather Service/Michael Garske) https://www.weather.gov/hgx/2021ValentineStorm

Sleet is another type of winter storm and it is sometimes difficult to distinguish from ice storms (even though sleet has more water in liquid state than ice storms). In any case, sleet and ice storms will cause very dangerous conditions when they accumulate on trees, power lines and the ground. Although winter storms occur most frequently in the northern, central, and western portions of the United States, it is possible for lower states to receive snow and experience ice storms periodically. For instance, Oklahoma experienced a major ice storm on October 27, 2020. This event knocked out power and destroyed countless trees/tree limbs. Making matters worse, southern states may have extra challenges dealing with ice storms since they often lack necessary snow removal/de‐icing equipment. Such storms can create major traffic problems as was the case from Amarillo to the Dallas‐Fort Worth metropolitan area in January 2007. Cars and semi‐trucks were stranded on the highway for hours and even days. Sadly, the slippery roads can prove deadly. During the early morning hours of February 11, 2021, the I‐35W freeway iced‐over just to the north of Fort Worth Texas. At least 5 people died when 133 vehicles were involved in a pile up that spanned a half a mile. The incident required that 65 people be treated at local hospitals.