Severe Storms

A severe storm is a violent weather phenomenon that has a specific structure, often associated with heavy precipitation and air circulating in a cyclonic or anticyclonic manner. Winds affected by a storm are often of high velocity, a factor used to differentiate among storm stages.

Winter Storms

A storm is a disturbed state of the atmosphere that impacts the ground surface with powerful and potentially destructive weather. Some storms are confined to certain seasons or locations, and others can occur anywhere. Among the storms occurring in the atmosphere are winter storms, thunderstorms, tornadoes, and tropical cyclones. Winter storms, in particular, can be severe, destructive, and life-threatening. Winter storms like ice storms and blizzards can be particularly catastrophic.

The term “ice storm” is given to rain that falls from the atmosphere in liquid form and freezes when it comes in contact with a surface having a temperature of 0 degrees Celsius or lower. This form of precipitation is also known as freezing rain. For freezing rain to occur, the surface temperature of the ground must be below 0 degrees Celsius, and above-freezing temperatures must be present aloft. Snowflakes fall through the layer of warmer air and are melted. The resulting raindrops then cool as they pass through the colder air near the surface, losing most or all of their latent heat content such that they freeze quickly on impact with any cold surface. This kind of precipitation produces layers of solid ice, called glaze, that coat streets, trees, automobiles, and power lines, sometimes to thicknesses of more than two centimeters. The added weight of the ice has been sufficient to collapse vulnerable structures and can cause major power disruptions when electrical grid towers fall under the additional load.

A blizzard is characterized by a strong wind with a velocity of 56 kilometers per hour or higher, temperatures lower than –7 degrees Celsius, and enough snow to restrict visibility to less than 150 meters. The high winds, coupled with the low temperature, produce extreme windchill factors, while the amount of falling or blowing snow driven by the wind can produce total whiteout conditions. Blizzards are associated with midlatitude cyclones. The type of cyclone most likely to produce blizzard conditions is one with a surface low pressure connected with a low pressure in the upper air at the level of the jet stream.

The area of the heaviest snowfall in a cyclone is within about two hundred kilometers north of the low-pressure center. The heavy snow results from moist air from the south turning counterclockwise around the low-pressure center. Farther to the south, usually along the cold front, sleet and freezing rain may occur. Sleet, like freezing rain, occurs when above-freezing temperatures are present aloft; only the raindrops freeze before reaching the surface. The strongest winds are behind the cold front, and blizzard conditions are most likely to occur there.

Thunderstorms

A thunderstorm is a violent disturbance in the atmosphere and typically occurs along a weather front. These storms are especially frequent when a cold front moves into a mass of warm, moist air. Associated features of thunderstorms include lightning and thunder, occasionally hail, and frequently heavy precipitation, although, in dry climates, precipitation at the surface may not occur. When temperatures in the atmosphere decrease rapidly with height, the atmosphere is unstable. Moreover, if the air is moist, a considerable amount of energy is stored inactively in the water vapor, and this energy will be released when the vapor changes to liquid water or ice. When this moist, unstable air is given an initial lift by unequal heating of the ground surface, a mountain range, or an advancing front, a rising air current is set in motion. As long as the rising air is less dense than the surrounding air, it will continue to rise. As the water vapor is condensed, air density is decreased, and a towering thunderhead cloud forms.

Conditions favorable for the formation of thunderstorms most often occur in warm, moist tropical air. For this reason, thunderstorms form most frequently in states bordering the Gulf of Mexico and are also frequent around the Great Lakes region in North America. Thunderstorms are divided into two basic types, termed local and organized thunderstorms. Local storms are isolated, scattered, and usually short-lived. Normally, they occur on warm summer afternoons around the peak daily temperature. Organized thunderstorms are long-lived and occur over larger areas than local storms. They form in rows called squall lines, along cold fronts, occasionally along warm fronts, and adjacent to mountain peaks.

The initial stage of thunderstorm formation is the cumulus stage, in which the cloud is dominated by updrafts. Precipitation does not occur in this stage, but as the cloud gets larger, the updrafts get stronger and more widespread. In the top of the cloud, where liquid water and ice crystals are abundant and where buoyancy is less, a downdraft is initiated. As soon as the downdraft starts, the second stage in the life cycle is reached, called the mature stage.

The most violent weather in the thunderstorm occurs during the mature stage. A strong updraft and downdraft both exist in the cloud formation, and heavy rain is produced in the downdraft side. Also associated with the downdraft are strong wind gusts at the surface. Depending on the strength of upper winds and downdraft currents, surface winds may range from cool, gentle breezes to strong blasts of air. Gradually, the downdraft spreads throughout the cloud, and the dissipating stage is reached. In this stage, the storm is characterized by weak downdrafts and light rain.

Hazards associated with thunderstorms other than strong winds and heavy rain include lightning and hail. Lightning occurs because of the separation of positive and negative charges within clouds, between clouds, and between the clouds and the ground. When the electrical potential of a charge is sufficient to overcome the insulating effect of the air, a lightning stroke results. Thunder is caused by a rapid expansion of air as a lightning bolt—which is several times hotter than the sun’s surface—passes through it. Hail is formed as an ice crystal is buffeted about within a cloud, and successive layers of ice are added to the developing hailstone as supercooled water is encountered within the cloud. The eventual size of a hailstone depends on the length of time it undergoes these conditions during its passage through the cloud.

Tornadoes and Hurricanes

Tornadoes are also associated with severe thunderstorms. They are small, powerful storms usually less than half a kilometer in diameter, though at times, they may extend for a kilometer or more. A tornado may have the shape of a funnel, rope, or cylinder extending from the base of a thunderstorm cloud to the surface of the ground. The tone of the tornado depends on its background, the debris, and the condensed moisture within it. A blue sky behind a tornado makes it appear dark, whereas intense rain behind it makes it look white. The funnel will appear dark when it is filled with debris and dust sucked up from the surface.

The motion of a tornado is highly variable, averaging 65 kilometers per hour, but some tornadoes have been observed to travel as fast as 110 kilometers per hour. Inside, the storm winds, which almost always turn in a counterclockwise direction, may whirl around the center in excess of 500 kilometers per hour. Within a tornado are smaller, more intense vortices called suction vortices. There may be anywhere from one to three such vortices in a tornado. When there is more than one suction vortex, they rotate around a common center and may account for total destruction in one area, while an area only meters away sustains little or no damage.

Tornadoes occur in many parts of the world, but the topography and pressure patterns of the central United States are especially suited to tornado formation, such that the region extending northeast from Texas through southwestern Ontario is often referred to as “Tornado Alley.” In this region, the Rocky Mountains block winds from the west, while the cold, dry jet stream flows over the mountains to meet warm, moist air flowing northward from the Gulf of Mexico due to high pressure in the Atlantic Ocean at 30 degrees north latitude. When unstable air rises, the extreme vertical contrast between the two air masses provides conditions for an explosive upward movement of air. Another aspect of the tornado is the low pressure in the center. Pressure here often approaches 800 millibars, whereas average sea-level pressure is 1,013 millibars.

Tornado occurrence follows the seasonal migration of the jet stream. In winter, the jet stream is closer to the Gulf of Mexico, making the Gulf states more prone to tornadoes. In spring, the Southern Plains states are most likely to experience tornadoes; in late spring and summer, the Northern Plains and the eastern United States are most prone. May is the peak month of occurrence. Because of its size, Texas has more tornadoes than any other state, but when storms are averaged over the area, Oklahoma ranks first.

Hurricanes are tropical cyclones that occur in the Atlantic Ocean and the Gulf of Mexico. Their counterparts in the Pacific Ocean are typically referred to as cyclones and typhoons. A hurricane goes through a four-stage development. Stage 1 is the tropical disturbance, in which a low-pressure center has some clouds and precipitation but no enclosed isobars (lines connecting points of equal pressure) and only light winds. The second stage is the tropical depression, with lower pressure in the center and at least one enclosed isobar but with winds less than 60 kilometers per hour. Third is the tropical storm stage, with winds between 60 and 120 kilometers per hour around a low-pressure center with several enclosed isobars. The storm is given a name at this stage. Fourth is the tropical cyclone or hurricane stage, with pronounced rotation around a central core or eye. Winds are sustained at speeds in excess of 120 kilometers per hour, circling as bands toward the center. Hurricanes form over warm oceans and derive most of their energy from water with temperatures greater than 26.5 degrees Celsius. These storms form only in late summer and fall. Because the motion of a hurricane is affected by the Coriolis force, hurricanes are not seen to form closer than 5 degrees of latitude to the equator, where the Coriolis force is not sufficient to induce rotation. Hurricanes have no fronts and are smaller than a midlatitude cyclone, while central pressures are lower and winds are stronger.

Study of Storms

One of the first major investigations into the nature of thunderstorms was undertaken from 1946 to 1947 during what was called the Thunderstorm Project. The project consisted of flying instrument-bearing aircraft through thunderstorms to obtain various kinds of data. The study also was augmented by the collection of data from various instruments such as radar, radiosondes, and ground-based instruments. Much was learned about thunderstorm structure, internal activity, and life cycle from this investigation. It was largely through the Thunderstorm Project that the cumulus, mature, and dissipating stages in a thunderstorm’s life cycle were identified. Aircraft are also used to fly through hurricanes for the purpose of collecting storm data.

One device used in data collection in the upper atmosphere is the rawinsonde, which is a package of weather instruments attached to a balloon that is sent aloft. The rawinsonde has instruments to record temperature and humidity, a reflector for collecting wind data by ground-based radar, and a transmitter to send data to recorders in the weather station. Another valuable meteorological tool is radar, which can be used to detect and observe storms hundreds of kilometers away from a station. A radar set sends out pulses of radio waves through an antenna that rebounds from objects such as raindrops, cloud drops, ice crystals, and hail. A radio receiver intercepts the returning pulses between transmitted pulses.

Doppler radar is a particularly useful forecasting tool, especially for severe thunderstorms and tornadoes. Doppler radar sends out continuous radio waves instead of pulses and uses the Doppler effect on reflected radio signals to measure the speed of objects moving toward or away from the radar antenna. Targets such as cloud drops, raindrops, and other liquid or solid particles reflect radio waves so the direction of air movement can be discerned. Thus, air that is whirling within a storm can be detected by the appearance of shortened wavelength on one side of a central point and lengthened wavelength on the other side. Doppler radar can identify and locate a tornado within a thunderstorm as early as fifteen to twenty minutes before the funnel touches the ground. A network of Doppler radar stations was established across North America, providing meteorologists with a great deal of information regarding storms and weather patterns.

Satellites are another useful tool for detecting and observing middle-latitude cyclones, thunderstorms, and hurricanes. Several meteorological satellites have been placed in orbit over the years, beginning with the Television Infrared Observation Satellites (TIROS). Satellites such as the Applications Technology Satellites (ATS) and Synchronous Meteorological Satellites (SMS) are in geosynchronous orbit, which means they make one revolution of Earth in a twenty-four-hour period so that they remain over the same location on the surface. These satellites sense conditions on and above Earth and send coded information back to weather stations. The information is transformed into weather images that are frequently shown on television. Images and measurements are taken in both the visible and infrared wavelength ranges. Both wavelengths can produce visible images, but infrared sensors detect the temperature differences of objects. These images provide information on midlatitude cyclones, thunderstorms, and hurricanes.

Significance

Storms occur in the United States nearly every day. They can be quite destructive and often life-threatening, although they are also beneficial as essential components of the water cycle. One dangerous aspect of storms is lightning. About twenty-eight people are killed in the United States yearly from lightning strikes.

Thunderstorms, on an annual basis, can cause more cumulative damage than other storms simply because there are so many of them. In addition to lightning, thunderstorms can produce heavy downpours of rain, which can cause flash flooding. Hailstorms are also associated with thunderstorms. Hail often destroys crops and can cause damage to buildings, automobiles, and other structures. Strong gusts of wind on the downdraft side of a thunderstorm often do appreciable damage to vegetation, especially trees, and to buildings. Downbursts—air that rushes out of a thunderstorm downdraft and spreads out laterally near the surface—are particularly hazardous to aircraft taking off or landing and have caused several plane crashes.

Tornadoes are generally associated with severe thunderstorms, although they also occur in conjunction with hurricanes. They are difficult to forecast and often appear quickly, without warning. More tornadoes occur annually across the contiguous United States than in any other country. According to the National Oceanic and Atmospheric Administration, there was an average of 1,251 tornadoes per year between 1991–2010, with an average of 1,300 annual tornadoes in the 2020s. Deaths from tornadoes average eighty per year, based on National Weather Service figures, and property damage is typically valued in the millions of dollars. The conditions for the formation of storms that generate tornadoes migrate with the movement of the jet stream, and the month of May has historically had more tornadoes than any other, although June has more days on which tornadoes occur.

Hurricanes are normally limited to late summer and fall, although they have occurred as late as December. Global climate changes and rising ocean temperatures have brought about a significant extension of the hurricane season. Hurricanes are not as intense as tornadoes, but being thousands of times larger and of much longer duration, they cause more damage. Their strength is designated by the Saffir-Simpson Hurricane Wind Scale, with category 1 having the least powerful winds, at 119 to 153 kilometers per hour, and category 5 having the most powerful winds, at 252 kilometers per hour or more. Hurricanes form most frequently in the Gulf of Mexico and the Atlantic Ocean, but similar storms also form in the Pacific Ocean. The most damaging aspect of a hurricane is the tidal surge that accompanies it. Surges have been as high as 8.5 meters and have carried floods inland for several kilometers. The damage from Hurricane Camille in 1969, which had a storm surge of more than 7.5 meters, totaled some $1.5 billion. In 2005, Hurricane Katrina struck the Louisiana coast as a category 3 storm, flooding the city of New Orleans; Katrina boasted sustained winds of almost 280 kilometers (175 miles) per hour, pushing a storm surge of up to 8.5 meters (28 feet). The storm killed more than 1,500 people, and property damage amounted to more than $214 billion. High-velocity winds also cause considerable damage, which is more widespread than surge damage. In 2017, Hurricanes Harvey, Irma, and Maria, all above category 3 at landfall, accounted for more than $265 billion in damage. In 2022, Hurricane Ian, a category 5 hurricane, became the costliest hurricane in Florida history after causing an estimated $112 billion in damage. The 2023 hurricane season was one of the most active years on record, with the fourth most named storms of any year. An average season consists of fourteen storms, but 2023 saw twenty named storms.

Ice storms occur most often in the eastern and southern states and have been responsible for power outages when power lines break and towers collapse under the added weight of accumulated glaze. Ice storms are also very hazardous to transportation due to slippery road conditions. Blizzards occur in the northern United States. Blizzard winds can reach hurricane force, and travelers can become stranded because of drifting snow and low visibility. Because of the cold temperatures, people lost in blizzards frequently suffer severe frostbite or die from hypothermia.

Principal Terms

blizzard: a winter storm characterized by cold wind having a minimum velocity of 56 kilometers per hour, large amounts of blowing snow, and low levels of visibility

Doppler radar: a radar method that uses the Doppler shift to measure the speed of targets moving either toward or away from the radar

Doppler shift: a phenomenon in which the wavelength of electromagnetic radiation (or other type of wave) is lengthened by reflection from a surface moving away from the source or shortened by reflection from a surface moving toward the source

downburst: a severe localized downward outflowing of air and associated wind shear below a thunderstorm

glaze: a coating of ice formed on exposed objects by the freezing of a film of supercooled water deposited by rain, drizzle, or fog

hurricane: a severe tropical cyclone, typically between 500 and 600 kilometers in diameter, with winds over 65 knots (74 miles or 120 kilometers) per hour

ice storm: a storm characterized by a fall of freezing rain, with the formation of glaze on objects below

rawinsonde: a radiosonde with a radar target attached so that it can be tracked for the collection of wind information

tornado: a violently rotating column of air that forms and extends downward from a cumulonimbus cloud and has the appearance of a funnel, rope, or column that touches the ground

vortex: the center of a whirling or rotating fluid, typically with low pressure in the center due to loss of the fluid mass in that locus by centrifugal force

Bibliography

Ahrens, C. Donald. Essentials of Meteorology: An Invitation to the Atmosphere. 9th ed. Cengage Learning Inc., 2023.

"Annual 2023 Tornadoes Report." National Centers for Environmental Information, 2024, www.ncei.noaa.gov/access/monitoring/monthly-report/tornadoes/202313. Accessed 30 July 2024.

Bluestein, Howard B. Tornado Alley: Monster Storms of the Great Plains. Oxford UP, 2006.

Dunlop, Storm. The Weather Identification Handbook. Lyons, 2003.

Halverson, Jeffrey B. An Introduction to Severe Storms and Hazardous Weather. Taylor & Francis Group, 2024.

Holton, James R., and Gregory J. Hakim. An Introduction to Dynamic Meteorology. 6th ed. Academic, 2019.

"Ian Is Costliest Hurricane in Florida History, Caused $112B in Damage in US: NOAA." NBC 6 South Florida, 3 Apr. 2023, www.nbcmiami.com/news/local/ian-is-costliest-hurricane-in-florida-history-caused-112b-in-damage-in-us-noaa/3007223. Accessed 29 Sept. 2023.

Lighthill, James, and Robert Pearce, eds. Monsoon Dynamics. Cambridge UP, 2009.

Mooney, Chris. Storm World: Hurricanes, Politics, and the Battle over Global Warming. Harcourt, 2007.

Office of Climate, Water, and Weather Services. "Weather Fatalities 2016." National Weather Service, Analyze, Forecast and Support Office, National Oceanic and Atmospheric Administration, US Dept. of Commerce, 11 May 2017, www.nws.noaa.gov/om/hazstats.shtml. Accessed 28 Mar. 2018.

Rosenfeld, Jeffery. Eye of the Storm: Inside the World’s Deadliest Hurricanes, Tornadoes and Blizzards. Cambridge: Basic, 2003.

"Saffir-Simpson Hurricane Wind Scale." National Hurricane Center, National Oceanic and Atmospheric Administration, US Dept. of Commerce, www.nhc.noaa.gov/aboutsshws.php. Accessed 1 Aug. 2024.

Samaras, Tim, et al. Tornado Hunter: Getting Inside the Most Violent Storms on Earth. Natl. Geographic Soc., 2009.

Schneider, Bonnie. Extreme Weather: A Guide to Surviving Flash Floods, Tornadoes, Hurricanes, Heat Waves, Snowstorms, Tsunamis and Other Natural Disasters. Palgrave, 2012.

Schneider, Stephen Henry, and Michael D. Mastrandrea, eds. Encyclopedia of Climate and Weather. 2nd ed. 3 vols. Oxford UP, 2011.

"Tornadoes." National Weathe Service, www.weather.gov/media/pah/Skywarn/TORNADOsafety.pdf. Accessed 1 Aug. 2024.