Severe weather is any dangerous meteorological or hydro-meteorological phenomena, of varying duration, with risk of causing major damage, serious social disruption and loss of human life.[1] While types of severe weather phenomena can vary depending on the latitude, atitude, topography, and atmospheric conditions of a region, general forms of severe weather include: thunderstorms, hurricanes, hailstorms, tornadoes, heavy precipitation, and damaging downburst winds. More localized severe weather phenomena are characterized by blizzards, snowstorms, ice storms, and duststorms that can only occur at certain regions. Extreme temperature variations caused by heat waves and cold waves can also be considered forms of severe weather. The term severe weather is generally used to describe significant weather occurrences which develop during strong to severe thunderstorms, tropical cyclones, or extratropical cyclones.

Extratropical cyclones can contain phenomena such as squall lines within their warm sector, tornadoes near their warm front, snow storms within their comma-head precipitation pattern, as well as heavy lake-effect snows and ground blizzards in their wake. Along the east coast of North America, extratropical cyclones which develop from fall into the spring are known as nor’easters. During the warm season, tropical cyclones move across the tropics and subtropics of the globe, bringing heavy rainfall, high winds, and significant storm surges to places near their path. Severe thunderstorms contain hazards such as high winds, hail, tornadoes, and lightning, which can also cause outbreaks of wildfires. Severe weather can occur within larger thunderstorm complexes such as squall lines and other types of mesoscale convective systems.

There are frequent incorrect usages between the terms of severe weather and extreme weather, as there is a tendency to refer the two terms as synonyms.

A thunderstorm, also known as an electrical storm, a lightning storm, a hailstorm, or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth’s atmosphere known as thunder.[1] The meteorologically-assigned cloud type associated with the thunderstorm is the cumulonimbus. Thunderstorms are usually accompanied by strong winds, heavy rain and sometimes snow, hail, or no precipitation at all. Those which cause hail to fall are known as hailstorms. Thunderstorms may line up in a series or rainband, known as a squall line, and strong or severe thunderstorms may rotate, known as supercells. While most thunderstorms move with the mean wind flow through the layer of the troposphere in which they occupy, vertical wind shear causes a deviation in their course at a right angle to the wind shear direction.

Thunderstorms can generally form and develop in any geographic location, perhaps most frequently within areas located at mid-latitude when warm moist air front collides and border cool air fronts.[2] Thunderstorms are responsible for the development and formation of many severe weather phenomena. Thunderstorms, and the phenomena that occurs along with it, can produce numerous risks and hazards to populations and landscapes. Damages that result from thunderstorms are mainly inflicted by downburst winds, large hailstones, and flash flooding caused by heavy precipitation. Stronger thunderstorm cells are capable of producing tornadoes and waterspouts.

There are four types of thunderstorms: single cell, multicell cluster, multicell lines, and supercells. Supercell thunderstorms are the strongest and the most associated with severe weather phenomena. Mesoscale convective systems formed by favorable vertical wind shear within the tropics and subtropics are responsible for the development of hurricanes. Dry thunderstorms, with no precipitation, can cause the outbreak of wildfires with the heat generated from the cloud-to-ground lightning that accompanies them. A variety of methods are used to study thunderstorms, such as weather radar, weather stations, and video photography. Past civilizations held various myths concerning thunderstorms and their development as late as the Eighteenth Century.

A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones feed on heat released when moist air rises, resulting in condensation of water vapor contained in the moist air. They are fueled by a different heat mechanism than other cyclonic windstorms such as nor’easters, European windstorms, and polar lows, leading to their classification as “warm core” storm systems. Tropical cyclones originate in the doldrums near the equator, about 10° away from it.

The term “tropical” refers to both the geographic origin of these systems, which form almost exclusively in tropical regions of the globe, and their formation in maritime tropical air masses. The term “cyclone” refers to such storms’ cyclonic nature, with counterclockwise rotation in the Northern Hemisphere and clockwise rotation in the Southern Hemisphere. Depending on its location and strength, a tropical cyclone is referred to by names such as hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, and simply cyclone.

While tropical cyclones can produce extremely powerful winds and torrential rain, they are also able to produce high waves and damaging storm surge as well as spawning tornadoes. They develop over large bodies of warm water, and lose their strength if they move over land. This is why coastal regions can receive significant damage from a tropical cyclone, while inland regions are relatively safe from receiving strong winds. Heavy rains, however, can produce significant flooding inland, and storm surges can produce extensive coastal flooding up to 40 kilometres (25 mi) from the coastline. Although their effects on human populations can be devastating, tropical cyclones can also relieve drought conditions. They also carry heat and energy away from the tropics and transport it toward temperate latitudes, which makes them an important part of the global atmospheric circulation mechanism. As a result, tropical cyclones help to maintain equilibrium in the Earth’s troposphere, and to maintain a relatively stable and warm temperature worldwide.

Many tropical cyclones develop when the atmospheric conditions around a weak disturbance in the atmosphere are favorable. The background environment is modulated by climatological cycles and patterns such as the Madden-Julian oscillation, El Niño-Southern Oscillation, and the Atlantic Multi-Decadal Mode. Others form when other types of cyclones acquire tropical characteristics. Tropical systems are then moved by steering winds in the troposphere; if the conditions remain favorable, the tropical disturbance intensifies, and can even develop an eye. On the other end of the spectrum, if the conditions around the system deteriorate or the tropical cyclone makes landfall, the system weakens and eventually dissipates. It is not possible to artificially induce the dissipation of these systems with current technology.

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