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For tornado outbreaks, see Tornado outbreak.
Severe weather is not to be confused with the term severe weathering.
A mesoscale discussion predicting a possible supercell thunderstorm phenomenon in the late evening, with risks of large hail
An example of a mesoscale discussion. In the United States, mesoscale discussions are issued by the Storm Prediction Center, discussing the severe weather threat in the area outlined by the discussion.
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Severe weather refers to 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.[2]

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.

Severe weather is often used interchangably with extreme weather, even though the two have distant meanings.

Terminology

While there is no universal definition of what severe weather means, it can generally be defined as any aspect of weather that can pose risks to life or property and those that require the attention of authorities. Some authorities simply define severe weather as any weather phenomena relating to severe thunderstorm.[3]

According to the World Meteorology Association, severe weather can be categorized into two groups based on how common they are able to form.[1] Thunderstorms, and the phenomena that accompany them, have a wide geographic range where they can form. These weather occurrences can be classified as general severe weather.[1] Blizzards and dust storms require more specific conditions to appear and therefore have a more limited geographic range. These forms of weather are classified as localized severe weather.[1]

The term severe weather is often used interchangeably with extreme weather. Extreme weather describes unusual weather events that uncommonly occur in a certain area.[4]

Cause

Diagram showing ingredients needed for severe weather. The red arrow shows the position of the low level jet stream, while the blue arrow shows the location of the upper level jet stream

Organized severe weather tends to occur in the same conditions which cause ordinary thunderstorms: atmospheric moisture, lift, and instability.[5] Clouds are little more than condensed water vapor, and thus high atmospheric moisture, signified by a high dew point, aids the development of the clouds which become thunderstorms. Instability is defined the tendency for a parcel of air near the Earth's surface to keep rising when forced upwards by a source of lift. When sufficient air starts to rise due to this instability, this air also cools and expands, and the moisture contained in the parcel condenses creating the cloud itself. Sources of lift include geographic features, such as mountains, and air mass boundaries such as cold fronts, warm fronts, and dry lines.

A wide variety of conditions can cause severe weather. While in general, the criteria above will produce generic thunderstorms, any of several factors can turn those thunderstorms severe; for instance, a pool of cold air aloft can aid in the development of large hail from seemingly innocuous thunderstorms. However, the most severe hail and tornadoes are produced by supercell thunderstorms, and the worst downbursts and derechos (straight-line winds) are produced by bow echoes. Both of these types of storms tend to form in environments high in wind shear.[5]

The severe weather phenomenon of a thunderstorm can cause a large amount of damage towards any landscape and environment. In terrestrial ecosystems, thunderstorms have the possibilities of causing forest fires, with the lightning strikes that accompanies them. The heavy precipitation produced by some thunderstorms can result in the phenomenon of flash flooding, which could inflict severe damage among the surrounding environments. A severe weather outbreak is typically considered to be 10 or more tornadoes, some of which will most likely be long tracked and violent, and many large hail and damaging wind reports. However, there is much dependence on the geographic size of the outbreak, such as whether it covers hundreds or thousands of square kilometers.[6]

Categories

See also: Wind shear

Severe thunderstorms can be put into three different categories. These are approaching severe, severe, and significant severe. Approaching severe is defined as hail between 1/2 and 3/4 inch diameter or winds between 50 and 58 M.P.H. (50 knots). In the United States, such storms will usually warrant a Significant Weather Alert. Severe is defined as hail 3/4 inch (one inch in some places) diameter or larger, winds 58 M.P.H. or stronger, or a tornado.[7] Significant severe is defined as hail 2 inch in diameter or larger, winds 75 M.P.H. (65 knots) or stronger, a tornado of strength EF2 or stronger, the occurrence of flash flood phenomena by heavy precipitation, or extreme temperatures.[1][8] Both severe and significant severe events will warrant a severe thunderstorm warning from the National Weather Service, Environment Canada, or the Australian Bureau of Meteorology if the event is in the United States, Canada, or Australia respectively. If a tornado is occurring or if it is likely one will occur, the severe thunderstorm warning will be superseded by a tornado warning, which warns for other elements of a severe thunderstorm as well as the tornado itself.

Types

Lightning

See also: Lightning
A return-stroke lightning.

Once the thunderclouds have been formed, the downdrafts present in the cumulonimbus cloud causes the remaining water molecules to freeze into solid states of ice and hail. The ice particles, containing a positive charge is separated by the updrafts, while the hail particles that contain a negative charge is moved by the downdrafts. This basically separates the differently charged particles. During the thunderstorm, the Earth's surface is composed of a positive charged. Because opposites attract, the negative charged particles on the lower cloud (caused by the downdraft) tries to contact the positive particles upon land. The negative charges, once strong enough to resist against air resistance flows downwards towards the Earth. This is called a stepped leader. The positive particles, being attracted to the stepped leader climbs up elevated objects of trees and other objects and contacts the negative particles. Once this happens, a strong electrical current inputs the positive charges into the cloud. This creates a return stroke of lightning.[9] The high voltage of electricity comprised within lightning is capable of releasing enough heat energy to ignite of a fire.[10]

High winds

Panorama of a strong shelf cloud, which precedes the onset of high winds

High winds are known to cause damage, depending upon their strength. Infrequent wind gusts can cause poorly-designed suspension bridges to sway. When wind gusts are at a similar frequency to the swaying of the bridge, the bridge can be destroyed easier, such as what occurred with the Tacoma Narrows Bridge in 1940.[11] Wind speeds as low as 23 knots (43 km/h) can lead to power outages due to tree branches disrupting the flow of energy through power lines.[12] While no species of tree is guaranteed to stand up to hurricane-force winds, those with shallow roots are more prone to uproot, and brittle trees such as eucalyptus, sea hibiscus, and avocado are more prone to damage.[13] Hurricane-force winds cause substantial damage to mobile homes, and begin to structurally damage homes with foundations. Winds of this strength due to downsloped winds off terrain have been known to shatter windows and sandblast paint from cars.[14] Once winds exceed 135 knots (250 km/h), homes completely collapse, and significant damage is done to larger buildings. Total destruction to man-made structures occurs when winds reach 175 knots (324 km/h). The Saffir-Simpson scale and Enhanced Fujita scale (TORRO scale in Europe) were designed to help estimate wind speed from the damage caused by high winds related to tropical cyclones and tornadoes, and vice versa.[15][16]

Wildfires

Wildfire in Yellowstone National Park produces a pyrocumulus cloud
See also: Wildfire and Dry lightning

Dry lightning is a term in the United States for lightning that occurs with no precipitation at the surface. This type of lightning is the most common natural cause of wildfires.[10] Pyrocumulus clouds produce lightning for the same reason that it is produced by cumulonimbus clouds. When the higher levels of the atmosphere are cooler, and the surface is warmed to extreme temperatures due to a wildfire, volcano, etc, convection will occur, and the convection produces lightning. Therefore, fire can beget dry lightning through the development of more dry thunderstorms which cause more fires.

Heat waves, droughts, cyclical climate changes such as El Niño, and other weather patterns can also increase the risk and alter the behavior of wildfires dramatically.[17] Years of precipitation followed by warm periods have encouraged more widespread fires and longer fire seasons.[18] Since the mid 1980s, earlier snowmelt and associated warming has also been associated with an increase in length and severity of the wildfire season in the Western United States.[19]

Fire intensity also increases during daytime hours. Burn rates of smoldering logs are up to five times greater during the day due to lower humidity, increased temperatures, and increased wind speeds.[20] Sunlight warms the ground during the day and causes air currents to travel uphill, and downhill during the night as the land cools. Wildfires are fanned by these winds and often follow the air currents over hills and through valleys.[21] Fires in Europe occur frequently during the hours of 12:00 p.m. and 2:00 p.m.[22] U.S. wildfire operations revolve around a 24-hour fire day that begins at 1000 hours due to the predictable increase in intensity resulting from the daytime warmth.[23]

Precipitation

Wet season storm at night in Darwin, Australia
See also: Precipitation (meteorology) and Wet season

In mid-latitudes, convective precipitation is intermittent and often associated with baroclinic boundaries such as cold fronts, squall lines, and warm fronts.[24] Elsewhere, the wet season is the time of year, covering one or more months, when most of the average annual rainfall in a region falls.[25] Areas with wet seasons are dispersed across portions of the tropics and subtropics.[26] Savanna climates and areas with monsoon regimes have wet summers and dry winters. Tropical rainforests technically do not have dry or wet seasons, since their rainfall is equally distributed through the year.[27] Some areas with pronounced rainy seasons will see a break in rainfall mid-season when the intertropical convergence zone or monsoon trough move poleward of their location during the middle of the warm season.[28] When the wet season occurs during the warm season, or summer, rain falls mainly during the late afternoon and early evening hours. The wet season is a time when air quality improves,[29] freshwater quality improves,[30][31] and vegetation grows significantly. Soil nutrients diminish and erosion increases.[28] Animals have adaptation and survival strategies for the wetter regime. Unfortunately, the previous dry season leads to food shortages into the wet season, as the crops have yet to mature. Developing countries have noted that their populations show seasonal weight fluctuations due to food shortages seen before the first harvest, which occurs late in the wet season.[32]

Tropical cyclones, a source of very heavy rainfall, consist of large air masses several hundred miles across with low pressure at the centre and with winds blowing inward towards the centre in either a clockwise direction (southern hemisphere) or counterclockwise (northern hemisphere).[33] Although cyclones can take an enormous toll in lives and personal property, they may be important factors in the precipitation regimes of places they impact, as they may bring much-needed precipitation to otherwise dry regions.[34] Areas in their path can receive a year's worth of rainfall from a tropical cyclone passage.[35]

Monsoons

Precipitation occurred during monsoon season can often cause flooding within landscapes.
Main article: Monsoons

Monsoons are seasonal wind shifts which lead to long-lasting wet seasons which produce a bulk of the annual precipitation in areas such as southeast Asia, Australia, western Africa, eastern South America, as well as Mexico. Widespread flooding can occur if rainfall becomes excessive,[36] which can lead to landslides and mudflows in mountainous areas.[37] Such floods cause rivers to leave their banks and homes to go underwater.[38] Floods can be exacerbated by fires during the previous dry season, which cause soils which are sandy or composed of loam to become hydrophobic, or repellent of water.[39] There are various ways government organizations help their residents deal with wet season floods. Flood plain mapping is conducted, which helps diagnose what areas are more prone to flooding.[40] Instructions on how to control erosion through outreach is also done via telephone or the internet.[41]

Flooding waters that occur during Monsoon seasons can often host numerous microorganisms of protist, bacterias, and virus. These disease-agent organisms thrive and spread within contaminated waters of Monsoon floodings.[42] Other disease carriers of mosquitoes and flies will lay their eggs within the contaminated bodies of water. If exposed to flood waters, these disease-agents have high possibility of entering the blood vessels of human bodies and have capabilities causing infections of food borne and waterborne diseases. Parasitic organisms that live off the nutrients of other organisms, that also be found contained within flood waters and also present a risk to infecting diseases.

Diseases that be infected with exposure to flood waters include: Malaria, Cholera, Typhoid, Hepatitis A, and the Common cold.[43]

Possible infections of trenchfoot can also occur when exposed to extended periods of time within flooded areas.[44] Trenchfoot develops when a person's feets is exposed with wet conditions for long periods of time. When infected with trenchfoot, a person's feet may feel itch or tingling sensations. If left untreated, swellings of blisters may appear along with the feel of numbness and pain. Skin upon infected areas may turn red when exposed to warm conditions. Dead skin cells presented in infected areas will soon peel off. Pain developed from leg cramps is also common with the infections of trenchfoot.[45]

Acid rain

Main article: Acid rain

Acid rain is any form of precipitation, where the water molecules presented have a unusual decrease in pH levels.[46] Acid rain can often occur during the weather phenomenon of a thunderstorm. The voltage of electricity release by lightning is powerful enough to split nitrogen molecules in the atmosphere into separate atoms.[47] These nitrogen atoms are reactive enough to form compounds with the water molecules presented in rain, forming nitric acid, a form of acid rain. The properties of acid rain are able to dissolve structure created out of minerals containing calcite (otherwise known as calcium carbonate). The acid rain reacts with the carbon molecules compost within the calcite, while also releasing calcium atoms. This process then wears and dissolves away structures made of limestone and other minerals containing calcite. Other that the damage it does to infrastructures and buildings of limestone, acid rain can also produce risk and harm towards marine wildlife through increases in acidity.[48] Tissues of vegetations are also presented with the risk of being damaged. Acid rain can also increase acidity in soil particles, which decrease the nutrients presented in harmed soil as well as encourage the presence of certain disease-agent microorganisms. Besides occurring within the presence of thunderstorms, acid rain can also occur in dense populated areas. Dense populated areas often have high usage of energy source, which releases greenhouse gas. This can mixed with water molecules presented in the atmosphere, creating acid rain. The gas particles released by volcanic eruptions can also allow acid rain to form.[49]

Local severe weather phenomenon

Dust storm

Main article: Dust storms
A massive dust storm cloud is close to enveloping a military camp as it rolls over Al Asad, Iraq, just before nightfall on April 27, 2005.

A dust storm is an unusual form of windstorm that is characterized by the existence of large quantities of sand and dust particles in which are carried by moving air.[50] Dust storms frequently develop during periods of droughts and over arid and semi-arid regions, when less moisture is being presented within soil particles, which reduces their density and cause it to be easier to lift by moving air. Dust storms generally occur for only a few minutes before subsiding.

Dust storms have numerous hazards and are capable of creating deaths when actions are not taken carefully. During the occurrence of a dust storm, visibility can be reduced dramatically, making it harder to see. This can cause exposure to these weather phenomena for extended periods of time. Reduction in visibility causes the most damage within traffic areas. Risks of collisions with other automobiles can occur within areas of dust storm occurrences. Aircraft in flight during the occurrence of dust storms can be also exposed to risks of crashing when flying along busy air routes or landing, due to the fast winds and reduction of visibility. The amount of oxygen intake by respiratory organs can decrease as well,[51] resulting in possibilities of suffocation. Damage can also be inflicted upon external optical organs of the eyes.[52] Dust storms can produce many issues for agricultural industries as well. Soil erosion is one of the most common hazards within dust storm phenomena, decreasing lands available for producing crops. Dust and sand particles associated in dust storms can cause severe processes of weathering to occur upon infrastructures and rock formation. Nearby bodies of water can be polluted by the settling of dust and sand, causing deaths along aquatic organisms. Decrease in exposure to radiation can affect plant growth, as well as provide insulation to the heat from infrared radiation, causing increases in temperature.

Dust and sand particles associated in dust storms can also be carried away from their original area thousands of miles to other geographic locations by moving air streams.[53] While this does provide hazards for countries receiving these dust and sand particles, they also provide many benefits to certain areas. Many rainforest, like the Amazon, rely on the settling of dust and sand particles from deserts to receive nutrients required for plant growth. All dust storms don't occur naturally. The lost of vegetation by human activities can also cause dust storms to occur. An example would be the Dust Bowl phenomenon that happened during the 1930s.

Dust storms can be classified into different weather systems based upon how they develop. Dust storms can form by bordering of cool and warm air fronts, resulting in the formation of gust system presented in dust storms. This type of dust storm can be consider an extratropical cyclone. Dust storms that form by the outflow boundary of a dissipating thunderstorm, like the haboob, can be consider a form of mesoscale convective complex.

Mesoscale convective systems

This graphic shows the conditions favorable for certain organized thunderstorm complexes, based upon CAPE and vertical wind shear values.
Main article: Mesoscale convective system

A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones,[54] and normally persists for several hours or more. A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines,[55] lake-effect snow events, polar lows, and mesoscale convective complexes (MCCs), and generally form near weather fronts. The type that forms during the warm season over land has been noted across North America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.

Forms of MCS that develop within the tropics use either the Intertropical Convergence Zone or monsoon troughs as a focus for their development, generally within the warm season between spring and fall. One exception is that of lake-effect snow bands, which form due to cold air moving across relatively warm bodies of water, and occurs from fall through spring. Polar lows are a second special class of MCS which form at high latitudes during the cold season. Once the parent MCS dies, later thunderstorm development can occur in connection with its remnant mesoscale convective vortex (MCV). Mesoscale convective systems are important to the United States rainfall climatology over the Great Plains since they bring the region about half of their annual warm season rainfall.

Thunderstorm development

Thunderstorms produce many hazards that put life and property at risk.

The phenomena of floods, hurricanes, tornadoes, and thunderstorms are considered to be the most destructive weather-related natural disaster to occur. Though these weather phenomena are all required to form and develop under different conditions and geographic location, they do have one thing in common. All are related to cumulonimbus clouds. This similarity between these weather events and their required condition of formation can be used to determine the most frequent and possible location of development and use the data to perhaps save lives. Thunderstorms are destructive weather phenomenon that are often characterized by the existence of powerful lightning and frequent precipitation. Thunderstorms can generally form and develop anywhere with the presence of thunderclouds. By understanding the development of a thundercloud, we can easily under the process of developing a thunderstorm.

Thunderclouds also have a wide distribution of occurrences. These forms of clouds are able to form under the circumstance where warm, moist air collides with cool air fronts.[56] Thunderclouds undergoes two stages of development: Cumulus and Mature.[57] During the stage of Cumulus, the heat generated from the Sun's radiation warms up the Earth's crust and the air close towards it. The warming of the air cause an updraft to form, allowing the warmed air to rise as warm air is less dense than normal oxygen. If the updraft contains water molecules, the rising of the updraft would cause the water molecules to condense forming a Cumulus cloud(there is less heat presented in higher attitudes). The continuation of condensation of the water molecules would allow the Cumulus cloud to reach greater volume.[58] This ends the Cumulus stage. The next stage of mature starts off by the expansion of the Cumulus cloud. As the Cumulus cloud continues to reach larger size, the water molecules composed within the cloud becomes too heavy for the Cumulus to hold. The water molecules then fall downwards as water is denser than air in the atmosphere. While this phenomenon undergoes, the cool air(or downdraft) enter the now-less dense Cumulus cloud. This process is known as entrainment. Downdrafts, being heavier than updrafts causes the Cumulus cloud to descend. While descending, the downdraft forces down water molecules that form the precipitation of the thunderstorm. Once this process is complete, the Cumulus cloud is then transformed into a Cumulonimbus cloud. The Cumulonimbus cloud is the basic foundation of a thundercloud.[59]

Tropical cyclones

The damage caused by Hurricane Andrew is a good example of the damage caused by a category 5 hurricane
Main article: Tropical cyclone

A tropical cyclone is a storm system characterized by a low pressure center and numerous thunderstorms that produce strong winds and flooding rain. A tropical cyclone feeds 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 and European windstorms, leading to their classification as "warm core" storm systems.[60]

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 their location and strength, tropical cyclones are referred to by other names, such as hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, or simply as a cyclone. Generally speaking, a tropical cyclone is referred to as a hurricane (from the name of the ancient Central American deity of wind, Huracan) in the Atlantic and eastern Pacific oceans, while they are termed cyclones in the south Pacific and Indian oceans.[61]

While tropical cyclones can produce extremely powerful winds and torrential rain, they are also able to produce high waves and damaging storm surge.[62] They develop over large bodies of warm water.[63] A tornado-like feature located in the eyewall, known as eyewall mesovortices. They are similar, in principle, to small "suction vortices" often observed in multiple-vortex tornadoes. In these vortices, wind speed can be up to 10% higher than in the rest of the eyewall. Eyewall mesovortices are most common during periods of intensification in tropical cyclones.[64] A hurricane’s heavy surf may cause harm towards organisms that are either close to or upon the surface of the water, such as coral reefs.[65] Tropical cyclone lose their strength as they move over land.[66] This is the reason 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.[34] 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.

Tornado

The F5 tornado that struck Elie, Manitoba in 2007.
Main article: Tornado

A tornado is a violent, dangerous, rotating column of air which is in contact with both the surface of the earth and a cumulonimbus cloud (otherwise known as a thundercloud) or, in rare cases, the base of a cumulus cloud. Tornadoes come in many sizes but are typically in the form of a visible condensation funnel, whose narrow end touches the earth and is often encircled by a cloud of debris and dust.[67]

Most tornadoes have wind speeds between 40 mph (64 km/h) and 110 mph (177 km/h), are approximately 250 feet (75 m) across, and travel a few miles (several kilometers) before dissipating. Some attain wind speeds of more than 300 mph (480 km/h), stretch more than a mile (1.6 km) across, and stay on the ground for dozens of miles (more than 100 km).[5][68][69]

Although tornadoes have been observed on every continent except Antarctica, 75% of them occur[70] in the United States.[71] The remaining tornadoes occur within regions at mid-latitude where warm and cool air fronts collide.[72] These regions include: southern Canada, south-central and eastern Asia, the Philippines, east-central South America, Southern Africa, northwestern and southeast Europe, western and southeastern Australia, and New Zealand.[73]

Tornadoes, despite being one of the most destructive weather phenomena ever to occur, generally have short lifespans. Most long-occurring tornadoes lasts no more than one hour. Because of this, less information is known about the development and formation of tornadoes.[74] Meteorologists currently have two explanations for the development of tornadoes.

The first explanation states that for tornadoes to begin to develop, two conditions must be first satisfied.[56] One, an invisible horizontal spinning effect must be first formed upon the Earth's surface. This is usually formed by sudden changes occurred in winds direction, known as wind shear. Second, a thundercloud, or occasionally a cumulus cloud, must be present. During a thunderstorm, the updrafts presented are occasionally powerful enough to lift the horizontal spinning row of air upwards, turning it into a vertical air column. This vertical air column then becomes the basic structure for the tornado. Tornadoes that forms in this way are often classified as weak tornadoes that generally last for less than 1–10 minutes.[56]

The second method of formation occurs during the occurrence of a supercell thunderstorm. This type of tornado forms by the updrafts present in the supercell thunderstorm. When winds occurring during this phenomenon increase and intensify, the force released can cause the updrafts to rotate. This rotating updraft is known as a mesocyclone.[75] For a tornado to form in this manner, a downdraft called the rear-flank downdraft enters the center of the tornado from the back. Cold air, being denser than warm air is able to penetrate through the updraft. The combination of the updraft and downdraft completes the development of a tornado. Tornadoes that form in this method are often classified as violent and are capable of lasting for more than one hour.[56]

Waterspout

Main article: Waterspout
Formation of numerous waterspouts in the Great Lakes region.

Waterspouts are severe weather phenomena, characterized by spiraling funnel-shaped wind currents that form over bodies of water, connecting to large cumulonimbus clouds. Waterspouts are generally defined as tornadoes, or more specifically, non-supercelled tornadoes that develop over bodies of water.[76] Waterspouts are generally weaker than most tornadoes, and are rarer in occurrence. Waterspouts frequently form in tropical areas close to the equator, but are extremely rare in areas of high latitude.[77] Most waterspouts have been known to develop during Summer periods, where warm temperatures over bodies of water cause water to evaporate, turning into warm water vapor that later develops into a cumulus cloud. When no winds are present, the updrafts created by the warming temperatures can gather and organize into what is known as a waterspout.[78] Waterspouts are not known for inflicting much damage, mainly because they are not commonly exposed to terrestrial environments. However, some waterspouts are capable of actually advancing upon terrestrial environments. Some waterspouts are known to produce wind speeds similar to those of hurricanes, and thus are capable of producing massive amounts of damage. Vegetation and weakly constructed buildings and other infrastructure can be destroyed by waterspouts. Automobiles can also be lifted when close enough to advancing waterspouts. Heavy precipitation may occur within experiencing areas as well, developed by the water molecules picked by the funnel wind currents. Waterspouts don't generally last every long upon terrestrial environments as the resisting forces of friction produced can easily dissipate such weather phenomena. Strong horizontal winds can also cause waterspouts to dissipate,[78] destroying the concentration of the updrafts. In aquatic environments, waterspouts can cause damage to coral reefs and can be a risk to any marine organisms close to the ocean surface.

Squall line

Cyclonic vortex over Pennsylvania with a trailing squall line.
Main article: Squall line

A squall line is an elongated line of severe thunderstorms that can form along and/or ahead of a cold front.[55][79] In the early 20th century, the term was used as a synonym for cold front.[80] The squall line contains heavy precipitation, hail, frequent lightning, strong straight line winds, and possibly tornadoes and waterspouts.[81] Severe weather, in form of strong straight-line winds can be expected in areas where the squall line itself is in the shape of a bow echo, within the portion of the line which bows out the most.[82] Tornadoes can be found along waves within a line echo wave pattern, or LEWP, where mesoscale low pressure areas are present.[83] Some bow echoes which develop within the summer season are known as derechos, and they move fast through large sections of territory.[84] On the back edge of the rain shield associated with mature squall lines, a wake low can form, which is a mesoscale low pressure area that forms behind the mesoscale high pressure system normally present under the rain canopy, which are sometimes associated with a heat burst.[85]

Hailstorms

Main articles: Hail and List of costly or deadly hailstorms

A hailstorm is any form of a storm that produces precipitating hailstones.[86] Hailstorms are generally capable of developing in any geographic location where thunderclouds are present; occurring mostly in dry conditions.[87] Most hailstorms develop in the presence of cumulonimbus clouds and other severe weather phenomena. The updrafts and downdrafts present within cumulonimbus clouds cause water molecules to freeze and solidify, creating hailstones and other forms of solid precipitation.[88] Due to their larger density, these hailstones become heavy enough to withstand the density of the cloud, and thus fall towards the ground. The downdrafts in cumulonimbus clouds can also cause increases in the speed of the falling hailstones. The term "hailstorm" is usually used to describe the existence of significant quantities or size of the hailstones presented in such event.

Hailstones can cause serious damage, notably to automobiles, aircraft, skylights, glass-roofed structures, livestock, and most commonly, crops.[89] Rarely, massive hailstones have been known to cause concussions or fatal head trauma. Hailstorms have been the cause of costly and deadly events throughout history. One of the earliest recorded incidents occurred around the 12th century in Wellesbourne, Britain.[90] The largest hailstone in terms of maximum circumference and length ever recorded in the United States fell in 2003 in Aurora, Nebraska, USA.[91]

Downburst

Downburst damages in a straight line. (Source NOAA)
Main article: Downburst

A downburst is created by an area of significantly rain-cooled air that, after hitting ground level, spreads out in all directions producing strong winds. Unlike winds in a tornado, winds in a downburst are directed outwards from the point where it hits land or water. "Dry downbursts" are associated with thunderstorms with very little rain,[92] while wet downbursts are created by thunderstorms with high amounts of rainfall. Microbursts and macrobursts are downbursts at very small and larger scales respectively.[93] Another variety, the heat burst, is created by vertical currents on the backside of old outflow boundaries and squall lines where rainfall is lacking. Heat bursts generate significantly higher temperatures due to the lack of rain-cooled air in their formation.[94] Downbursts create vertical wind shear or microburst which is dangerous to aviation.[95] Derecho are longer, usually more stronger, forms of downburst winds characterized by straight-lined windstorms.[96]

Polar low

A polar low is a small-scale, symmetric, short-lived atmospheric low pressure system (depression) that is found over the ocean areas poleward of the main polar front in both the Northern and Southern Hemispheres. The systems usually have a horizontal length scale of less than 1,000 kilometres (620 mi) and exist for no more than a couple of days. They are part of the larger class of mesoscale weather systems. Polar lows can be difficult to detect using conventional weather reports and are a hazard to high-latitude operations, such as shipping and gas and oil platforms. Polar lows have been referred to by many other terms, such as polar mesoscale vortex, Arctic hurricane, Arctic low, and cold air depression. Today the term is usually reserved for the more vigorous systems that have near-surface winds of at least 17 metres per second (38 mph).[97]

Lake-effect snow

Lake-effect precipitation coming off Lake Erie, as seen by NEXRAD radar, October 12–13, 2006
Main article: Lake-effect snow

Lake-effect snow is produced in the winter in the shape of one or more elongated bands when cold winds move across long expanses of warmer lake water, providing energy and picking up water vapor which freezes and is deposited on the lee shores.[98] The same effect over bodies of salt water is called ocean effect snow,[99] sea effect snow,[100] or even bay effect snow.[101] The effect is enhanced when the moving air mass is uplifted by the orographic effect of higher elevations on the downwind shores. This uplifting can produce narrow, but very intense bands of precipitation, which is deposited at a rate of many inches of snow per hour and often brings copious snowfall totals. The areas affected by lake-effect snow are called snowbelts. This effect occurs in many locations throughout the world, but is best known in the populated areas of the Great Lakes of North America.[102]

If the air temperature is not low enough to keep the precipitation frozen, it falls as lake-effect rain. In order for lake-effect rain or snow to form, the air moving across the lake must be significantly cooler than the surface air (which is likely to be near the temperature of the water surface). Specifically, the air temperature at the altitude where the air pressure is 850 millibars (or 1.5 kilometres (0.93 mi) altitude) should be 13 °C (24 °F) lower than the temperature of the air at the surface.[102] Lake-effect occurring when the air at 850 millibars is 25 °C (45 °F) colder than the water temperature can produce thundersnow, snow showers accompanied by lightning and thunder (due to the larger amount of energy available from the increased instability).[103]

Non-tropical storms

Extratropical cyclones

Extratropical cyclones, sometimes called "mid-latitude cyclones" or "wave cyclones", are a group of cyclones defined as synoptic scale low pressure weather systems that occur in the middle latitudes of the Earth (outside the tropics) having neither tropical nor polar characteristics, and are connected with fronts and horizontal gradients in temperature and dew point otherwise known as "baroclinic zones".[104] Extratropical cyclones are the everyday phenomena which, along with anticyclones, drive the weather over much of the Earth, producing anything from cloudiness and mild showers to heavy gales and thunderstorms.

Nor'easter

Main article: Nor'easter
Satellite image of the intense nor'easter responsible for the North American blizzard of 2006. Note the hurricane-like eye at the center.

A nor'easter (also "northeaster"; see below) is a synoptic-scale storm along the East Coast of the United States and Atlantic Canada. A nor'easter is so named because the winds in a nor'easter come from the northeast, especially in the coastal areas of the Northeastern United States and Atlantic Canada. More specifically, it describes a low pressure area whose center of rotation is just off the East Coast and whose leading winds in the left forward quadrant rotate onto land from the northeast. The precipitation pattern is similar to other extratropical storms. Nor'easters also can cause coastal flooding, coastal erosion, hurricane force winds, and heavy snow. Nor'easters can occur at any time of the year but are mostly known for their presence in the winter season.[105] Nor'easters can be devastating and damaging, especially in the winter months, when most damage and deaths are cold related, as nor'easters are known for bringing extremely cold air down from the Arctic air mass. Nor'easters thrive on the converging air masses; that is, the polar cold air mass and the warmer ocean water of the Gulf Stream.[105]

European windstorms

Main article: European windstorms

European windstorms are severe local windstorms within European that develop from moving winds from the North Atlantic. These windstorms are commonly associated with the destructive extratropical cyclones and their low pressure frontal systems.[106] European windstorms occur mainly within the seasons of autumn and winter. Severe European windstorms are often characterized by heavy and large amount of precipitation and their strong adiabatic process, where the systems does not relate with its nearby surroundings despite temperature differences.[107] Types of European windstorms includes the foehn, which is developed from dry and warm winds that descend downwards from the lee side of the Alps.[108]

Winter weather

Main articles: Snow storm and Blizzard
Damage caused by Lake Storm "Aphid" in October 2006

When heavy, wet snow with a snow-water equivalent (SWE) ratio of between 6:1 and 12:1 and a weight in excess of 10 pounds per square foot (~50 kg/m2)[109] piles onto trees or electricity lines, significant damage may occur on a scale usually associated with hurricanes.[110] An avalanche can occur upon a sudden thermal or mechanical impact upon snow that has accumulated on a mountain, which causes the snow to rush downhill en masse. Preceding an avalanche is a phenomenon known as an avalanche wind caused by the approaching avalanche itself, which adds to its destructive potential.[111] Large amounts of snow which accumulate on top of man-made structures can lead to structural failure.[112] During snowmelt, acidic precipitation which previously fell into the snow pack is released, which harms marine life.[113]

Blizzards are forms of winter storms. Situations that occur within blizzards often include high quantities of blowing snow and strong winds. These situations alone can reduce visibility by drastic amounts, making it extremely difficult to see. This situation can result in exposure to extended periods of time within blizzard and increase the chance of being lost. The strong winds being enforced within blizzards are able to create wind chills that would decrease the amount of heat presented within your bodies, making temperatures within blizzard feel a lot colder than reality. The effects of wind chills can result in frostbites and hypothermia. The process of frostbites can often cause the lost of certain body parts, such as fingers and toes upon limbs. Hypothermia would result in dramatic decreases in body temperature, which might cause uncontrollable actions of shivering, reductions in mental capability, and perhaps decreases in blood circulation rate. The winds presented in blizzards is also capable of damaging plants grown in areas of blizzards occurrences. Strong winds can also cause problems in urban environments, where power outrage may occur and pipes become frozen, cutting off fuel sources[114]

Ice storms

Main article: Ice storms

An ice storm is a windstorm developed by the presence of freezing liquid precipitation and the gradual increase in damaging ice.[115]

Trees destroyed and fallen by the freezing precipitation of an ice storm.

The accumulations of ice during the occurrence of an ice storm can be extremely destructive. Trees and vegetations can be destroyed within areas of ice storms, which in turn can bring down nearby power lines, causing the lost of heat and the access of communication.[116] Roofs of buildings and automobiles may be severely damaged. Gas pipes can become frozen or even damaged, causing possible gas leaks. High winds occurring within areas can influence direction of waves, which may cause tidal waves to occur and increase possibility of beach erosion. Possible avalanches may develop as well, due to the extra amount of ice being presented. Visibility can be reduce dramatically that can result in extended periods of time within experiencing areas. This is often then associated with the possibility of being presented with hypothermia and frostbites, resulting in the lose of limbs and damages inflicted on organs. The aftermath of such destructive ice storm can result in possibility of severe flooding. Sudden thawing in the ice upon surfaces can cause the high quantities of water to be displaced, especially when close towards lakes, rivers, and bodies of water.[117]

Ice storms are occasionally replaced with the usage of the term Silver storm, referring to the color of the freezing precipitation.[115] Ice storms occur throughout the winter season, at areas located on or above mid-latitude.

Extreme temperatures

Heat waves

Main article: Heat waves

Heat waves are long periods of abnormally high temperatures. There is generally no universal definition of a heatwave because of the variation within temperatures are different upon geographic locations.[96] Along with the excessive amount of heat, heatwaves are often also accompanied with high amount of humidity. These two characteristics of a heatwave are often associated with each other, increasing the apparent temperature, in this case the heat index, to dangerous levels.

Heat waves can often have severe effects upon the landscape, causing famine, destruction of vegetation, and possible deaths to livestock and wildlife.

Because heatwaves are not visible as other forms of severe weather are, like hurricanes, tornadoes, and thunderstorms, they are one of the less known severe weather.[118] These severe weather phenomena can cause destructive damage to populations. Crops and vegetations can be greatly affected by heatwaves due to the lack of water sources. The dried soil can increase the chances of erosion, decreasing lands available for agriculture. Outbreaks of wildfires can increase in frequency, being that the dry vegetations have higher chance of igniting. The evaporation of bodies of water can be devastating to marine populations, decreasing the size of the habitats available as well as amount of nutrition presented within these waters. Livestock and other terrestrial populations may decline as well. Human populations can be exposed to risk of hyperthermia, resulting in rapid increase in body temperature. Heat cramps, heat expansion, heat stroke, and dehydration can also be inflict upon human populations.[119] Power outages can also occur within areas experiencing heatwaves. Association with the urban heat island effect can increase temperatures even more, particularly overnight.[120]

Heat waves are generally capable of occurring at any geographic location. These weather phenomenon usually occur when large quantities of cool, dry air masses become stationary over an area for long periods of time. Warm, moist air from the Gulf of Mexico, the Mediterranean sea or any other large bodies of water then become presented underneath the stationary cool air mass. The cool air masses causes process of inversion to occur, which is unsuitable for clouds to develop. Without the presence of clouds, more severe radiation can be exposed upon an area, causing temperatures to increase at rapid rates.[121] With the presence of Global Warming, heatwaves will be most likely to increase in occurrences.

Cold waves

Main article: Cold wave

A cold wave is a weather phenomenon that is distinguished by a cooling of the air. Specifically, as used by the U.S. National Weather Service, a cold wave is a rapid fall in temperature within a 24 hour period requiring substantially increased protection to agriculture, industry, commerce, and social activities. The precise criterion for a cold wave is determined by the rate at which the temperature falls, and the minimum to which it falls. This minimum temperature is dependent on the geographical region and time of year.[122] Cold waves generally are capable of occurring any geological location and are formed by large cool air masses that accumulate over certain regions, caused by movements of air streams.[96]

A cold wave can cause death and injury to livestock and wildlife. Exposure to cold mandates greater caloric intake for all animals, including humans, and if a cold wave is accompanied by heavy and persistent snow, grazing animals may be unable to reach necessary food and water, and die of hypothermia or starvation. They often necessitate the purchase of foodstuffs at considerable cost to farmers to feed livestock.[96] Human population can be inflicted with possibility of frostbites when exposed to extended periods of time within the duration of a cold waves, resulting in the lose of limbs and damages to internal organs.

Extreme winter cold often causes poorly insulated water pipelines and mains to freeze. Even some poorly-protected indoor plumbing ruptures as water expands within them, causing much damage to property and costly insurance claims. Fires, paradoxically, become even more of a hazard during extreme cold. Water mains may break and water supplies may become unreliable, making firefighting more difficult.[96]

Cold waves that bring unexpected freezes and frosts during the growing season in mid-latitude zones can kill plants during the early and most vulnerable stages of growth, resulting in crop failure as plants are killed before they can be harvested economically. Such cold waves have caused famines. Cold waves can also cause soil particle to harden and freeze, making it harder for plants and vegetation to grow with these areas. At times as deadly to plants as drought, cold waves can leave a land in danger of later brush and forest fires that consume dead biomass. One extreme was the so-called Year Without a Summer of 1816, one of several years during the 1810s in which numerous crops failed during freakish summer cold snaps after volcanic eruptions that reduced incoming sunlight, and so reduce the amount of heat exposed upon the Earth surface.

See also

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