Severe weather
Severe weather is any dangerous meteorological phenomenon with the potential to cause damage, serious social disruption, or loss of life. These vary depending on the latitude, altitude, topography, and atmospheric conditions. High winds, hail, excessive precipitation, and wildfires are forms and effects, as are thunderstorms, downbursts, tornadoes, waterspouts, tropical cyclones, and extratropical cyclones. Regional and seasonal phenomena include blizzards, snowstorms, ice storms, and duststorms.
Severe weather is one type of extreme weather, which includes unexpected, unusual, severe, or unseasonal weather and is by definition rare for that location or time of the year. Due to the effects of climate change, the frequency and intensity of some of the extreme weather events are increasing, for example, heatwaves and droughts.
Terminology
Meteorologists have generally defined severe weather as any aspect of the weather that poses risks to life or property or requires the intervention of authorities. A narrower definition of severe weather is any weather phenomenon relating to severe thunderstorms.According to the World Meteorological Organization, severe weather can be categorized into two groups: general severe weather and localized severe weather. Nor'easters, European wind storms, and the phenomena that accompany them form over wide geographic areas. These occurrences are classified as general severe weather. Downbursts and tornadoes are more localized and therefore have a more limited geographic effect. These forms of weather are classified as localized severe weather.
The term severe weather is technically not the same phenomenon as extreme weather. Extreme weather describes unusual weather events that are at the extremes of the historical distribution for a given area.
Causes
Organized severe weather occurs under the same conditions that generate ordinary thunderstorms: atmospheric moisture, lift, and instability. A wide variety of conditions cause severe weather. Several factors can convert thunderstorms into severe weather. For example, a pool of cold air aloft may aid in the development of large hail from an otherwise innocuous-appearing thunderstorm. The most severe hail and tornadoes are produced by supercell thunderstorms, and the worst downbursts and derechos are produced by bow echoes. Both of these types of storms tend to form in environments with high wind shear.Floods, hurricanes, tornadoes, and thunderstorms are considered to be the most destructive weather-related natural disasters. Although these weather phenomena are all related to cumulonimbus clouds, they form and develop under different conditions and geographic locations. The relationship between these weather events and their formation requirements is used to develop models to predict the most frequent and possible locations. This information is used to notify affected areas and save lives.
Categories
Severe thunderstorms can be assessed in three different categories. These are "approaching severe", "severe", and "significantly severe".Approaching severe is defined as hail between diameter or winds between 50 and 58 mph. In the United States, such storms will usually warrant a Significant Weather Alert.
Severe is defined as hail diameter, winds, or a tornado.
Significant severe is defined as hail in diameter or larger, winds 75 mph or more, or a tornado of strength EF2 or stronger.
Both severe and significant severe events warrant a severe thunderstorm warning from the United States National Weather Service, the Environment Canada, the Australian Bureau of Meteorology, the Meteorological Service of New Zealand and the Meteorological Office UK, if the event occurs in those countries. If a tornado is occurring or is imminent, the severe thunderstorm warning will be superseded by a tornado warning in the United States and Canada.
A severe weather outbreak is typically considered to be when ten or more tornadoes, some of which will likely be long-tracked and violent, and many large hail or damaging wind reports occur within one or more consecutive days. Severity is also dependent on the size of the geographic area affected, whether it covers hundreds or thousands of square kilometers.
High winds
High winds are known to cause damage, depending upon their strength.Wind speeds as low as may lead to power outages when tree branches fall and disrupt power lines. Some species of trees are more vulnerable to winds. Trees with shallow roots are more prone to uproot, and brittle trees such as eucalyptus, sea hibiscus, and avocado are more prone to branch damage.
Wind gusts may cause poorly designed suspension bridges to sway. When wind gusts harmonize with the frequency of the swaying bridge, the bridge may fail as occurred with the Tacoma Narrows Bridge in 1940.
Hurricane-force winds, caused by individual thunderstorms, thunderstorm complexes, derechos, tornadoes, extratropical cyclones, or tropical cyclones can destroy mobile homes and structurally damage buildings with foundations. Winds of this strength due to downslope winds off terrain have been known to shatter windows and sandblast paint from cars.
Once winds exceed within strong tropical cyclones and tornadoes, homes completely collapse, and significant damage is done to larger buildings. Total destruction to man-made structures occurs when winds reach. The Saffir–Simpson scale for cyclones and Enhanced Fujita scale for tornadoes were developed to help estimate wind speed from the damage they cause.
Tornado
A dangerous rotating column of air in contact with both the surface of the earth and the base of a cumulonimbus cloud or a cumulus cloud, in rare cases. Tornadoes come in many sizes but typically form a visible condensation funnel whose narrowest end reaches the earth and surrounded by a cloud of debris and dust.Tornadoes' wind speeds generally average between and. They are approximately across and travel a few miles before dissipating. Some attain wind speeds in excess of, may stretch more than two miles across, and maintain contact with the ground for dozens of miles. The Enhanced Fujita Scale and the TORRO Scale are two examples of scales used to rate the strength, intensity and/or damage of a tornado.
Tornadoes, despite being one of the most destructive weather phenomena, are generally short-lived. A long-lived tornado generally lasts no more than an hour, but some have been known to last for 2 hours or longer. Due to their relatively short duration, less information is known about the development and formation of tornadoes.
Waterspout
Waterspouts are generally defined as tornadoes or non-supercell tornadoes that develop over bodies of water.Waterspouts typically do not do much damage because they occur over open water, but they are capable of traveling over land. Vegetation, weakly constructed buildings, and other infrastructure may be damaged or destroyed by waterspouts. Waterspouts do not generally last long over terrestrial environments as the friction produced easily dissipates the winds. Strong horizontal winds will cause waterspouts to dissipate as they disturb the vortex. While not generally as dangerous as "classic" tornadoes, waterspouts can overturn boats, and they can cause severe damage to larger ships.
Downburst and derecho
Downbursts are created within thunderstorms by significantly rain-cooled air, which, upon reaching ground level, spreads out in all directions and produce strong winds. Unlike winds in a tornado, winds in a downburst are not rotational but are directed outwards from the point where they strike land or water. "Dry downbursts" are associated with thunderstorms with very little precipitation, while wet downbursts are generated by thunderstorms with large amounts of rainfall. Microbursts are very small downbursts with winds that extend up to 2.5 miles from their source, while macrobursts are large-scale downbursts with winds that extend in excess of 2.5 miles. 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. Derechos are longer, usually stronger, forms of downburst winds characterized by straight-lined windstorms.Downbursts create vertical wind shear or microbursts, which are dangerous to aviation. These convective downbursts can produce damaging winds, lasting 5 to 30 minutes, with wind speeds as high as, and cause tornado-like damage on the ground. Downbursts also occur much more frequently than tornadoes, with ten downburst damage reports for every one tornado.
Squall line
A squall line is an elongated line of severe thunderstorms that can form along or ahead of a cold front. The squall line typically contains heavy precipitation, hail, frequent lightning, strong straight line winds, and possibly tornadoes or waterspouts. Severe weather in the form of strong straight-line winds can be expected in areas where the squall line forms a bow echo, in the farthest portion of the bow. Tornadoes can be found along waves within a line echo wave pattern where mesoscale low-pressure areas are present. Intense bow echoes responsible for widespread, extensive wind damage are called derechos, and move quickly over large territories. A wake low or a mesoscale low-pressure area forms behind the rain shield of a mature squall line and is sometimes associated with a heat burst.Squall lines often cause severe straight-line wind damage, and most non-tornadic wind damage is caused from squall lines. Although the primary danger from squall lines is straight-line winds, some squall lines also contain weak tornadoes.