In meteorology, a cyclone is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above. Cyclones are characterized by inward-spiraling winds that rotate about a zone of low pressure. The largest low-pressure systems are polar vortices and extratropical cyclones of the largest scale. Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie within the synoptic scale. Mesocyclones, tornadoes, and dust devils lie within the smaller mesoscale. Upper level cyclones can exist without the presence of a surface low, and can pinch off from the base of the tropical upper tropospheric trough during the summer months in the Northern Hemisphere. Cyclones have also been seen on extraterrestrial planets, such as Mars, Jupiter, and Neptune. Cyclogenesis is the process of cyclone formation and intensification. Extratropical cyclones begin as waves in large regions of enhanced mid-latitude temperature contrasts called baroclinic zones. These zones contract and form weather fronts as the cyclonic circulation closes and intensifies. Later in their life cycle, extratropical cyclones occlude as cold air masses undercut the warmer air and become cold core systems. A cyclone's track is guided over the course of its 2 to 6 day life cycle by the steering flow of the subtropical jet stream.
Surface weather analysis is a special type of weather map that provides a view of weather elements over a geographical area at a specified time based on information from ground-based weather stations.
Wind shear, sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal wind shear. Vertical wind shear is a change in wind speed or direction with a change in altitude. Horizontal wind shear is a change in wind speed with a change in lateral position for a given altitude.
A squall line, or more accurately a quasi-linear convective system (QLCS), is a line of thunderstorms, often forming along or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front. Linear thunderstorm structures often contain heavy precipitation, hail, frequent lightning, strong straight-line winds, and occasionally tornadoes or waterspouts. Particularly strong straight-line winds can occur where the linear structure forms into the shape of a bow echo. Tornadoes can occur along waves within a line echo wave pattern (LEWP), where mesoscale low-pressure areas are present. Some bow echoes can grow to become derechos as they move swiftly across a large area. On the back edge of the rainband associated with mature squall lines, a wake low can be present, on very rare occasions associated with a heat burst.
In meteorology, a low-pressure area, low area or low is a region where the atmospheric pressure is lower than that of surrounding locations. Low-pressure areas are commonly associated with inclement weather, while high-pressure areas are associated with lighter winds and clear skies. Winds circle anti-clockwise around lows in the northern hemisphere, and clockwise in the southern hemisphere, due to opposing Coriolis forces. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the atmosphere (aloft). The formation process of a low-pressure area is known as cyclogenesis. In meteorology, atmospheric divergence aloft occurs in two kinds of places:
Typhoon Tip, known in the Philippines as Typhoon Warling, was the largest and most intense tropical cyclone ever recorded. The forty-third tropical depression, nineteenth tropical storm, twelfth typhoon, and third super typhoon of the 1979 Pacific typhoon season, Tip developed out of a disturbance within the monsoon trough on October 4 near Pohnpei in Micronesia. Initially, Tropical Storm Roger to the northwest hindered the development and motion of Tip, though after the storm tracked farther north, Tip was able to intensify. After passing Guam, Tip rapidly intensified and reached peak sustained winds of 305 km/h (190 mph) and a worldwide record-low sea-level pressure of 870 hPa (25.69 inHg) on October 12. At its peak intensity, Tip was the largest tropical cyclone on record, with a wind diameter of 2,220 km (1,380 mi). Tip slowly weakened as it continued west-northwestward and later turned to the northeast, in response to an approaching trough. The typhoon made landfall in southern Japan on October 19, and became an extratropical cyclone shortly thereafter. Tip's extratropical remnants continued moving east-northeastward, until they dissipated near the Aleutian Islands on October 24.
A tropical wave, in and around the Atlantic Ocean, is a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which moves from east to west across the tropics, causing areas of cloudiness and thunderstorms. Tropical waves form in the easterly flow along the equatorial side of the subtropical ridge or belt of high air pressure which lies north and south of the Intertropical Convergence Zone (ITCZ). Tropical waves are generally carried westward by the prevailing easterly winds along the tropics and subtropics near the equator. They can lead to the formation of tropical cyclones in the north Atlantic and northeastern Pacific basins. A tropical wave study is aided by Hovmöller diagrams, a graph of meteorological data.
Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere. Cyclogenesis is an umbrella term for at least three different processes, all of which result in the development of some sort of cyclone, and at any size from the microscale to the synoptic scale.
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, 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, lake-effect snow events, polar lows, and mesoscale convective complexes (MCCs), and generally forms near weather fronts. The type that forms during the warm season over land has been noted across North and South America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.
Tropical Storm Danielle was a strong tropical storm that made landfall on Virginia in the 1992 Atlantic hurricane season. The fourth storm of the season, Danielle was one of two tropical cyclones in the year to make landfall in the United States, the other being Hurricane Andrew. It formed out of a stationary trough of low pressure on September 18 near the coast of North Carolina. The system quickly reached tropical storm status, and Danielle looped to the west due to a change in steering currents. Tropical Storm Danielle reached a peak of 65 mph (105 km/h) winds before weakening and hitting the Delmarva Peninsula. The storm quickly dissipated over land.
Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of producing anything from cloudiness and mild showers to severe gales, thunderstorms, blizzards, and tornadoes. These types of cyclones are defined as large scale (synoptic) low pressure weather systems that occur in the middle latitudes of the Earth. In contrast with tropical cyclones, extratropical cyclones produce rapid changes in temperature and dew point along broad lines, called weather fronts, about the center of the cyclone.
Severe Tropical Cyclone Justin was a tropical cyclone of the 1996–97 Australian region cyclone season that caused seven deaths and had a major economic impact in northern Queensland, Australia. It had a long 3+1⁄2-week life from 6 March to 24 March 1997. Peaking as a Category 3 cyclone and making landfall as a Category 2, it caused significant damage in the Cairns region, which it approached on two occasions. Houses were undermined by huge waves, a marina and boats were severely damaged, roads and bridges suffered from flood and landslide damage and huge losses were inflicted on sugar cane, fruit and vegetable crops. The death toll in Queensland was seven including five on a yacht which sank. There were 26 who died in Papua New Guinea, which was also severely affected. Total estimated costs from damages in Australia were $190 million.
A mesovortex is a small-scale rotational feature found in a convective storm, such as a quasi-linear convective system, a supercell, or the eyewall of a tropical cyclone. Mesovortices range in diameter from tens of miles to a mile or less and can be immensely intense.
The Braer Storm was the most intense extratropical cyclone ever recorded over the northern Atlantic Ocean. Developing as a weak frontal wave on 8 January 1993, the system moved rapidly northeast. The combination of the absorption of a second low-pressure area to its southeast, a stronger than normal sea surface temperature differential along its path, and the presence of a strong jet stream aloft led to a rapid strengthening of the storm, with its central pressure falling to an estimated 914 hPa on 10 January. Its strength was well predicted by forecasters in the United Kingdom, and warnings were issued before the low initially developed.
The following is a glossary of tropical cyclone terms.
Hurricane Bawbag, also known as Cyclone Friedhelm was an intense extratropical cyclone which brought hurricane-force winds to Scotland at the beginning of December 2011. The storm also brought prolonged gales and rough seas to the rest of the British Isles, as well as parts of Scandinavia. On 8 December, winds reached up to 165 mph (266 km/h) at elevated areas, with sustained wind speeds of up to 80 mph (130 km/h) reported across populous areas. The winds uprooted trees and resulted in the closure of many roads, bridges, schools and businesses. Overall, the storm was the worst to affect Scotland in 10 years, though a stronger storm occurred less than a month afterwards, on 3 January 2012. Although the follow-up storm was more intense, the winter of 2011–12 is usually remembered for Bawbag among Scots.
Pacific Northwest windstorms, sometimes colloquially known as Big Blows, are extratropical cyclones which form in the Pacific basin, and affect land areas in the Pacific Northwest of the United States and British Columbia, Canada. They form as cyclonic windstorms associated with areas of low atmospheric pressure that track across the North Pacific Ocean towards western North America. Deep low pressure areas are relatively common over the North Pacific. They are most common in the winter months. On average, the month when most windstorms form is November or December.
The 1960 North Indian Ocean cyclone season featured two deadly tropical cyclones that killed approximately 20,000 people collectively in East Pakistan. The Indian subcontinent divides the North Indian Ocean into two areas: the Bay of Bengal to the east and the Arabian Sea to the west. The official Regional Specialized Meteorological Centre in this basin is the India Meteorological Department (IMD), while the Joint Typhoon Warning Center releases unofficial advisories. On average, five storms form in the North Indian Ocean every season with dual peaks in activity during May and November. Cyclones that occurred between 45°E and 100°E were included in seasonal records by the IMD.
The following is a glossary of tornado terms. It includes scientific as well as selected informal terminology.
This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.
