Wind shear, sometimes referred to as wind gradient, is a difference in wind speed or direction over a short distance in the atmosphere. Atmospheric wind shear is described as either vertical or horizontal wind shear. Vertical wind shear is a change in wind direction with change in altitude. Horizontal wind shear is a change in wind speed with change in lateral position for a given altitude. Wind shear is a microscale meteorological phenomenon occurring over a small distance, but it can be associated with mesoscale or synoptic scale weather features such as squall lines and cold fronts, it is observed near microbursts and downbursts caused by thunderstorms, areas of locally higher low-level winds referred to as low level jets, near mountains, radiation inversions that occur due to clear skies and calm winds, wind turbines, sailboats. Wind shear has significant effects on control of an aircraft, it has been a sole or contributing cause of many aircraft accidents. Wind shear is sometimes experienced by pedestrians at ground level when walking across a plaza towards a tower block and encountering a strong wind stream, flowing around the base of the tower.
Sound movement through the atmosphere is affected by wind shear, which can bend the wave front, causing sounds to be heard where they would not, or vice versa. Strong vertical wind shear within the troposphere inhibits tropical cyclone development, but helps to organize individual thunderstorms into longer life cycles which can produce severe weather; the thermal wind concept explains how differences in wind speed at different heights are dependent on horizontal temperature differences, explains the existence of the jet stream. Wind shear refers to the variation of wind over vertical distances. Airplane pilots regard significant wind shear to be a horizontal change in airspeed of 30 knots for light aircraft, near 45 knots for airliners at flight altitude. Vertical speed changes greater than 4.9 knots qualify as significant wind shear for aircraft. Low level wind shear can affect aircraft airspeed during take off and landing in disastrous ways, airliner pilots are trained to avoid all microburst wind shear.
The rationale for this additional caution includes: microburst intensity can double in a minute or less, the winds can shift to excessive cross wind, 40–50 knots is the threshold for survivability at some stages of low-altitude operations, several of the historical wind shear accidents involved 35–45 knots microbursts. Wind shear is a key factor in the creation of severe thunderstorms; the additional hazard of turbulence is associated with wind shear. Weather situations where shear is observed include: Weather fronts. Significant shear is observed when the temperature difference across the front is 5 °C or more, the front moves at 30 knots or faster; because fronts are three-dimensional phenomena, frontal shear can be observed at any altitude between surface and tropopause, therefore be seen both horizontally and vertically. Vertical wind shear above warm fronts is more of an aviation concern than near and behind cold fronts due to their greater duration. Upper-level jet streams. Associated with upper level jet streams is a phenomenon known as clear air turbulence, caused by vertical and horizontal wind shear connected to the wind gradient at the edge of the jet streams.
The CAT is strongest on the anticyclonic shear side of the jet next to or just below the axis of the jet. Low-level jet streams; when a nocturnal low-level jet forms overnight above the Earth's surface ahead of a cold front, significant low level vertical wind shear can develop near the lower portion of the low level jet. This is known as nonconvective wind shear since it is not due to nearby thunderstorms. Mountains; when winds blow over a mountain, vertical shear is observed on the lee side. If the flow is strong enough, turbulent eddies known as "rotors" associated with lee waves may form, which are dangerous to ascending and descending aircraft. Inversions; when on a clear and calm night, a radiation inversion is formed near the ground, the friction does not affect wind above the top of the inversion layer. The change in wind can be 40 knots in speed. A nocturnal low level jet can sometimes be observed, it tends to be strongest towards sunrise. Density differences cause additional problems to aviation.
Downbursts. When an outflow boundary forms due to a shallow layer of rain-cooled air spreading out near ground level from the parent thunderstorm, both speed and directional wind shear can result at the leading edge of the three dimensional boundary; the stronger the outflow boundary is, the stronger the resultant vertical wind shear will become. Weather fronts are boundaries between two masses of air of different densities, or different temperature and moisture properties, which are convergence zones in the wind field and are the principal cause of significant weather. Within surface weather analyses, they are depicted using various colored symbols; the air masses differ in temperature and may differ in humidity. Wind shear in the horizontal occurs near these boundaries. Cold fronts feature narrow bands of thunderstorms and severe weather, may be preceded by squall lines and dry lines. Cold fronts are sharper surface boundaries with more significant horizontal wind shear than warm fronts; when a front becomes stationary, it can degenerate into a line which separates regions of differing wind speed, known as a shear line, though the wind direction across the fron
Hurricane Wilma was the most intense tropical cyclone recorded in the Atlantic basin, the second-most intense tropical cyclone recorded in the Western Hemisphere, after Hurricane Patricia in 2015. Part of the record-breaking 2005 Atlantic hurricane season, which included three of the ten most intense Atlantic hurricanes Wilma was the twenty-second storm, thirteenth hurricane, sixth major hurricane, fourth Category 5 hurricane, the second-most destructive hurricane of the 2005 season. A tropical depression formed in the Caribbean Sea near Jamaica on October 15, headed westward, intensified into a tropical storm two days which abruptly turned southward and was named Wilma. Wilma continued to strengthen, became a hurricane on October 18. Shortly thereafter, explosive intensification occurred, in only 24 hours, Wilma became a Category 5 hurricane with wind speeds of 185 mph. Wilma's intensity leveled off after becoming a Category 5 hurricane, winds had decreased to 150 mph before it reached the Yucatán Peninsula on October 20 and 21.
After crossing the Yucatán, Wilma emerged into the Gulf of Mexico as a Category 2 hurricane. As it began accelerating to the northeast, gradual re-intensification occurred, the hurricane was upgraded to Category 3 status on October 24. Shortly thereafter, Wilma made landfall in Florida with winds of 120 mph; as Wilma was crossing Florida, it weakened back to a Category 2 hurricane, but again re-intensified as it reached the Atlantic Ocean. The hurricane intensified into a Category 3 hurricane for the last time, before weakening while accelerating northeastward. By October 26, Wilma transitioned into an extratropical cyclone southeast of Nova Scotia. Wilma made several landfalls, with the most destructive effects felt in the Yucatán Peninsula of Mexico and the U. S. state of Florida. At least 62 deaths were reported and damage totaled to $27.4 billion, of which $19 billion occurred in the United States. After Wilma, no other major hurricane made landfall in the contiguous United States until Hurricane Harvey made landfall in southern Texas on August 26, 2017, ending a record period of 11 years 10 months.
During this time, major Atlantic hurricanes occurred more than average. After Wilma, no hurricane struck the state of Florida until Hurricane Hermine did so nearly 11 years in 2016, no major hurricane struck Florida until Hurricane Irma made landfall in early September 2017. During mid-October 2005, a large area of disturbed weather developed across much of the Caribbean Sea, as a lower-tropospheric low interacted with a broad area of disturbed weather, aided by an upper-level low across the region. A broad area of low pressure developed on October 13 to the southeast of Jamaica, became more concentrated as upper-level wind shear decreased. Dvorak classifications began on October 14, by late October 15 the surface circulation in the system became well enough defined, with sufficiently organized deep convection, for the National Hurricane Center to designate the system as Tropical Depression Twenty-Four while located about 220 mi east-southeast of Grand Cayman; the depression drifted southwestward because of the influence of two ridges to its north and with warm sea surface temperatures and a favorable upper-level environment, it strengthened into Tropical Storm Wilma on October 17.
Development was slow, due to the large size of the storm and a flat pressure gradient. However, convection organized, from October 18 through October 19, Wilma underwent explosive deepening over the open waters of the Caribbean Sea. Around 12:00 UTC on October 18, the system intensified into a hurricane. In a 30‑hour period, the pressure dropped from 982 mbar to the record-low of 882 mbar, while the winds increased to 185 mph. During its intensification on October 19, the hurricane's eye shrank to as small as 2.3 mi in diameter, becoming the smallest eye seen in a tropical cyclone. On October 20, Wilma weakened below Category 5 intensity due to an eyewall replacement cycle, began to turn towards the northwest, further slowed its movement. Late on October 21, Wilma made landfall on the island of Cozumel, Quintana Roo, at around 21:45 UTC with maximum sustained winds of 150 mph and again made a second landfall on the Mexican mainland six hours and only weaker. Wilma continued to drift towards the north over the Yucatán Peninsula, although it weakened to a moderate hurricane while over land, it reemerged over the southern Gulf of Mexico on October 23 around 00:00 UTC.
Despite Wilma spending 24 hours over land, it reemerged with little intensity lost, began to re-intensify shortly after. This was due to its large size and because the majority of its circulation remained over the warm waters of the northwest Caribbean and Gulf of Mexico. A powerful trough accelerated its forward motion, its large eye remained well-organized, Wilma intensified, despite increasing amounts of wind shear producing winds of 125 mph, before making landfall on Cape Romano, Florida, as a 120 mph major hurricane. Wilma crossed the state in about 4.6 hours and weakened to a Category 2 hurricane, with winds of 110 mph, after entering the Atlantic Ocean near Jupiter, Florida. Key West flooded homes; the Lower Keys experienced an unusual flood: it occurred twice. First, as the storm approached Florida, it pushed water across the keys from south to north. A
Puerto Rico the Commonwealth of Puerto Rico and called Porto Rico, is an unincorporated territory of the United States located in the northeast Caribbean Sea 1,000 miles southeast of Miami, Florida. An archipelago among the Greater Antilles, Puerto Rico includes the eponymous main island and several smaller islands, such as Mona and Vieques; the capital and most populous city is San Juan. The territory's total population is 3.4 million. Spanish and English are the official languages. Populated by the indigenous Taíno people, Puerto Rico was colonized by Spain following the arrival of Christopher Columbus in 1493, it was contested by French and British, but remained a Spanish possession for the next four centuries. The island's cultural and demographic landscapes were shaped by the displacement and assimilation of the native population, the forced migration of African slaves, settlement from the Canary Islands and Andalusia. In the Spanish Empire, Puerto Rico played a secondary but strategic role compared to wealthier colonies like Peru and New Spain.
Spain's distant administrative control continued up to the end of the 19th century, producing a distinctive creole Hispanic culture and language that combined indigenous and European elements. In 1898, following the Spanish–American War, the United States acquired Puerto Rico under the terms of the Treaty of Paris. Puerto Ricans have been citizens of the United States since 1917, enjoy freedom of movement between the island and the mainland; as it is not a state, Puerto Rico does not have a vote in the United States Congress, which governs the territory with full jurisdiction under the Puerto Rico Federal Relations Act of 1950. However, Puerto Rico does have one non-voting member of the House called a Resident Commissioner; as residents of a U. S. territory, American citizens in Puerto Rico are disenfranchised at the national level and do not vote for president and vice president of the United States, nor pay federal income tax on Puerto Rican income. Like other territories and the District of Columbia, Puerto Rico does not have U.
S. senators. Congress approved a local constitution in 1952, allowing U. S. citizens on the territory to elect a governor. Puerto Rico's future political status has been a matter of significant debate. In early 2017, the Puerto Rican government-debt crisis posed serious problems for the government; the outstanding bond debt had climbed to $70 billion at a time with 12.4% unemployment. The debt had been increasing during a decade long recession; this was the second major financial crisis to affect the island after the Great Depression when the U. S. government, in 1935, provided relief efforts through the Puerto Rico Reconstruction Administration. On May 3, 2017, Puerto Rico's financial oversight board in the U. S. District Court for Puerto Rico filed the debt restructuring petition, made under Title III of PROMESA. By early August 2017, the debt was $72 billion with a 45% poverty rate. In late September 2017, Hurricane Maria made landfall in Puerto Rico; the island's electrical grid was destroyed, with repairs expected to take months to complete, provoking the largest power outage in American history.
Recovery efforts were somewhat slow in the first few months, over 200,000 residents had moved to the mainland State of Florida alone by late November 2017. Puerto Rico is Spanish for "rich port". Puerto Ricans call the island Borinquén – a derivation of Borikén, its indigenous Taíno name, which means "Land of the Valiant Lord"; the terms boricua and borincano derive from Borikén and Borinquen and are used to identify someone of Puerto Rican heritage. The island is popularly known in Spanish as la isla del encanto, meaning "the island of enchantment". Columbus named the island San Juan Bautista, in honor of Saint John the Baptist, while the capital city was named Ciudad de Puerto Rico. Traders and other maritime visitors came to refer to the entire island as Puerto Rico, while San Juan became the name used for the main trading/shipping port and the capital city; the island's name was changed to "Porto Rico" by the United States after the Treaty of Paris of 1898. The anglicized name was used by the U.
S. government and private enterprises. The name was changed back to Puerto Rico by a joint resolution in Congress introduced by Félix Córdova Dávila in 1931; the official name of the entity in Spanish is Estado Libre Asociado de Puerto Rico, while its official English name is Commonwealth of Puerto Rico. The ancient history of the archipelago, now Puerto Rico is not well known. Unlike other indigenous cultures in the New World which left behind abundant archeological and physical evidence of their societies, scant artifacts and evidence remain of the Puerto Rico's indigenous population. Scarce archaeological findings and early Spanish accounts from the colonial era constitute all, known about them; the first comprehensive book on the history of Puerto Rico was written by Fray Íñigo Abbad y Lasierra in 1786, nearly three centuries after the first Spaniards landed on the island. The first known settlers were the Ortoiroid people, an Archaic Period culture of Amerindian hunters and fishermen who migrated from the South American mainland.
Some scholars suggest their settlement dates back about 4,000 years. An archeological dig in 1990 on the island of Vieques found the remains of a man, designated as the "Puerto Ferro Man", dated to around 2000 BC; the Ortoiroid were displaced
The Bahamas, known as the Commonwealth of The Bahamas, is a country within the Lucayan Archipelago. The archipelagic state consists of more than 700 islands and islets in the Atlantic Ocean, is located north of Cuba and Hispaniola, northwest of the Turks and Caicos Islands, southeast of the U. S. state of Florida, east of the Florida Keys. The capital is Nassau on the island of New Providence; the designation of "the Bahamas" can refer either to the country or to the larger island chain that it shares with the Turks and Caicos Islands. The Royal Bahamas Defence Force describes the Bahamas territory as encompassing 470,000 km2 of ocean space; the Bahamas is the site of Columbus's first landfall in the New World in 1492. At that time, the islands were inhabited by the Lucayans, a branch of the Arawakan-speaking Taíno people. Although the Spanish never colonised the Bahamas, they shipped the native Lucayans to slavery in Hispaniola; the islands were deserted from 1513 until 1648, when English colonists from Bermuda settled on the island of Eleuthera.
The Bahamas became a British crown colony in 1718. After the American Revolutionary War, the Crown resettled thousands of American Loyalists in the Bahamas. Africans constituted the majority of the population from this period; the slave trade was abolished by the British in 1807. Subsequently, the Bahamas became a haven for freed African slaves. Today, Afro-Bahamians make up nearly 90% of the population; the Bahamas became an independent Commonwealth realm in 1973 with Elizabeth II as its queen. In terms of gross domestic product per capita, the Bahamas is one of the richest countries in the Americas, with an economy based on tourism and finance; the name Bahamas is most derived from either the Taíno ba ha ma, a term for the region used by the indigenous Native Americans, or from the Spanish baja mar reflecting the shallow waters of the area. Alternatively, it may originate from a local name of unclear meaning; the word The constitutes an integral part of the short form of the name and is, capitalised.
So in contrast to "the Congo" and "the United Kingdom", it is proper to write "The Bahamas." The name The Bahamas is thus comparable with certain non-English names that use the definite article, such as Las Vegas or Los Angeles. The Constitution of the Commonwealth of The Bahamas, the country's fundamental law, capitalizes the "T" in "The Bahamas." Taino people moved into the uninhabited southern Bahamas from Hispaniola and Cuba around the 11th century, having migrated there from South America. They came to be known as the Lucayan people. An estimated 30,000 Lucayans inhabited the Bahamas at the time of Christopher Columbus's arrival in 1492. Columbus's first landfall in the New World was on an island; some researchers believe this site to be present-day San Salvador Island, situated in the southeastern Bahamas. An alternative theory holds that Columbus landed to the southeast on Samana Cay, according to calculations made in 1986 by National Geographic writer and editor Joseph Judge, based on Columbus's log.
Evidence in support of this remains inconclusive. On the landfall island, Columbus exchanged goods with them; the Spanish forced much of the Lucayan population to Hispaniola for use as forced labour. The slaves suffered from harsh conditions and most died from contracting diseases to which they had no immunity; the population of the Bahamas was diminished. In 1648, the Eleutherian Adventurers, led by William Sayle, migrated from Bermuda; these English Puritans established the first permanent European settlement on an island which they named Eleuthera—the name derives from the Greek word for freedom. They settled New Providence, naming it Sayle's Island after one of their leaders. To survive, the settlers salvaged goods from wrecks. In 1670, King Charles II granted the islands to the Lords Proprietors of the Carolinas in North America, they rented the islands from the king with rights of trading, appointing governors, administering the country. In 1684 Spanish corsair Juan de Alcon raided Charles Town.
In 1703, a joint Franco-Spanish expedition occupied the Bahamian capital during the War of the Spanish Succession. During proprietary rule, the Bahamas became a haven for pirates, including Blackbeard. To put an end to the'Pirates' republic' and restore orderly government, Great Britain made the Bahamas a crown colony in 1718 under the royal governorship of Woodes Rogers. After a difficult struggle, he succeeded in suppressing piracy. In 1720, Rogers led local militia to drive off a Spanish attack. During the US War of Independence in the late 18th century, the islands became a target for US naval forces under the command of Commodore Esek Hopkins. US Marines occupied the capital of Nassau for 2 weeks. In 1782, following the British defeat at Yorktown, a Spanish fleet appeared off the coast of Nassau; the city surrendered without a fight. Spain returned possession of the Bahamas to Great Britain the following year, u
Port-au-Prince is the capital and most populous city of Haiti. The city's population was estimated at 987,310 in 2015 with the metropolitan area estimated at a population of 2,618,894; the metropolitan area is defined by the IHSI as including the communes of Port-au-Prince, Cite Soleil, Carrefour, Pétion-Ville. The city of Port-au-Prince is on the Gulf of Gonâve: the bay on which the city lies, which acts as a natural harbor, has sustained economic activity since the civilizations of the Arawaks, it was first incorporated under French colonial rule in 1749. The city's layout is similar to that of an amphitheatre, its population is difficult to ascertain due to the rapid growth of slums in the hillsides above the city. The city was catastrophically affected by a devastating earthquake in 2010, with large numbers of structures damaged or destroyed. Haiti's government estimated the death toll to be 230,000, it is said that a captain named de Saint-André named the area in 1706, after he sailed into the bay in a ship named Le Prince, hence Port-au-Prince to mean, "Port of the Prince."
However, the port and the surrounding region continued to be known as Hôpital, but the islets in the bay had been known as Les îlets du Prince as early as 1680. French colonial commissioner Étienne Polverel named the city Port-Républicain on 23 September 1793 "in order that the inhabitants be kept continually in mind of the obligations which the French Revolution imposed on them." It was renamed back to Port-au-Prince by Jacques I, Emperor of Haiti. When Haiti was divided between a kingdom in the north and a republic in the south, Port-au-Prince was the capital of the republic, under the leadership of Alexandre Pétion. Henri Christophe renamed the city Port-aux-Crimes after the assassination of Jacques I at Pont Larnage. Prior to the arrival of Christopher Columbus, the island of Hispaniola was inhabited by people known as the Taíno, who arrived in 2600 BC in large dugout canoes, they are believed to come from what is now eastern Venezuela. By the time Columbus arrived in 1492 AD, the region was under the control of Bohechio, Taíno cacique Xaragua.
He, like his predecessors, feared settling too close to the coast. Instead, the region served as a hunting ground; the population of the region was 400,000 at the time, but the Taínos were gone within 30 years of the arrival of the Spaniards. With the arrival of the Spaniards, the Amerindians were forced to accept a protectorate, Bohechio, childless at death, was succeeded by his sister, wife of the cacique Caonabo; the Spanish insisted on larger tributes. The Spanish colonial administration decided to rule directly, in 1503, Nicolas Ovando governor, set about to put an end to the régime headed by Anacaona, he invited her and other tribal leaders to a feast, when the Amerindians had drunk a good deal of wine, he ordered most of the guests killed. Anacaona was spared. Through violence and murders, the Spanish settlers decimated the native population. Direct Spanish rule over the area having been established, Ovando founded a settlement not far from the coast named Santa Maria de la Paz Verdadera, which would be abandoned several years later.
Not long thereafter, Ovando founded Santa Maria del Puerto. The latter was first burned by French explorers in 1535 again in 1592 by the English; these assaults proved to be too much for the Spanish colonial administration, in 1606, it decided to abandon the region. For more than 50 years, the area, today Port-au-Prince saw its population drop off drastically, when some buccaneers began to use it as a base, Dutch merchants began to frequent it in search of leather, as game was abundant there. Around 1650, French flibustiers, running out of room on the Île de la Tortue began to arrive on the coast, established a colony at Trou-Borded; as the colony grew, they set up a hospital not far from the coast, on the Turgeau heights. This led to the region being known as Hôpital. Although there had been no real Spanish presence in Hôpital for well over 50 years, Spain retained its formal claim to the territory, the growing presence of the French flibustiers on ostensibly Spanish lands provoked the Spanish crown to dispatch Castilian soldiers to Hôpital to retake it.
The mission proved to be a disaster for the Spanish, as they were outnumbered and outgunned, in 1697, the Spanish government signed the Treaty of Ryswick, renouncing any claims to Hôpital. Around this time, the French established bases at Ester and Gonaïves. Ester was a rich village, inhabited by merchants, equipped with straight streets. On the other hand, the surrounding region, Petite-Rivière, was quite poor. Following a great fire in 1711, Ester was abandoned, yet the French presence in the region continued to grow, soon afterward, a new city was founded to the south, Léogâne. While the first French presence in Hôpital, the region to contain Port-au-Prince was that of the flibustiers.
The Windward Islands known as the Islands of Barlovento, are the southern larger islands of the Lesser Antilles, within the West Indies. They lie south of the Leeward Islands between latitudes 10° and 16° N and longitudes 60° and 62° W; as a group they start from Dominica and reach southward to the north of Trinidad and Tobago and west of Barbados. The Windward Islands are called such because they were more windward to sailing ships arriving to the New World than the Leeward Islands, given that the prevailing trade winds in the West Indies blow east to west; the trans-Atlantic currents and winds that provided the fastest route across the ocean brought these ships to the rough dividing line between the Windward and Leeward islands. Dominica is the dividing line between the Leeward islands. Guadeloupe and islands to the north became known as the "Leeward Islands". Vessels in the Atlantic slave trade departing from the British Gold Coast and Gulf of Guinea in Africa would first encounter the southeasternmost "Windward" islands of the Lesser Antilles in their west-northwesterly heading to final destinations in the Caribbean and North and Central America.
The chain of Windward Islands forms a part of the easternmost boundary of the Caribbean Sea. Most of the present "Windward Islands" were once colonial island territories of France known as the French West Indies; the Windward Islands are as follows: Dominica Martinique Saint Lucia Saint Vincent and the Grenadines Grenada Trinidad and Tobago Leeward Islands Southern Caribbean Lesser Antilles topics Windward Islands topics Leeward Islands topics Windward Islands cricket team Chisholm, Hugh, ed.. "Windward Islands". Encyclopædia Britannica. 26. Cambridge University Press. P. 716
A low-pressure area, depression or cyclone is a region on the topographic map where the atmospheric pressure is lower than that of surrounding locations. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the troposphere; the formation process of a low-pressure area is known as cyclogenesis. Within the field of meteorology, atmospheric divergence aloft occurs in two areas; the first area is on the east side of upper troughs, which form half of a Rossby wave within the Westerlies. A second area of wind divergence aloft occurs ahead of embedded shortwave troughs, which are of smaller wavelength. Diverging winds aloft ahead of these troughs cause atmospheric lift within the troposphere below, which lowers surface pressures as upward motion counteracts the force of gravity. Thermal lows form due to localized heating caused by greater sunshine over deserts and other land masses. Since localized areas of warm air are less dense than their surroundings, this warmer air rises, which lowers atmospheric pressure near that portion of the Earth's surface.
Large-scale thermal lows over continents help drive monsoon circulations. Low-pressure areas can form due to organized thunderstorm activity over warm water; when this occurs over the tropics in concert with the Intertropical Convergence Zone, it is known as a monsoon trough. Monsoon troughs reach their southerly extent in February; when a convective low acquires a well-hot circulation in the tropics it is termed a tropical cyclone. Tropical cyclones can form during any month of the year globally, but can occur in either the northern or southern hemisphere during December. Atmospheric lift will generally produce cloud cover through adiabatic cooling once the air becomes saturated as it rises, although the low-pressure area brings cloudy skies, which act to minimize diurnal temperature extremes. Since clouds reflect sunlight, incoming shortwave solar radiation decreases, which causes lower temperatures during the day. At night the absorptive effect of clouds on outgoing longwave radiation, such as heat energy from the surface, allows for warmer diurnal low temperatures in all seasons.
The stronger the area of low pressure, the stronger the winds experienced in its vicinity. Globally, low-pressure systems are most located over the Tibetan Plateau and in the lee of the Rocky mountains. In Europe, recurring low-pressure weather systems are known as "depressions". Cyclogenesis is the development and strengthening of cyclonic circulations, or low-pressure areas, within the atmosphere. Cyclogenesis is the opposite of cyclolysis, has an anticyclonic equivalent which deals with the formation of high-pressure areas—anticyclogenesis. Cyclogenesis is an umbrella term for several different processes, all of which result in the development of some sort of cyclone. Meteorologists use the term "cyclone" where circular pressure systems flow in the direction of the Earth's rotation, which coincides with areas of low pressure; the largest low-pressure systems are cold-core polar cyclones and extratropical cyclones which lie on the synoptic scale. Warm-core cyclones such as tropical cyclones and polar lows lie within the smaller mesoscale.
Subtropical cyclones are of intermediate size. Cyclogenesis can occur from the microscale to the synoptic scale. Larger-scale troughs called Rossby waves, are synoptic in scale. Shortwave troughs embedded within the flow around larger scale troughs are smaller in scale, or mesoscale in nature. Both Rossby waves and shortwaves embedded within the flow around Rossby waves migrate equatorward of the polar cyclones located in both the Northern and Southern hemispheres. All share one important aspect, that of upward vertical motion within the troposphere; such upward motions decrease the mass of local atmospheric columns of air, which lowers surface pressure. Extratropical cyclones form as waves along weather fronts due to a passing by shortwave aloft or upper level jet streak before occluding in their life cycle as cold-core cyclones. Polar lows are small-scale, short-lived atmospheric low-pressure systems that occur over the ocean areas poleward of the main polar front in both the Northern and Southern Hemispheres.
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, they are vigorous systems. Tropical cyclones form due to latent heat driven by significant thunderstorm activity, are warm-core with well-defined circulations. Certain criteria need to be met for their formation. In most situations, water temperatures of at least 26.5 °C are needed down to a depth of at least 50 m. Another factor is rapid cooling with height, which allows the release of the heat of condensation that powers a tropical cyclone. High humidity is needed in the lower-to-mid troposphere. Low amounts of wind shear are needed. Lastly, a formative tropical cyclone needs a pre-existing system of disturbed weather, although without a circulation no cyclonic development will take place. Mesocyclones form as warm core cyclones over land, c