Atlantic Hurricanes

An Atlantic hurricane or tropical storm is a tropical cyclone that forms in the Atlantic Ocean, primarily between the months of June and November. A hurricane differs from a cyclone or typhoon only on the basis of location. A hurricane is a storm that occurs in the Atlantic Ocean and northeastern Pacific Ocean, a typhoon occurs in the northwestern Pacific Ocean, and a cyclone occurs in the South Pacific Ocean or Indian Ocean.

Tropical cyclones can be categorized by intensity. Tropical storms have one-minute maximum sustained winds of at least 39 mph (34 knots, 17 m/s, 63 km/h), while hurricanes have one-minute maximum sustained winds exceeding 74 mph (64 knots, 33 m/s, 119 km/h).[2] Most North Atlantic tropical storms and hurricanes form between June 1 and November 30.[3] The United States National Hurricane Center monitors the basin and issues reports, watches, and warnings about tropical weather systems for the North Atlantic Basin as one of the Regional Specialized Meteorological Centers for tropical cyclones, as defined by the World Meteorological Organization.

In recent times, tropical disturbances that reach tropical storm intensity are named from a predetermined list. Hurricanes that result in significant damage or casualties may have their names retired from the list at the request of the affected nations in order to prevent confusion should a subsequent storm be given the same name. On average, in the North Atlantic basin (from 1966 to 2009) 11.3 named storms occur each season, with an average of 6.2 becoming hurricanes and 2.3 becoming major hurricanes (Category 3 or greater). The climatological peak of activity is around September 10 each season.

In March 2004, Catarina was the first hurricane-intensity tropical cyclone to be recorded in the Southern Atlantic Ocean. Since 2011, the Brazilian Navy Hydrographic Center has started to use the same scale as the North Atlantic Ocean for tropical cyclones in the South Atlantic Ocean and assign names to those which reach 35 kn (65 km/h; 40 mph)

Tropical cyclones are steered by the surrounding flow throughout the depth of the troposphere (the atmosphere from the surface to about eight miles (12 km) high). Neil Frank, former director of the United States National Hurricane Center, used the analogies such as "a leaf carried along in a stream" or a "brick moving through a river of air" to describe the way atmospheric flow affects the path of a hurricane across the ocean. Specifically, air flow around high pressure systems and toward low pressure areas influences hurricane tracks.

In the tropical latitudes, tropical storms and hurricanes generally move westward with a slight tendency toward the north, under the influence of the subtropical ridge, a high pressure system that usually extends east–west across the subtropics. South of the subtropical ridge, surface easterly winds (blowing from east to west) prevail. If the subtropical ridge is weakened by an upper trough, a tropical cyclone may turn poleward and then recurve, or curve back toward the northeast into the main belt of the Westerlies. Poleward (north) of the subtropical ridge, westerly winds prevail and generally steer tropical cyclones that reach northern latitudes toward the east. The westerlies also steer extratropical cyclones with their cold and warm fronts from west to east.

Generally speaking, the intensity of a tropical cyclone is determined by either the storm's maximum sustained winds or lowest barometric pressure. The following table lists the most intense Atlantic hurricanes in terms of their lowest barometric pressure. In terms of wind speed, Hurricane Allen (in 1980) was the strongest Atlantic tropical cyclone on record, with maximum sustained winds of 190 mph (305 km/h). However, these measurements are suspect since instrumentation used to document wind speeds at the time would likely succumb to winds of such intensity. Nonetheless, their central pressures are low enough to rank them among the strongest recorded Atlantic hurricanes.

Owing to their intensity, the strongest Atlantic hurricanes have all attained Category 5 classification. Hurricane Opal, the strongest Category 4 hurricane recorded, intensified to reach a minimum pressure of 916 mbar (hPa; 27.05 inHg), a pressure typical of Category 5 hurricanes. Nonetheless, the pressure remains too high to list Opal as one of the ten strongest Atlantic tropical cyclones. Hurricane Wilma is the strongest Atlantic hurricane recorded, after reaching an intensity of 882 mbar (hPa; 26.05 inHg) in October 2005; this also made Wilma the strongest tropical cyclone worldwide outside of the West Pacific, where seven tropical cyclones have been recorded to intensify to lower pressures. However, this was later superseded by Hurricane Patricia in 2015 in the east Pacific, which had a pressure reading of 872 mbar. Preceding Wilma is Hurricane Gilbert, which had also held the record for most intense Atlantic hurricane for 17 years. The 1935 Labor Day hurricane, with a pressure of 892 mbar (hPa; 26.34 inHg), is the third strongest Atlantic hurricane and the strongest documented tropical cyclone prior to 1950. Since the measurements taken during Wilma and Gilbert were documented using dropsonde, this pressure remains the lowest measured over land.

Hurricane Rita is the fourth strongest Atlantic hurricane in terms of barometric pressure and one of three tropical cyclones from 2005 on the list, with the others being Wilma and Katrina at first and seventh respectively. However, with a barometric pressure of 895 mbar (hPa; 26.43 inHg), Rita is the strongest tropical cyclone ever recorded in the Gulf of Mexico. Mitch and Dean share intensities for the eighth strongest Atlantic hurricane at 905 mbar (hPa; 26.73 inHg). The tenth place for most intense Atlantic tropical cyclone is Hurricane Maria listed to have deepened to a pressure as low as 908 mbar (hPa; 26.81 inHg).

Many of the strongest recorded tropical cyclones weakened prior to their eventual landfall or demise. However, three of the storms remained intense enough at landfall to be considered some of the strongest landfalling hurricanes – three of the ten hurricanes on the list constitute the three most intense Atlantic landfalls in recorded history. The 1935 Labor Day hurricane made landfall at peak intensity, making it the most intense Atlantic landfall. Though it weakened slightly before its eventual landfall on the Yucatán Peninsula, Hurricane Gilbert maintained a pressure of 900 mbar (hPa; 26.58 inHg) at landfall, as did Camille, making their landfalls tied as the second strongest. Similarly, Hurricane Dean made landfall on the peninsula, though it did so at peak intensity and with a higher barometric pressure; its landfall marked the fourth strongest in Atlantic hurricane history.

Climatology serves to characterize the general properties of an average season and can be used as one of many other tools for making forecasts. Most storms form in warm waters several hundred miles north of the equator near the Intertropical convergence zone from tropical waves. The Coriolis force is usually too weak to initiate sufficient rotation near the equator. Storms frequently form in the warm waters of the Gulf of Mexico, the Caribbean Sea, and the tropical Atlantic Ocean as far east as the Cape Verde Islands, the origin of strong and long-lasting Cape Verde-type hurricanes. Systems may also strengthen over the Gulf Stream off the coast of the eastern United States, wherever water temperatures exceed 26.5 °C (79.7 °F).

Although most storms are found within tropical latitudes, occasionally storms will form further north and east from disturbances other than tropical waves such as cold fronts and upper-level lows. These are known as baroclinically induced tropical cyclones. There is a strong correlation between Atlantic hurricane activity in the tropics and the presence of an El Niño or La Niña in the Pacific Ocean. El Niño events increase the wind shear over the Atlantic, producing a less-favorable environment for formation and decreasing tropical activity in the Atlantic basin. Conversely, La Niña causes an increase in activity due to a decrease in wind shear.

According to the Azores High hypothesis by Kam-biu Liu, an anti-phase pattern is expected to exist between the Gulf of Mexico coast and the North American Atlantic coast. During the quiescent periods (3000–1400 BC, and 1000 AD to present), a more northeasterly position of the Azores High would result in more hurricanes being steered toward the Atlantic coast. During the hyperactive period (1400 BC to 1000 AD), more hurricanes were steered towards the Gulf coast as the Azores High was shifted to a more southwesterly position near the Caribbean. Such a displacement of the Azores High is consistent with paleoclimatic evidence that shows an abrupt onset of a drier climate in Haiti around 3200 14C years BP, and a change towards more humid conditions in the Great Plains during the late-Holocene as more moisture was pumped up the Mississippi Valley through the Gulf coast. Preliminary data from the northern Atlantic coast seem to support the Azores High hypothesis. A 3000-year proxy record from a coastal lake in Cape Cod suggests that hurricane activity has increased significantly during the past 500–1000 years, just as the Gulf coast was amid a quiescent period of the last millennium.

The season in which the most tropical storms formed on record is the 2020 Atlantic hurricane season (30). However, 2005 was the one in which the most hurricanes formed on record (15).

The 2005 Atlantic hurricane season has had the most major hurricanes on record (7). The 1950 Atlantic hurricane season and 1961 Atlantic hurricane season were once thought to have 8 and 7 respectively, but re-analysis showed that several storms during both seasons were weaker than thought, and thus the record is now held by the 2005 season.

The least active season on record since 1946 (when the database is considered more reliable) was the 1983 Atlantic hurricane season, with four tropical storms, two hurricanes, and one major hurricane. Overall, the 1914 Atlantic hurricane season remains the least active, with only one documented storm.

The most intense hurricane (by barometric pressure) on record in the North Atlantic basin was Hurricane Wilma (2005) (882 mbar).

The largest hurricane (in gale diameter winds) on record to form in the North Atlantic was Hurricane Sandy with a gale diameter of 870 miles (1,400 km)

The longest-lasting hurricane was the 1899 San Ciriaco hurricane, which lasted for 27 days and 18 hours as a tropical cyclone.

The longest-tracked hurricane was Hurricane Faith, which traveled for 6,850 miles (11,020 km) as a tropical cyclone. Faith is also the northernmost moving tropical cyclone in the Atlantic basin.

The most tornadoes spawned by a hurricane was 127 by Hurricane Ivan (2004 season).

The strongest landfalling hurricane was the Labor Day Hurricane of 1935 (892 hPa).

The deadliest hurricane was the Great Hurricane of 1780 (22,000 fatalities).

The deadliest hurricane to make landfall on the continental United States was the Galveston Hurricane in 1900 which may have killed up to 12,000 people.

The most damaging hurricane was both Hurricane Katrina and Hurricane Harvey of the 2005 and 2017 seasons, respectively, both of which caused $125 billion in damages in their respective years. However, when adjusted for inflation, Katrina is the costliest with $161 billion.

The quickest forming hurricane was Hurricane Humberto in 2007. It was a minimal hurricane that formed and intensified faster than any other tropical cyclone on record before landfall. Developing on September 12, 2007, in the northwestern Gulf of Mexico, the cyclone rapidly strengthened and struck High Island, Texas, with winds of about 90 mph (150 km/h) early on September 13.

Proxy records based on paleotempestological research have revealed that major hurricane activity along the Gulf of Mexico coast varies on timescales of centuries to millennia. Few major hurricanes struck the Gulf coast during 3000–1400 BC and again during the most recent millennium. These quiescent intervals were separated by a hyperactive period during 1400 BC and 1000 AD, when the Gulf coast was struck frequently by catastrophic hurricanes and their landfall probabilities increased by 3–5 times. This millennial-scale variability has been attributed to long-term shifts in the position of the Azores High, which may also be linked to changes in the strength of the North Atlantic Oscillation.

According to the Azores High hypothesis, an anti-phase pattern is expected to exist between the Gulf of Mexico coast and the Atlantic coast. During the quiescent periods, a more northeasterly position of the Azores High would result in more hurricanes being steered towards the Atlantic coast. During the hyperactive period, more hurricanes were steered towards the Gulf coast as the Azores High was shifted to a more southwesterly position near the Caribbean. Such a displacement of the Azores High is consistent with paleoclimatic evidence that shows an abrupt onset of a drier climate in Haiti around 3200 14C years BP, and a change towards more humid conditions in the Great Plains during the late-Holocene as more moisture was pumped up the Mississippi Valley through the Gulf coast. Preliminary data from the northern Atlantic coast seem to support the Azores High hypothesis. A 3,000-year proxy record from a coastal lake in Cape Cod suggests that hurricane activity increased significantly during the past 500–1000 years, just as the Gulf Coast was amid a quiescent period during the last millennium. Evidence also shows that the average latitude of hurricane impacts has been steadily shifting northward, towards the Eastern Seaboard over the past few centuries. This change has been sped up in modern times due to the Arctic Ocean heating up especially much from fossil fuel-caused global warming.

The number and strength of Atlantic hurricanes may undergo a 50–70 year cycle, also known as the Atlantic Multidecadal Oscillation. Nyberg et al. reconstructed Atlantic major hurricane activity back to the early eighteenth century and found five periods averaging 3–5 major hurricanes per year and lasting 40–60 years, and six other averaging 1.5–2.5 major hurricanes per year and lasting 10–20 years. These periods are associated with the Atlantic multidecadal oscillation. Throughout, a decadal oscillation related to solar irradiance was responsible for enhancing/dampening the number of major hurricanes by 1–2 per year.