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Definition

What is a hurricane?

A hurricane is the regional name for a tropical cyclone that forms in the North Atlantic Ocean basin. A hurricane is a rotating storm system that has a low pressure center surrounded by a spiral of thunderstorms. A hurricane starts as an atmospheric disturbance that produces a weakly-organized system of thunderstorms. If conditions are ideal than the storm will strengthen into a tropical depression (a low-pressure center that has winds <39 mph) [1], a tropical storm (wind speeds between 39 and 74 mph), or a hurricane (wind speeds >74 mph).

There are certain conditions that must be met for a hurricane to form. These include:

  1. Warm ocean water that is at least 27°C and at least 45 meters (150 feet) deep,
  2. Little to no wind shear (limited variation in wind speed/direction with height)
  3. An unstable atmosphere capable of producing strong thunderstorms is needed [2]
  4. A deep layer of humid air
  5. Enough Coriolis force that will allow the system to develop a spinning motion are also necessary [2].

A hurricane is an organized storm system that has winds >74 mph. It has a very low atmospheric pressure at the center of circulation (known as the eye). The eye of the hurricane is typically cloud free and has light winds because it is a region of weak sinking air [2]. The fastest winds in a hurricane are typically found in the eyewall, an area of strong convection that is close to the eye. A series of rainbands surround the eye due to the cyclonic convergence of air towards the low pressure in the eye. These rainbands can cause significant rainfall, but typically have weaker winds than the eyewall [2],[3].

How do Hurricanes Form?

Hurricane formation or cyclogenesis commonly starts from easterly waves that form in sub-Saharan Africa and make their way west across the Atlantic Ocean [2]. Easterly waves have the "cyclonic vorticity" or rotation and the atmospheric instability that is necessary for tropical cyclone genesis [2]. Not all easterly waves develop into hurricanes [2]. Those that do, experience an increase in convection which leads to closed circulation near the surface [2]. When the easterly wave develops a closed circulation, it is classified as a tropical depression [2]. As the depression gains strength, it eventually develops into a tropical storm which then turns into a hurricane, if conditions are favorable [2]. Storms that form from easterly waves are called Cape Verde hurricanes [1]. Atlantic hurricanes can also be influenced by tropical upper-tropospheric troughs in the central subtropical Atlantic [1]. Regardless of where they form, storms need the right environmental conditions to continue to strengthen and grow, these include:

(1) Ocean temperature
Hurricanes require warm ocean water that is at least 27°C and at least 45 meters (150 feet) deep. Warm ocean waters are a source of energy through evaporation that acts as fuel during hurricane formation and development [3]. Ocean temperatures must reach at least 26-27°C for cyclones to form [1],[3]. The surface layer of warm water must be deep to prevent colder water at greater depths from being mixed into the surface [1]. An increase in sea surface temperature (SST) may also lead to an increase in tropical cyclone intensity [4],[5].

(2) Limited wind shear
The difference in speed between winds at two different heights is a measure of wind shear [1]. Vertical wind shear occurs when winds in a vertical column of air blow at different speeds [1]. This difference in wind speed with height prevents the storm from maintaining symmetry in its circulation [2]. Wind shear can also weaken a storm by allowing heat energy to escape or by allowing dry air to enter the storm [2]. The greater the difference in winds at two altitudes, the greater the wind shear and the lower the lIkelihood of storm formation and/or development [1],[6]. Typically, wind shear >8.5 m s−1 will weaken tropical cyclones and prevent them from strengthening [1]. Wind shear > 10 m s−1 prevents the development of tropical cyclones [1]. Aiyyer and Thorncroft [6] found that approximately 50% of variability in tropical cyclone activity in the main development region (between 10° and 20°N) in the Atlantic Ocean is due to vertical wind shear. Wind shear is one of the main factors that causes tropical cyclones to weaken [6]. Generally, wind shear is greater as you move away from the tropics [2].

(3) Unstable atmosphere
An unstable atmosphere is required for a hurricane to form. The atmosphere tends to be most unstable in the Caribbean and Gulf of Mexico. Atmospheric instability tends to peak during the warm season, particularly in September.

(4) Humidity
Humid air provides a storm with the water vapor it needs to form and develop [1]. Relative humidity plays a large role in determining whether a storm can form [1],[2]. In the tropics, humidity levels tend to be high close to the surface but then decrease with increases in altitude [2]. A humid middle troposphere is very favorable for tropical cyclone genesis. Without the presence of a deep layer of humid air, dry air can prevent storm formation or weaken a storm that has already formed [2]. Storms also tend to get larger when the atmosphere is more moist [7].

Warm humid air provides a storm with the energy necessary to form and sustain itself [2]. The concept of the "heat engine" by Carnot helps illustrate how heat helps to fuel a tropical cyclone [2]. In Carnot's concept of a "heat engine", the heat engine is composed of a source of heat and a heat sink [3]. In the case of a tropical cyclone, the warm ocean acts as a heat source and the cyclone's upper atmosphere is its heat sink [2]. Warm air tends to rise when it is warmer than the surrounding air. When warm humid air or water vapor condenses, it releases energy in the form of latent heat [2]. When latent heat is released, it warms the air and clouds around it and causes additional uplift [2]. The greater the temperature difference between the heat source and the heat sink, the more efficient the "heat engine" will be [2].

(5) Vorticity

Vorticity or a spinning motion is vital for storm formation [2]. In the Northern Hemisphere, it causes hurricanes to rotate in a counterclockwise direction due to the Coriolis force [3]. The Coriolis force is strongest at the poles and weakest at the equator (0°), therefore, storms cannot form close to the equator because of the weak Coriolis force [1].

When do hurricanes form?

June 1st to November 30th is known as hurricane season in the North Atlantic Ocean. However, tropical cyclones can form at any time when conditions are conducive.

Intensity?

The hurricane strength or hurricane intensity can either be measured based on the central pressure or the maximum sustained wind speed. The wind speed is most commonly used to describe the intensity of a hurricane. The Saffir-Simpson Scale is a "hurricane potential damage scale" [1] based on maximum sustained (one-minute) wind speeds. The Saffir-Simpson Scale has five categories. Category 1 hurricanes have winds of 74 to 95 mph, while Category 5 hurricanes have winds of 155 mph or greater [1],[2].

Hurricane tracks and forward speed

Due to the shape (and length) of the Texas coastline, there is no typical path that hurricanes take when they make landfall in Texas. As shown in the figure, there is a slight tendency for storms to be moving in a northerly or northwesterly direction at landfall. Corpus Christi provides a good example of this since many of the storm that have affected this area moved from south to north or from southeast to northeast.

The forward speed (or translation speed) of hurricanes can also be highly variable. On average these storms are moving at ~16 km/h (10 mph) at landfall. However, some storms move very slowly or stall, while others move very quickly. The forward speed of the storm is determined by upper level steering winds and interaction between the tropical cyclone and mid-latitude systems.

Hurricane Track SpeedDistribution of daily TC direction of movement (a) and Histogram of TC translation speed for all TCs that influenced Texas between 1950-2009 (b) (created by Zhu and Quiring [8]).

Reference

[1] Elsner, J.B. and A.B. Kara, Hurricanes of the North Atlantic: climate and society. 1999, New York: Oxford University Press. 488.

[2] Emanuel, K.A., Divine wind :the history and science of hurricanes. 2005, New York: Oxford University Press.

[3] Emanuel, K., Tropical cyclones. Annual review of earth and planetary sciences, 2003. 31(1): p. 75.

[4] Landsea, C.W., A CLIMATOLOGY OF INTENSE (OR MAJOR) ATLANTIC HURRICANES. Monthly Weather Review, 1993. 121(6): p. 1703-1713.

[5] Michaels, P.J., P.C. Knappenberger, and R.E. Davis, Sea-surface temperatures and tropical cyclones in the Atlantic basin. Geophys. Res. Lett., 2006. 33(9): p. L09708.

[6] Aiyyer, A.R. and C. Thorncroft, Climatology of Vertical Wind Shear over the Tropical Atlantic. Journal of Climate, 2006. 19(12): p. 2969-2983.

[7] Hill, K.A. and G.M. Lackmann, Influence of Environmental Humidity on Tropical Cyclone Size. Monthly Weather Review, 2009. 137(10): p. 3294-3315.

[8] Zhu, L., and S. M. Quiring, Variations in tropical cyclone precipitation in Texas (1950 to 2009), J. Geophys. Res. Atmos., 118, 3085–3096, 2013.

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