Environmental Conditions Leading to Hurricanes |
Hurricanes need the right environmental conditions to continue to strengthen and grow, these include:
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].
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 cause tropical cyclones to weaken [6]. Generally, wind shear is greater as you move away from the tropics [2].
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.
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].
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].
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].
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 cause tropical cyclones to weaken [6]. Generally, wind shear is greater as you move away from the tropics [2].
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.
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].
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].
Acknowledgements & Credits: Click here for literature cited in this section, "What's a Hurricane?" This material is based upon work supported by the Texas Department of Public Safety's Division of Emergency Management. Background photo courtesy of Mark Moran (Creative Commons license CC BY 2.0)
© 2019 Jennifer L. Irish & Steven M. Quiring. All rights reserved.
© 2019 Jennifer L. Irish & Steven M. Quiring. All rights reserved.