Chapter 12. Tropical Storms and Hurricanes

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CHAPTER 12. Tropical Storms and Hurricanes
Chapter Overview:

The chapter details the initiation, movement, structure, and types of tropical cyclones. In addition, causes of damage are described and related to hurricane morphology.

Chapter at a Glance:

Hurricanes are responsible for astonishing amounts of property damage and loss of life in many regions of the world.

• Hurricanes Around the Globe - Atlantic and eastern Pacific tropical cyclones are known as hurricanes while over the western Pacific they are referred to as typhoons. Over the Indian Ocean and Australia they are known as cyclones. The western North Pacific has the highest frequency of tropical cyclones for the globe with an annual average of 16 typhoons. Conversely, the south Atlantic produces none, as the basin is too small to initiate cyclogenesis.
• The Tropical Setting - A subsidence inversion, or trade wind inversion, forms on the eastern side of the subtropical anticyclones. Below this a marine layer of cool, moist air resides. The marine layer is shallowest and the inversion lowest towards the eastern basin edges where cold water upwelling and cold ocean currents dwell. Towards the western edges of the ocean basins the marine layer warms with corresponding surface temperatures and expands to greater heights. Convection results in large cumulonimbus clouds over these regions as opposed to the eastern areas where only stratus may exist.
• Hurricane Characteristics - By definition, hurricanes, the most powerful of all storms, have sustained winds of 120 km/hr (74 mph) or greater. Although of lesser intensity than tornadoes, hurricanes have a much larger spatial domain and life span, making them much more devastating. Average diameters are approximately 600 km (350 mi) and central pressures average about 950 mb but may be as low as 870 mb. Most energy attained by hurricanes stems from latent heat released by condensation. Therefore, hurricanes occur where warm waters abound and during the times of highest sea surface temperatures. For the Northern Hemisphere this is typically in August and September, January to March for the Southern Hemisphere. The morphology of a hurricane consists of a central eye surrounded by large cumulonimbus thunderstorms occupying the adjacent eye wall. Outward from the eye wall exist cloud bands of cumulonimbus thunderstorms. Weak uplift and low precipitation regions separate the cloud bands. Pressure differences into the center of the storm are about twice as great as the average mid-latitude cyclone resulting in strong sustained winds. Unlike mid-latitude cyclones, hurricanes are warm cored lows as a result of adiabatic expansion of in-rushing air. This results in only sight horizontal temperature differences toward the eye. Latent heat release from condensation causes the eye to be much warmer than the surrounding storm. Because the system is warm cored, the horizontal pressure gradient with altitude decreases slowly. At about the 400 mb level, pressures within the storm are approximate to that outside. From 400 mb to the tropopause, pressures within the storm exceed those outside so that the upper portion of the storm rotates anticyclonically while the lower portions rotate cyclonically. The upper portions of the storm are also blanketed by a cirrus cloud cap as a result of low temperatures at these great heights.
A. The Eye and the Eye Wall - The most distinctive feature of a hurricane is the clear central eye. This area is indicative of descending air and light winds. The average eye is about 25 km (15 mi) in diameter but some may be as small as 6 km (3.5 mi) or as large as 100 km (60 mi). A shrinking eye is typically indicative of storm intensification. The eye wall is immediately adjacent the eye and is composed of the strongest winds, the largest clouds, and the heaviest precipitation. Rainfall rates may be as high as 2500 mm/day (100") under the eye wall. Sinking air warming adiabatically causes air in the eye to be warmer than elsewhere. Additionally, relative humidities are lower in this region due to the higher temperatures.
• Hurricane Formation - Tropical disturbances often begin in the eastern ocean basins as disorganized clusters of thunderstorms. Some disturbances form in association with mid-latitude troughs migrating toward lower latitudes but most form from convection associated with the ITCZ. Easterly waves or undulations in the trade wind pattern spawn hurricanes in the Atlantic. Because of weak pressure gradients in low latitudes, easterly waves are depicted by plotting streamlines, or lines of wind direction. The waves typically stretch between 2000 and 3000 km (1200 to 1800 mi). Close fitting streamlines east of the wave axis indicate zones of convergence while west of the axis streamlines indicate divergence. Thus the disturbance is located upstream of the wave axis. Atlantic hurricanes form from seedlings emanating from western Africa. These disturbances move westward in the trade wind flow and intensify as waters warm beneath them. The waves take about one week to traverse the Atlantic as average speed is about 15 to 35 km/hr (10-20 mph). Only about 10% of all disturbances intensify into more organized rotating storms. When at least one closed isobar is present the disturbance is classified as a tropical depression. Upon further intensification to wind speeds of 60 km/hr (37 mph) the storm is officially classified as a tropical storm. Hurricane status is gained when winds reach or exceed 120 km/hr (74 mph). A high percentage of depressions become tropical storms and an even higher percentage of those attain hurricane status.
A. Conditions Necessary for Hurricane Formation - Hurricanes only form over deep water layers with surface temperatures in excess of 27^oC (81^oF) as hurricane energy is derived from latent heat release and associated evaporation of water. Poleward of about 20^o water temperatures are usually below this threshold. Additionally, hurricanes are most frequent in late summer and early fall as water temperatures are maximized. The Coriolis force is also an important contributor to hurricane formation. As such, systems typically form poleward of about 5^o latitude. An unstable atmosphere is also necessary and this typically occurs toward the central to western ocean basins as trade wind inversions and cool ocean surfaces dominate eastward. Finally, strong vertical shear must be absent for hurricane formation to progress. Upon formation, hurricanes are self-propagating but are limited by the supply of latent heat from warm ocean waters.

• Hurricane Movement and Dissipation - Movement of tropical cyclones is dependent upon the stage of development. Tropical disturbances and depressions are largely regulated by trade wind flow and simply move westward. At the tropical storm stage, trade wind importance diminishes as upper level winds and ocean temperatures become more influential. Movement of tropical storms and hurricanes is essentially parabolic, however, movement may be highly erratic in particular storms. For Atlantic storms, once the storms gain latitude they recurve toward the northeast through the influence of surface and upper level westerlies.
• Destruction by Hurricanes - Hurricane winds cause excessive damage even to well built buildings. Heavy rainfall is also responsible for large amounts of property damage. The storm surge is responsible for a large percentage of damage along coastal regions. Storm surges occur as water piles due to heavy winds and low atmospheric pressure and increase with storm intensity. Hurricane Camille caused a storm surge of 7 m (23') along the Mississippi coast. Additionally, high surf occurs atop the surge, further increasing damage potential. Winds and surge are typically most intense in the right front quadrant of the storm. Here wind speeds combine with the speed of the storm’s movement to create the area of highest potential impact. This area also produces the greatest frequency of tornadoes within the hurricane due to frictional drag of lower atmospheric winds upon landfall.
• Hurricane Forecasts and Advisories - The National Hurricane Center is responsible for predicting, and tracking Atlantic and east Pacific hurricanes. Data are gathered through satellite observations, surface observations, and aircraft using dropsondes. Statistical, dynamic, and hybrid computer models running on supercomputers assist in future track position and storm intensity predictions. Future positions are given along six-hour trajectories with accuracy decreasing as lead-time increases.
A. Hurricane Watches and Warnings - A hurricane watch is administered if an approaching hurricane is predicted to reach land in more than 24 hours. If the time frame is less, a hurricane warning is given. The erratic nature of the systems leads to difficulties in exact prediction, warning, and evacuation of prone areas.
B. Hurricane Intensity Scale - The Saffir-Simpson scale classifies hurricanes into five categories based on central pressures, maximum sustained winds speeds, and storm surge. Devastating hurricanes, categories 4 and 5, are rare events. Only two category five storms, hurricanes Camille (1969) and Gilbert (1988), have made landfall within the US since 1900.
Chapter Boxes:

12-1 Special Interest: The Naming of Hurricanes - Names are given to tropical cyclones when they reach tropical storm intensity. Names begin with the letters of the alphabet and progress through the order of the alphabet. For each year, the first tropical storm will begin with the letter A. Names proceed through a six-year list with names frequently reused unless a particular storm is noteworthy, then the name is retired. This practice began in WWII but was officially adopted by the NWS in the early 1950s.
12-2 Special Interest: Recent Notable Hurricanes - The period between 1995 and 2002 was marked by an unusually high number of Atlantic tropical storms and hurricanes, with many making landfall. Of the eight seasons, all but two had at least 8 hurricanes, well above the average of 5.8. The section details the most notable of these storms including; Hurricane Mitch (1998), Hurricane Floyd (1999), Tropical Storm Allison (2001), and Hurricane Lili (2002).
12-3 Special Interest: Seasonal and Long-Term Hurricane Forecasts - Long-term and seasonal hurricane forecasting is based upon a number of variables. These include present El Niño conditions, rainfall amounts in western Africa, temperature, stratospheric and upper-tropospheric winds, and Caribbean Sea air pressure. Results have been mixed. Other techniques examine long-term trends. For the Atlantic, no long-term trends in frequency are evident, although particular years and periods show higher frequencies than others. Future estimates revolve around potential impacts of human related global warming. Some argue that a warmer environment will produce higher frequencies of tropical cyclones while others argue the opposite.
12-4 Special Interest: The Galveston Hurricane of 1900 - The Galveston hurricane of 1900 is the single deadliest natural disaster in US history. Over 6000 deaths occurred on Galveston Island as a massive storm surge inundated an island that has a peak elevation of 3m (9'). Storm warnings were reported as much as two days from the landfall date, but few took heed, as many believed that a Caribbean storm could not track across the Gulf of Mexico. Another judgment error resulted as many locals thought the island protected by the gently sloping seafloor near the island. As a result, few people evacuated the island. Residents were trapped on the island once the storm arrived, and few remained alive.

Related Web Sites:

Hurricane, Tropical Storm, Tropical Cyclone, Tropical Depression, Easterly Wave, Hurricane Watch: 

Eye Wall, Storm Surge:

Typhoon:

Media Enrichment:

ME12.1 - Interactive Exercise, Tropical Cyclones

ME12.2 - Weather in Motion, A fly through of Hurricane Mitch

ME12.3 - Weather in Motion, Hurricane Slice

ME12.4 - Weather in Motion, Hurricane Eye Wall

ME12.5 - Weather in Motion, Hurricane Dennis

ME12.6 - Weather in Motion, Storm Surge

ME12.7 - Weather in Motion, Hurricane Damage

ME12.8 - Weather in Motion, Interview with Chase Plane Pilot

Key Terms:

hurricane marine layer easterly waves hurricane watch

typhoon eye tropical depression hurricane warning

cyclone eye wall tropical storm Saffir-Simpson scale

trade wind inversion tropical disturbance storm surge
Review Questions:

1. Describe the geographic distribution of hurricanes, typhoons, and cyclones. What environmental conditions at these locations favor the development of such storms?

Collectively, all the above names refer to tropical cyclones. As the name implies, tropical cyclones originate within low latitude, “tropical” regions. The storms typically form in association with rapid convection associated with the ITCZ. Tropical cyclones traverse east to west, carried in the global north-or south-east trade winds as they move toward higher latitudes. The storms feed off of massive amounts of latent heat of condensation such that warm tropical waters, and associated evaporation, are required. The storms develop best towards the western edges of the ocean basins where the depth of the marine layer is most extensive. The western North Pacific sees the highest frequency of tropical cyclones, while the South Atlantic produces none.
2. Which region has the greatest incidence of major tropical storms?
The western North Pacific has the highest frequency of tropical cyclones for the globe with an annual averages of 16 typhoons.
3. What is the trade wind inversion, and what impact does it have on the formation of hurricanes?
A trade wind inversion is a subsidence inversion which forms on the eastern side of the subtropical anticyclones. Below this a marine layer of cool, moist air resides. The marine layer is shallowest and the inversion lowest towards the eastern basin edges where cold water upwelling and cold ocean currents dwell. This suppresses convection and the development of tropical cyclones.
4. Describe the size, sea level air pressure, and wind speed of a typical hurricane.
Average diameters are approximately 600 km (350 mi) and central pressures average about 950 mb but may be as low as 870 mb. To be classified as a hurricane, a storm must have sustained winds of 120 km/hr (74 mph) or greater.
5. When are hurricanes most likely to form?
During the warm season. In the northern hemisphere the “hurricane season” extends from June 1 to November 30.
6. Describe the cloud and precipitation patterns associated with hurricanes, including those associated with the eye and eye wall.

Tropical cyclones usually consist of a central eye surrounded by large cumulonimbus thunderstorms that comprise the eye wall. The eye is the area of lowest pressure and it is about 25 km in diameter on average. Within the eye, winds are relatively calm and descending. Thus, few clouds, if any, exist. The eye wall is the area of greatest winds and precipitation as this is where the greatest uplift of air and largest thunderstorms occur. Extending outward from the eye are swirling cloud bands of cumulonimbus thunderstorms. Weak uplift and low precipitation regions separate these bands.

7. Describe the various ways in which hurricanes differ from midlatitude cyclones.
Unlike mid-latitude cyclones, hurricanes are warm cored lows as a result of adiabatic expansion of in-rushing air. This results in only slight horizontal temperature differences toward the eye. Latent heat release from condensation causes the eye to be much warmer than the surrounding storm.
8. What are tropical disturbances, and how are they influenced by easterly waves?
Tropical disturbances often begin in the eastern ocean basins as disorganized clusters of thunderstorms. Some disturbances form from convection associated with the ITCZ. Easterly waves or undulations in the trade wind pattern spawn hurricanes in the Atlantic. The waves are depicted by plotting streamlines, or lines of wind direction.
9. Describe the characteristics that distinguish tropical disturbances, tropical depressions, tropical storms, and hurricanes.
Tropical disturbances are essentially clusters of thunderstorms. When a disturbance has at least one closed isobar it is upgraded to a depression. Upon further intensification to wind speeds of 60 km/hr (37 mph) the storm is officially classified as a tropical storm. Hurricane status is gained when winds reach or exceed 120 km/hr (74 mph).
10. What ocean surface characteristics are required for the intensification of storms into hurricanes and the maintenance of hurricanes?
Hurricanes form only over deep water layers with surface temperatures in excess of 27^oC (81^oF) as hurricane energy is derived from latent heat release and associated evaporation of water.
11. Is there a “typical” path that hurricanes take after having formed? Explain.
Yes, a parabolic one. Once storms gain latitude, they recurve toward the northeast through the influence of surface and upper level westerlies.
12. What feature associated with hurricanes causes the greatest destruction to coastal regions? Is this also true of inland regions?
The storm surge is responsible for a large percentage of damage along coastal regions. Storm surges occur as water piles due to heavy winds and low atmospheric pressure and increase with storm intensity. Inland regions see most damage from high winds, heavy rainfall, and occasional tornadoes.
13. Why is the right-hand side of a hurricane (relative to its direction of movement) the most dangerous?
Because wind speeds combine with the speed of the storm’s movement to create the area of highest potential impact.
14. Where are tornadoes most likely to be imbedded in a hurricane?
In the right-front quadrant due to frictional drag of lower atmospheric winds upon landfall.
15. What are hurricane watches and warnings? Are they exact corollaries to tornado watches and warnings?
A hurricane watch is administered if an approaching hurricane is predicted to reach land in more than 24 hours. If the time frame is less, a hurricane warning is given. The erratic nature of the systems leads to difficulties in exact prediction, warning, and evacuation of prone areas. Tornado watches and warnings are more reliable overall due to more predictable aspects of formation.
16. Why are forecasters concerned with issuing hurricane advisories for areas that do not eventually get hit by a hurricane?

Because the consequences would be more catastrophic if no warning were given and the storm changed course and struck an unprepared location.

17. What is the highest hurricane category on the Saffir-Simpson scale? How frequently do hurricanes of that magnitude occur?
Category 5 hurricanes are the strongest storms on the Saffir-Simpson scale. These are very rare events as only two category five storms made landfall within the US since 1900.

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