Chapter 11 -- Hurricanes

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Hurricane Hurricane Center Hurricanes from Book Long Island Severe Weather Guide

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Land-Fall Hurricanes

Tropical Weather -


The vast majority of hurricane deaths and damage are caused by relatively infrequent, yet powerful storms.

Most hurricanes form between the latitudes of 5° and 20° over all tropical oceans except the South Atlantic and eastern South Pacific. The North Pacific has the greatest number of storms, averaging 20 per year. In the western Pacific, hurricanes are called typhoons, and in the Indian Ocean, they are referred to as cyclones.

A steep pressure gradient generates the rapid, inward spiraling winds of a hurricane. As the warm, moist air approaches the core of the storm, it turns upward and ascends in a ring of cumulonimbus towers and forms a doughnut-shaped wall called the eye wall. At the very center of the storm, called the eye, the air gradually descends, precipitation ceases, and winds subside.

A hurricane is a heat engine fueled by the latent heat liberated when huge quantities of water vapor condense. They develop most often in late summer when ocean waters have reached temperatures of 27°C (80°F) or higher and are thus able to provide the necessary heat and moisture to the air. The initial stage of a tropical storm's life cycle, called a tropical disturbance, is a disorganized array of clouds that exhibits a weak pressure gradient and little or no rotation. Tropical disturbances that produce many of the strongest hurricanes that enter the western North Atlantic and threaten North America often begin as large undulations or ripples in the trade winds known as easterly waves.

Atlantic Hurricane/Tropical season June 1st to November 30th

Formation of a Hurricane Video

Tropical Structure -

Tropical Cyclone Classification

Tropical cyclones with an organized system of clouds and thunderstorms with a defined circulation, and maximum sustained winds of 38 mph (61 km/h) or less are called "tropical depressions". Once the tropical cyclone reaches winds of at least 39 mph (63 km/h) they are typically called a "tropical storm" and assigned a name.

If maximum sustained winds reach 74 mph (119 km/h), the cyclone is called:

A hurricane in the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, and the South Pacific Ocean east of 160°E, (The word hurricane comes from the Carib Indians of the West Indies, who called this storm a huracan. Supposedly, the ancient Tainos tribe of Central America called their god of evil "Huracan". Spanish colonists modified the word to hurricane.),

A typhoon in the Northwest Pacific Ocean west of the dateline (super typhoon if the maximum sustained winds are at least 150 mph / 241 km/h),
A severe tropical cyclone in the Southwest Pacific Ocean west of 160°E or Southeast Indian Ocean east of 90°E,
A severe cyclonic storm in the North Indian Ocean, and
A tropical cyclone in the Southwest Indian Ocean.

Hurricanes are further classified according to their wind speed. The Saffir-Simpson Hurricane Wind Scale is a 1-5 rating based on the hurricane's present intensity. This scale only addresses the wind speed and does not take into account the potential for other hurricane-related impacts, such as storm surge, rainfall-induced floods, and tornadoes.

Tropical Depression:
A tropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 33 kt (38 mph or 62 km/hr) or less.

Miles per hour Knots Kilometer/hr or Kph
Under 39 mph (30-38mph) Under 33 knots Under 62 Km/hr

Tropical Storm:
A tropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) ranges from 34 kt (39 mph or 63 km/hr) to 63 kt (73 mph or 118 km/hr).

Miles per hour Knots Kilometer/hr or Kph
39-73 mph 34-63 knots 63- 118 Km/hr

Hurricane / Typhoon:
A tropical cyclone in which the maximum sustained surface wind (using the U.S. 1-minute average) is 64 kt (74 mph or 119 km/hr) or more. The term hurricane is used for Northern Hemisphere tropical cyclones east of the International Dateline to the Greenwich Meridian. The term typhoon is used for Pacific tropical cyclones north of the Equator west of the International Dateline.

Miles per hour Knots Kilometer/hr or Kph
74 mph or greater 64 knots or greater 119 Km/hr or greater

Saffir-Simpson Scale
Saffir-Simpson Scale
Minor Modification to Saffir-Simpson Hurricane Wind Scale For the 2012 Hurricane Season

Tropical Symbolshurricane scale

hurricane scale

Each year, only a few tropical disturbances develop into full-fledged hurricanes that require minimum wind speeds of 119 kilometers per hour. When a cyclone's strongest winds do not exceed 61 kilometers per hour, it is called a tropical depression. When winds are between 61 and 119 kilometers per hour, the cyclone is termed a tropical storm. Hurricanes diminish in intensity whenever they (1) move over ocean waters that cannot supply warm, moist tropical air, (2) move onto land, or (3) reach a location where large-scale flow aloft is unfavorable.

Tropical Storm Names -


The Right Side of the StormAs a general rule of thumb, the hurricane's right side (relative to the direction it is travelling) is the most dangerous part of the storm because of the additive effect of the hurricane wind speed and speed of the larger atmospheric flow (the steering winds). The increased winds on the right side increase the storm surge described in the Hazards section of this module. Tornadoes are also more common here.Looking at the figure above, pretend you are standing behind the hurricane with your back to the steering flow. In this case, the right side is the eastern section of the hurricane. (If it were travelling east to west, the right side would be the north section.) The winds around the hurricane's eye are moving in a counterclockwise fashion. At Point A, the hurricane winds are nearly in line with the steering wind, adding to the strength of the winds. For example, if the steering currents are 30 mph and the average hurricane winds are 100 mph, the wind speed would be 130 mph at Point A. On the other hand, the winds at Point B are moving opposite those of the steering wind and therefore slow to 70 mph (100 - 30 mph). Courtesy

Although damage caused by a hurricane depends on several factors, including the size and population density of the area affected and the near-shore bottom configuration, the most significant factor is the strength of the storm itself. The Saffir-Simpson scale ranks the relative intensities of hurricanes. A 5 on the scale represents the worst storm possible, and a 1 is the least severe. Damage caused by hurricanes can be divided into three classes: (1) storm surge, which is most intense on the right side of the eye where winds are blowing toward the shore, occurs when a dome of water 65 to 80 kilometers (40 to 50 miles) wide sweeps across the coast near the point where the eye makes landfall, (2) wind damage, and (3) inland freshwater flooding, which is caused by torrential rains that accompany most hurricanes.


Storm Surge Animation

storm tide
Storm Surge:

What is storm surge?
Storm surges are frequently the most devastating element of a hurricane. As a hurricane’s winds spiral around and around the storm, they push water into a mound at the storm’s center. This mound of water becomes dangerous when the storm reaches land because it causes flooding along the coast. The water piles up, unable to escape anywhere but on land as the storm carries it landward. A hurricane will cause more storm surge in areas where the ocean floor slopes gradually. This causes major flooding.
As you watch the storm-surge animations, notice the effect that the physical geography of each coastline has on storm surge. Also, note the waves on top of the ocean's surface. Wind, waves, and sea-level rise all contribute to storm-surge damage.

Storm Surge Maps
NOAA's SLOSH (Sea, Lake, and Overland Surge from Hurricanes) storm surge model.

North Atlantic hurricanes develop in the trade winds, which generally move these storms from east to west. Today, because of early warning systems that help detect and track hurricanes, the number of deaths associated with these violent storms have been greatly reduced. Because the tropical and subtropical regions that spawn hurricanes consist of enormous areas of open oceans, meteorological data from these vast regions are provided primarily by satellites. Other important sources of hurricane information are aircraft reconnaissance, radar, and remote, floating instruments called data buoys. Using data from the observational tools, meteorologists can issue an announcement, called a hurricane watch, aimed at specific coastal areas threatened by a hurricane, generally within 36 hours. By contrast, a hurricane warning is issued when sustained winds of 119 kilometers per hour or higher are expected within a specified coastal area in 24 hours or less. Two important factors in the watch-and-warning decision process are (1) adequate lead time and (2) attempting to keep overwarning at a minimum.

Profile of a Hurricane
whirling tropical cyclone
wind speeds exceeding 200 mph
extreme low pressure
most form between latitude 5 and 20 degrees
warm, moist air rushes inward, turns upward and ascends in a ring of cumulonimbus clouds
eye - center of the storm
storm surge

Hurricane Formation and Decay
develop most often in late summer when ocean waters have reached higher temperatures (>27 C) June 1st to November 30th
development not well understood - starts with smaller tropical cyclone
diminishes when it moves over cooler ocean water or land

Detection and Tracking
aircraft reconnaissance
data buoys positioned along east coast and Gulf of Mexico

  There are many websites where you can find background information on hurricanes. You'll want to learn about how hurricanes form, which means learning about global wind patterns, and areas of high and low air pressure, among other things. The following web sites are good places to start:

  • Test your knowledge of hurricane formation with this interactive applet (requires Java). You can drag a hurricane around to areas with different water temperature and see what happens to it!
    Whittaker, T. and S. Ackerman, 2005. "Hurricane Applet," Weatherwise, University of Wisconsin [accessed May 17, 2006]
  • This is one of many online sources of historical hurricane data. We chose this website because the track maps include index information at selected data points so that you can easily correlate position and date (the one drawback is that the background color of the maps is black):
    Unisys, 2004. "Atlantic Tropical Storm Tracking by Year," Unisys Weather [accessed May 17, 2006]
  • The National Data Buoy Center has current and historical meterological data from a network of continuous monitoring buoys:
    NDBC, 2006. "National Data Buoy Center," NDBC, National Oceanic and Atmospheric Administration [accessed May 17, 2006]
  • For calculating distances on your hurricane track map, try this longitude and latitude distance calculator:, 2006. "Length of a Degree of Latitude and Longitude," [accessed May 17, 2006]