Community Hurricane Preparedness, 2nd Edition

Types of Tropical Cyclones

Hurricane Rita and Sea Surface Temperatures, September 21, 2005. Orange and red depict ocean waters that are
82°F or warmer—areas warm enough for hurricanes to form.

A hurricane is a type of tropical cyclone, a generic term for a low pressure system that forms in the tropics or subtropics and is accompanied by thunderstorms organized into rainbands. In the Northern Hemisphere, tropical cyclones have a counterclockwise circulation of winds near the earth’s surface. Tropical cyclones do not have cold or warm fronts attached; those systems are called extratropical cyclones or winter storms.

Tropical cyclones are classified into four types, based on their wind speeds. These are terms commonly used by forecasters and you'll encounter them throughout this module:

• Tropical Depression: Maximum sustained winds of 38 miles per hour (mph) or 33 knots or less (a knot is a nautical mile per hour, abbreviated kt; 1 knot = 1.15 mph)
• Tropical Storm: Maximum sustained winds of 39-73 mph (34-63 kt)
• Hurricane: Maximum sustained winds of 74 mph (64 kt) or higher
• Major Hurricane: Maximum sustained winds of 111 mph (96 kt) or higher

Once a tropical storm forms, NHC gives it a name from the list for the current year. (Click here for a link to the current list of tropical cyclone names.) Each list is reused every six years, although names of hurricanes that have resulted in substantial damage or death are retired. The letters Q, U, X, Y, and Z are not included in the Atlantic list because of the scarcity of names beginning with those letters. If more than 21 named tropical cyclones occur in the Atlantic basin in a season, additional storms are named from the Greek alphabet: Alpha, Beta, Gamma, Delta, and so on.

Each list alternates between male and female names. Names utilized are English, Spanish, or French, reflecting the diversity of communities throughout North America, Central America, and the Caribbean that are impacted by these cyclones.

In the rest of this section, we'll talk about how, when, and where tropical cyclones form and their structure and intensity, which relate to the hazards that accompany these storms.

Hurricane Intensity

Hurricanes are categorized according to the strength of their winds using the Saffir–Simpson Hurricane Wind Scale in which a Category 1 storm has the lowest wind speeds, while a Category 5 hurricane has the strongest.

A "major" hurricane is classified as Category 3 or greater. The animation below shows the damage that can be expected from different category winds. These are relative terms, because lower category storms can sometimes inflict greater damage than higher category storms, depending on where they strike and the particular hazards they bring. In fact, tropical storms can also produce significant damage and loss of life, mainly due to rainfall–induced flooding that often occurs a considerable distance inland.

Life Cycle

In its early stages, a tropical system appears on the satellite image as a relatively unorganized cluster of thunderstorms. If weather and ocean conditions are favorable, the system can undergo genesis and become a tropical depression (number 1 in the figure). At this point, the storm begins to take on the familiar spiral appearance due to the flow of the winds and the rotation of the earth. Once it moves over land, the storm begins to weaken, although its impacts may continue for days.

Composite imagery of Hurricane Rita from a tropical depression (1), tropical storm (2), hurricane (3), and major hurricane (4 and 5)

Birth

Tropical cyclones form over warm waters from pre-existing weather disturbances. Some of these disturbances emerge every three or four days from the coast of Africa as "tropical waves," as seen in this animated satellite image.

Tropical cyclones can also form from the trailing ends of cold fronts that move into the tropics and stall, or occasionally from upper–level low pressure circulations that work their way down to the surface.

Ingredients for the development of a tropical cyclone

The process by which a tropical cyclone forms and subsequently strengthens into a hurricane depends on at least six conditions, some of which are shown in the figure above.

1. A pre-existing disturbance providing some initial spin and flow into the system (the horizontal orange arrows)
2. Warm (at least 80°F/26.5°C) ocean temperatures to a depth of about 150 ft (~50 m)
3. Relatively light winds that do not change much in direction and speed up through the atmosphere (called "low wind shear")
4. A location at least 300 miles (~500 km) from the equator. Tropical cyclones require some of the spin provided by the earth's rotation-called the Coriolis force-which is zero on the equator.
5. An atmosphere that cools enough with height to allow thunderstorms to develop
6. An atmosphere that is moist enough to fuel the thunderstorms

Heat and energy for the storm are gathered by the disturbance through contact with warm ocean waters. The winds near the ocean surface spiral into the disturbance’s low pressure area. The warm ocean waters add moisture and heat to the air, which rises. As the moisture condenses into drops, more heat is released, contributing additional energy to power the storm. Bands of thunderstorms form, and the storm’s cloud tops rise higher into the atmosphere. If the winds at these high levels remain relatively light (little or no wind shear), the storm can remain intact and continue to strengthen.

Growth & Maturity

Composite imagery of Hurricane Rita from a tropical depression (1), tropical storm (2), hurricane (3), and major hurricane (4 and 5)

As the bands of thunderstorms contribute additional heat and moisture to the storm, it can strengthen to tropical storm status (number 2 in the figure) . The storm becomes a hurricane when winds reach a minimum of 74 mph (64 kt) (numbers 3, 4, and 5 in the figure). At this time, the cloud-free hurricane eye (number 4 in the figure) typically forms because rapidly sinking air at the center dries and warms the area.

During their life span, tropical storms and hurricanes can last for more than two weeks over the ocean. They can also change direction, sometimes hitting the same state more than once. For example, in 2008 Tropical Storm Fay made eight landfalls-including four in Florida. The storm produced torrential rainfall and a total of 82 tornadoes across five states.

Storm’s End

Just as many factors contribute to the birth of a hurricane or tropical storm, there are many reasons why a hurricane begins to decay. Wind shear can tear the hurricane apart. Moving over cooler water or into drier areas can lead to weakening as well. Landfall typically shuts off the main moisture source, and the surface circulation can be reduced by friction when it passes over land.

However, once a storm makes landfall, the danger isn't over. As the system moves inland, impacts from the storm including heavy rainfall, inland and river flooding, tornadoes, and high winds often continue to pose a significant threat. In fact, although storm surge poses the primary coastal threat from hurricanes, inland (or freshwater) flooding accounts for a substantial portion of the deaths from tropical cyclones.

Note too that a weakening hurricane or tropical storm can reintensify if it moves back over warm water or interacts with mid–latitude frontal systems. For example, Hurricane Ike in 2008 re–intensified as an extratropical low (that is, one that has lost its tropical characteristics) over the Ohio Valley a day after moving through Texas. The reinvigorated storm produced wind gusts to hurricane force in parts of Kentucky and Ohio, and its remnants caused approximately $4.4 billion in damages and killed at least 28 people.

Something to Think About

Tracks for all North Atlantic (since 1859) and Eastern North Pacific (since 1949) hurricanes

The larger atmospheric flow in which tropical and subtropical cyclones form generally travels in a west-northwest direction. That flow can easily bring hurricanes toward the East Coast, but it generally carries them away from the West Coast. Also, the waters of the Gulf Stream along the East Coast are quite warm (over 80°F/27°C) in comparison to those along the West Coast, which rarely get above 75°F (24°C). Remember that hurricanes need warm water to both develop and maintain their intensity.

Structure

A hurricane is more than a point on a weather map, and its path is more than a line, as shown in this graphic depicting the winds around Hurricane Ike. It is a large system that affects a wide area, requiring that precautions be taken far from where the eye is predicted to come ashore. This section talks about the different parts of the hurricane and will help you better understand the hurricane hazards discussed in the next section.

Hurricane Features

The main features of a hurricane are the rainbands on its outer edges, the eye, and the eyewall. Air spirals in toward the center in a counter-clockwise pattern, and out the top in the opposite direction. In the very center of the storm, air sinks, forming the cloud-free eye. Click on the graphic to explore these features.

Eye

Details of the hurricane eye’s structure

The hurricane’s center is a relatively calm, clear area usually 20-40 miles across (about 30-65 km), but sometimes as tiny as 5 miles(8 km) or as large as 60 miles (95 km). People in the midst of a hurricane are often amazed at how the incredibly fierce winds and rain can suddenly stop and the sky clear when the eye comes over them. Then, just as quickly, the winds and rain begin again, but this time from the opposite direction.

Hurricane Wilma (left: 10/19/2005, right: 10/24/2005)-note difference in eye size between the 5 days

Eyewall

The wall of clouds surrounding the clear eye in Hurricane Katrina

The dense wall of thunderstorms surrounding the eye-the eyewall, or donut-shaped ring of thunderstorms around the calm eye-has the strongest winds within the storm. Changes in the structure of the eye and eyewall can cause changes in the wind speed, which is an indicator of the storm’s intensity. The eye can grow or shrink in size, and double (concentric) eyewalls can form, as shown in the animation below.

Concentric eyewalls typically occur in major hurricanes, where an outer eyewall forms and surrounds the inner eyewall, as shown in this animation of Hurricane Ike (2008). The outer eyewall usually kills off and replaces the inner eyewall and then contracts. While the hurricane is going through this eyewall cycle, the storm tends to weaken and then can restrengthen again once a single eyewall is again in place. These eyewall cycles can take as little as 12 hours or as much as a couple days to complete.

Spiral Rainbands

A view of Hurricane Ike's rain bands from space

The storm’s outer rainbands (often with hurricane or tropical storm-force winds) can extend a few hundred miles from the center. These dense bands of thunderstorms, which spiral slowly counterclockwise, range in width from a few miles to tens of miles and can extend hundreds of miles from the center. Typical hurricanes are about 300 miles (about 500 km) wide, but, as seen in the images below, there is considerable variation. Hurricane Floyd’s rainbands (1999) stretched over 400 miles (about 600 km). However, Hurricane Andrew’s (1992) rainbands reached only 100 miles (160 km) out from the eye. Hurricane rainbands are also the favored location for hurricane-spawned tornadoes to occur.

The spiral rainbands of Hurricane Floyd (left, 1999) versus the more compact Hurricane Andrew (right, 1992)

Right Side

As a general rule of thumb, the hurricane’s right front side (relative to the direction it is traveling) is the most dangerous part of the storm because the strongest winds and greatest risk from storm surge are usually found to the right of the center. Tornadoes are also more common here.

The winds are different between the right and left sides of the hurricane because of the combined effect of the hurricane wind speed and speed of the larger atmospheric flow (the steering winds). Looking at the graphic below, the right side is the eastern section of the hurricane. (If it were traveling east to west, the right side would be the north section.) The winds around the hurricane’s eye are moving counterclockwise. At Point A, the hurricane winds are nearly in line with the steering wind, which typically increases the strength of the winds. On the other hand, the winds at Point B are moving opposite those of the steering wind and therefore would be expected to be slower on this side. NHC forecasts take this effect into account in determining official wind estimates. The wind value provided is the strongest wind anywhere in the hurricane's circulation.

Note, however, that this is just a general rule. Some hurricanes have features that can have equal or stronger damaging winds on the left side of the storm. Also, because of uncertainties in forecast track, communities on the left side of the forecast track need to plan for the full brunt of the storm.

Hurricane Size

As mentioned previously, hurricane size can vary considerably. Size is not necessarily an indication of hurricane intensity or damage potential. For example, Hurricane Katrina (2005) was a very large Category 3 storm (400 miles in diameter, or about 650 km) when it made landfall in Louisiana/Mississippi. Hurricane Charley (2004), in contrast, was a tiny Category 4 storm (150 miles or about 240 km) that at the time became the second most costly hurricane in U.S. history, behind Andrew (1992) which was also small in size (250 miles wide, or about 400 km) but a devastating Category 5 hurricane. As we'll see in the "Hazards" section, damage is a complex function of size, intensity, and location.

Hurricane Katrina, 2005, (left) was a Category 3 hurricane whose incredible devastation was due partly to its large size and the location where it came ashore. Hurricane Charley, 2004, (right) was a much smaller, but more intense storm (Category 4) that also caused significant damage.

Do not focus on the location and track of the center, because the hurricane’s destructive winds and rains cover a wide swath. Hurricane-force winds can extend outward to about 25 miles (40 km) from the storm center of a small hurricane and to more than 150 miles (240 km) for a large one. The area over which tropical storm-force winds occur is even greater. Typically, these winds range from 125 to 175 miles (200-280 km) out from the center, although in a large hurricane they can extend as far out as about 300 miles (~500 km).

Circulation & Movement

In the northern hemisphere, hurricane winds circulate around the center in a counter-clockwise fashion. This means that the wind direction at your location depends on where the hurricane’s eye is. A boat north of the hurricane's eye would experience winds from the east, while a boat to the south of the eye would have westerly winds.

Path & Speed

A tropical cyclone’s movement is mainly determined by the surrounding wind flows that steer it. The air in which the tropical cyclone is embedded is a constantly moving and changing "river" of air. Other features in that flow, such as high and low pressure systems, can greatly alter the speed and the path of the cyclone. In turn, it can modify the environment around the storm.

In the animation below, you can investigate how changes in position of lows, highs, and a hurricane itself can affect the path of the hurricane.

Some tropical cyclones follow a fairly straight course, while others loop and wobble along their path. For example, in 2004, Hurricane Jeanne (below) moved northward through the eastern Bahamas as a tropical storm and looked as if it might recurve harmlessly back out to sea. However, over the next few days it intensified while making a clockwise loop, before making landfall in Florida as a major hurricane.

Hurricane Jeanne (2004)

These changeable paths can be difficult to forecast, and they have important consequences for hazards. Typically, a tropical cyclone’s forward speed averages around 15 mph (24 km/h). However, some tropical cyclones stall, often causing devastatingly heavy rain. For example, Tropical Storm Allison (2001) moved slowly over eastern Texas and dumped over 30 inches (76 cm) of rain in Houston, causing extremely damaging flooding (see figure below). Others can accelerate to more than 60 mph (97 km/h). Hurricane Hazel (1954) hit North Carolina on the morning of 15 October; fourteen hours later it reached Toronto, Canada where it caused 80 deaths.

Storm total precipitation, Tropical Storm Allison (2001)

Frequency

Each year, about eleven tropical storms develop over the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. Many of these remain over the ocean. Annually, about six of these storms become hurricanes, and only two become major hurricanes sometime in their lifetime owing to the difficulty in getting all the right conditions for intensification to Category 3 status or higher. Roughly two hurricanes a year strike the United States coastline anywhere from Texas to Maine.

Fewer hurricanes in a given year, however, do not necessarily mean a lesser threat of disaster. Intense hurricanes and hurricanes that do significant damage certainly have occurred in years with much below-average hurricane activity. For example, in 1992 there were only 7 tropical storms, four hurricanes, and 1 major hurricane-Andrew, which devastated southern Florida.

The modern era of hurricane monitoring in the Atlantic basin began around 1944 when aircraft first flew into hurricanes to monitor their position and intensity. However, the most reliable data have been collected since the 1960's when weather satellite imagery became available. The graph above shows that the number of hurricanes in the modern era has ranged from as few as 2 in 1982 to as many as 15 in 2005. Many years have had no landfalling hurricanes in the U.S., while the most to hit the country in one year was 6 (1985).

Season

The official hurricane season for the Atlantic Basin (the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico) is from 1 June to 30 November. As seen in the graph, the season's peak is from mid-August to late October. However, tropical storms and hurricanes can occur outside of hurricane season.

Researchers produce seasonal forecasts but these have somewhat limited skill and tell you nothing about where storms will make landfall, when they will strike, or how intense they will be. There also does not appear to be any link between storm activity early in the hurricane season and activity in the rest of the period, although over periods of many years hurricanes have cycles of greater and lesser activity.

Something to Think About:

Although solar radiation does peak in June, it takes a while for the oceans to absorb that heat and reach their warmest temperatures. The sun warms the ocean surface first, and these temperatures mix with the cooler water below. It takes time for the ocean to warm to a great enough depth to support tropical cyclone development. Remember also that hurricane formation requires a number of ingredients, not just warm ocean waters. That combination of ingredients usually comes together in summer and early fall.

Origin Zones

Where a hurricane forms and the prevailing steering current determine where the storm may threaten land. As shown in this graphic, nearly all portions of the U.S. Gulf and Atlantic coastlines have been impacted by hurricanes over the last several decades. This highlights the vulnerability to hurricanes that exists for nearly every community on the East and Gulf Coasts.

The zones where hurricanes form and the tracks they take are generally related to the time of year. Consequently, different areas of the country have a greater risk during different months although, again, patterns can vary considerably from year to year. The figures below show the zones of origin and tracks for different months during the hurricane season. These are average conditions-be aware that hurricanes can originate in different locations and travel much different paths from the average. Nonetheless, having a sense of the general pattern can give you a better picture of the average hurricane season for your area.

During the early (June-July) season, hurricane activity is typically quiet and what systems do form tend to occur in a fairly restricted area over the Gulf of Mexico or westernmost Atlantic Ocean.

During the August-September timeframe, decreased wind shear and increased water temperatures allow for more hurricanes to form anywhere throughout the North Atlantic, Caribbean, and Gulf of Mexico.

October can see substantial amount of hurricane activity, though they typically form in the western Caribbean or western Atlantic and tend to have erratic tracks that often move quickly off toward the northeast.

Finally, in the late hurricane season (November), activity dies down and the focus shifts to the western Atlantic.

Questions

Question 5

Looking at the cloud motion in the animation, which of the following is mostly likely occurring at the surface?

a) LA has winds from the north and FL has winds from the west

b) LA has winds from the south and FL has winds from the north

c) LA has winds from the north and FL has winds from the south

d) LA has winds from the west and FL has winds from the south

The correct answer is c.

LA has north winds and FL has south winds. The clouds circulate counter-clockwise around the hurricane, so over LA they are moving from land toward the sea (winds from the north), while clouds over FL are moving from the sea toward the land (winds from the south).