Hurricanes

The ingredients for a hurricane include a pre-existing weather disturbance, warm tropical oceans, moisture, and relatively light winds aloft. If the right conditions persist long enough, they can combine to produce the violent winds, incredible waves, torrential rains, and floods we associate with this phenomenon.

 Each year, an average of eleven tropical storms develop over the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. Many of these remain over the ocean and never impact the U.S. coastline. Six of these storms become hurricanes each year. In an average 3-year period, roughly five hurricanes strike the US coastline, killing approximately 50 to 100 people anywhere from Texas to Maine. Of these, two are typically "major" or "intense" hurricanes (a category 3 or higher storm on the Saffir-Simpson Hurricane Scale).

What is a Hurricane?
A hurricane is a type of tropical cyclone, which is a generic term for a low pressure system that generally forms in the tropics. The cyclone is accompanied by thunderstorms and, in the Northern Hemisphere, a counterclockwise circulation of winds near the earth's surface. Tropical cyclones are classified as follows:

Tropical Depression
An organized system of clouds and thunderstorms with a defined surface circulation and maximum sustained winds* of 38 mph (33 kt**) or less

Tropical Storm
An organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds of 39-73 mph (34-63 kt)

Hurricane
An intense tropical weather system of strong thunderstorms with a well-defined surface circulation and maximum sustained winds of 74 mph (64 kt) or higher.

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

Tropical Cyclone Introduction

A tropical cyclone is a warm-core, low pressure system without any "front" attached, that develops over the tropical or subtropical waters, and has an organized circulation. Depending upon location, tropical cyclones have different names around the world. In the:

• Atlantic/Eastern Pacific Oceans - hurricanes
• Western Pacific - typhoons
• Indian Ocean - cyclones

Regardless of what they are called, there are several favorable environmental conditions that must be in place before a tropical cyclone can form. They are:

• Warm ocean waters (at least 80°F / 27°C) throughout a depth of about 150 ft. (46 m).
• An atmosphere which cools fast enough with height such that it is potentially unstable to moist convection.
• Relatively moist air near the mid-level of the troposphere (16,000 ft. / 4,900 m).
• Generally a minimum distance of at least 300 miles (480 km) from the equator.
• A pre-existing near-surface disturbance.
• Low values (less than about 23 mph / 37 kph) of vertical wind shear between the surface and the upper troposphere. Vertical wind shear is the change in wind speed with height.

• Easterly Waves: Also called tropical waves, this is an inverted trough of low pressure moving generally westward in the tropical easterlies. A trough is defined as a region of relative low pressure. The majority of tropical cyclones form from easterly waves.
  
  
• West African Disturbance Line (WADL): This is a line of convection (similar to a squall line) which forms over West Africa and moves into the Atlantic Ocean. WADL's usually move faster than tropical waves.
  
  
• TUTT: A TUTT (Tropical Upper Tropospheric Trough) is a trough, or cold core low in the upper atmosphere, which produces convection. On occasion, one of these develops into a warm-core tropical cyclone.
  
  
• Old Frontal Boundary: Remnants of a polar front can become lines of convection and occasionally generate a tropical cyclone. In the Atlantic Ocean storms, this will occur early or late in the hurricane season in the Gulf of Mexico or Caribbean Sea.

Once a disturbance forms and sustained convection develops, it can become more organized under certain conditions. If the disturbance moves or stays over warm water (at least 80°F), and upper level winds remain weak, the disturbance can become more organized, forming a depression.

The warm water is one of the most important keys as it is water that powers the tropical cyclone (see image above right). As water vapor (water in the gaseous state) rises, it cools. This cooling causes the water vapor to condense into a liquid we see as clouds. In the process of condensation, heat is released. This heat warms the atmosphere making the air lighter still which then continues to rise into the atmosphere. As it does, more air moves in near the surface to take its place which is the strong wind we feel from these storms.

Therefore, once the eye of the storm moves over land will begin to weaken rapidly, not because of friction, but because the storm lacks the moisture and heat sources that the ocean provided. This depletion of moisture and heat hurts the tropical cyclone's ability to produce thunderstorms near the storm center. Without this convection, the storm rapidly diminishes.

Tropical Cyclone Formation Basin

Given that sea surface temperatures need to be at least 80°F (27°C) for tropical cyclones form, it is natural that they form near the equator. However, with only the rarest of occasions, these storms do not form within 5° latitude of the equator. This is due to the lack of sufficient Coriolis Force, the force that causes the cyclone to spin. However, tropical cyclones form in seven regions around the world.

Tropical Cyclone Structure

The main parts of a tropical cyclone are the rainbands, the eye, and the eyewall. Air spirals in toward the center in a counter-clockwise pattern in the northern hemisphere (clockwise in the southern hemisphere), and out the top in the opposite direction. In the very center of the storm, air sinks, forming an "eye" that is mostly cloud-free.

The Eye

The hurricane's center is a relatively calm, generally clear area of sinking air and light winds that usually do not exceed 15 mph (24 kph) and is typically 20-40 miles (32-64 km) across. An eye will usually develop when the maximum sustained wind speeds go above 74 mph (119 kph) and is the calmest part of the storm.

But why does an eye form? The cause of eye formation is still not fully understood. It probably has to do with the combination of "the conservation of angular momentum" and centrifugal force. The conservation of angular momentum means is objects will spin faster as they move toward the center of circulation. So air increases it speed as it heads toward the center of the tropical cyclone. One way of looking at this is watching figure skaters spin. The closer they hold their hands to the body, the faster they spin. Conversely, the farther the hands are from the body the slower they spin. In tropical cyclone, as the air moves toward the center, the speed must increase.

However, as the speed increases, an outward-directed force, called the centrifugal force, occurs because the wind's momentum wants to carry the wind in a straight line. Since the wind is turning about the center of the tropical cyclone, there is a pull outward. The sharper the curvature, and/or the faster the rotation, the stronger is the centrifugal force.

Around 74 mph (119 kph) the strong rotation of air around the cyclone balances inflow to the center, causing air to ascend about 10-20 miles (16-32 km) from the center forming the eyewall. This strong rotation also creates a vacuum of air at the center, causing some of the air flowing out the top of the eyewall to turn inward and sink to replace the loss of air mass near the center.

This sinking air suppresses cloud formation, creating a pocket of generally clear air in the center. People experiencing an eye passage at night often see stars. Trapped birds are sometimes seen circling in the eye, and ships trapped in a hurricane report hundreds of exhausted birds resting on their decks. The landfall of hurricane Gloria (1985) on southern New England was accompanied by thousands of birds in the eye.

The sudden change of very strong winds to a near calm state is a dangerous situation for people ignorant about a hurricane's structure. Some people experiencing the light wind and fair weather of an eye may think the hurricane has passed, when in fact the storm is only half over with dangerous eyewall winds returning, this time from the opposite direction within a few minutes.

The Eyewall

Where the strong wind gets as close as it can is the eyewall. The eyewall consists of a ring of tall thunderstorms that produce heavy rains and usually the strongest winds. 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.

Rainbands

Curved bands of clouds and thunderstorms that trail away from the eye wall in a spiral fashion. These bands are capable of producing heavy bursts of rain and wind, as well as tornadoes. There are sometimes gaps in between spiral rain bands where no rain or wind is found.

In fact, if one were to travel between the outer edge of a hurricane to its center, one would normally progress from light rain and wind, to dry and weak breeze, then back to increasingly heavier rainfall and stronger wind, over and over again with each period of rainfall and wind being more intense and lasting longer. 

Tropical Cyclone Size

Typical hurricane strength tropical cyclones are about 300 miles (483 km) wide although they can vary considerably. as shown in the two enhanced satellite images below. Size is not necessarily an indication of hurricane intensity. Hurricane Andrew (1992), the second most devastating hurricane to hit the United States, next to Katrina in 2005, was a relatively small hurricane.

On record, Typhoon Tip (1979) was the largest storms with gale force winds (39 mph/63 km/h) that extended out for 675 miles (1087 km) in radius in the Northwest Pacific on 12 October, 1979. The smallest storm was Tropical Cyclone Tracy with gale force winds that only extended 30 miles (48 km) radius when it struck Darwin, Australia, on December 24, 1974.

However, the hurricane's destructive winds and rains cover a wide swath. Hurricane-force winds can extend outward more than 150 miles (242 km) for a large one. The area over which tropical storm-force winds occur is even greater, ranging as far out as almost 300 miles (483 km) from the eye of a large hurricane.

The strongest hurricane on record for the Atlantic Basin is Hurricane Wilma (2005). With a central pressure of 882 mb (26.05"), Wilma produced sustained winds of 175 mph (280 km/h).

Storm Surge 

Storm Surge vs. Storm Tide

• STORM SURGE is an abnormal rise of water generated by a storm, over and above the predicted astronomical tide.
• STORM TIDE is the water level rise during a storm due to the combination of storm surge and the astronomical tide.

Storm Surge vs. Storm Tide

Factors Impacting Surge

Wind and Pressure Components of Hurricane Storm Surge

The maximum potential storm surge for a particular location depends on a number of different factors. Storm surge is a very complex phenomenon because it is sensitive to the slightest changes in storm intensity, forward speed, size (radius of maximum winds-RMW), angle of approach to the coast, central pressure (minimal contribution in comparison to the wind), and the shape and characteristics of coastal features such as bays and estuaries.

Other factors which can impact storm surge are the width and slope of the continental shelf. A shallow slope will potentially produce a greater storm surge than a steep shelf. For example, a Category 4 storm hitting the Louisiana coastline, which has a very wide and shallow continental shelf, may produce a 20-foot storm surge, while the same hurricane in a place like Miami Beach, Florida, where the continental shelf drops off very quickly, might see an 8 or 9-foot surge. 

Adding to the destructive power of surge, battering waves may increase damage to buildings directly along the coast. Water weighs approximately 1,700 pounds per cubic yard; extended pounding by frequent waves can demolish any structure not specifically designed to withstand such forces. The two elements work together to increase the impact on land because the surge makes it possible for waves to extend inland.

Although elevated, this house in North Carolina could not withstand the 15 ft (4.5 m) of storm surge that came with Hurricane Floyd (1999)

Additionally, currents created by tides combine with the waves to severely erode beaches and coastal highways. Buildings that survive hurricane winds can be damaged if their foundations are undermined and weakened by erosion.

Beachfront road and boardwalk damaged by Hurricane Jeanne (2004)

In confined harbors, the combination of storm tides, waves, and currents can also severely damage marinas and boats. In estuaries and bayous, salt water intrusion endangers the public health, kills vegetation, and can send animals, such as snakes and alligators, fleeing from flooded areas.

Damaged boats in a marina

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 kph) or less are called "tropical depressions". Once the tropical cyclone reaches winds of at least 39 mph (63 kph) they are typically called a "tropical storm" and assigned a name.

If maximum sustained winds reach 74 mph (119 kph), 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 kph),
  
  
• 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
  
  
• Just 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.

Earlier versions of this scale – known as the Saffir-Simpson Hurricane Scale – incorporated central pressure and storm surge as components of the categories. However, hurricane size (extent of hurricane-force winds), local bathymetry (depth of near-shore waters), topography, the hurricane's forward speed and angle to the coast also affect the surge that is produced.

For example, the very large Hurricane Ike (with hurricane force winds extending as much as 125 miles (200 kilometers) from the center) in 2008 made landfall in Texas as a Category 2 hurricane and had peak storm surge values of about 20 feet (6 meters). In contrast, tiny Hurricane Charley (with hurricane force winds extending at most 25 miles (40 kilometers) from the center) struck Florida in 2004 as a Category 4 hurricane and produced a peak storm surge of only about 7 feet (2.1 meters). These storm surge values were substantially outside of the ranges suggested in the original scale.

To help reduce public confusion about the impacts associated with the various hurricane categories as well as to provide a more scientifically defensible scale, the storm surge ranges, flooding impact and central pressure statements were removed from the scale and only peak winds are now employed.

Saffir-Simpson Hurricane Scale

Category One Hurricane (Sustained winds 74-95 mph, 64-82 kt, or 119-153 km/hr). Very dangerous winds will produce some damage

People, livestock, and pets struck by flying or falling debris could be injured or killed. Older (mainly pre-1994 construction) mobile homes could be destroyed, especially if they are not anchored properly as they tend to shift or roll off their foundations. Newer mobile homes that are anchored properly can sustain damage involving the removal of shingle or metal roof coverings, and loss of vinyl siding, as well as damage to carports, sunrooms, or lanais. Some poorly constructed frame homes can experience major damage, involving loss of the roof covering and damage to gable ends as well as the removal of porch coverings and awnings. Unprotected windows may break if struck by flying debris. Masonry chimneys can be toppled. Well-constructed frame homes could have damage to roof shingles, vinyl siding, soffit panels, and gutters. Failure of aluminum, screened-in, swimming pool enclosures can occur. Some apartment building and shopping center roof coverings could be partially removed. Industrial buildings can lose roofing and siding especially from windward corners, rakes, and eaves. Failures to overhead doors and unprotected windows will be common. Windows in high-rise buildings can be broken by flying debris. Falling and broken glass will pose a significant danger even after the storm. There will be occasional damage to commercial signage, fences, and canopies. Large branches of trees will snap and shallow rooted trees can be toppled. Extensive damage to power lines and poles will likely result in power outages that could last a few to several days. Hurricane Dolly (2008) is an example of a hurricane that brought Category 1 winds and impacts to South Padre Island, Texas.

Category Two Hurricane (Sustained winds 96-110 mph, 83-95 kt, or 154-177 km/hr). Extremely dangerous winds will cause extensive damage

Category Three Hurricane (Sustained winds 111-130 mph, 96-113 kt, or 178-209 km/hr).

Category Four Hurricane (Sustained winds 131-155 mph, 114-135 kt, or 210-249 km/hr).

Category Five Hurricane (Sustained winds greater than 155 mph, greater than 135 kt, or greater than 249 km/hr).

• Hurricane Katrina

Credit: NOAA, NASA, National Weather Service