About Hurricanes

An Overview

Have you ever experienced a hurricane in person? If not, then the chances are that you have seen pictures of hurricanes and their effects of hurricanes on television, the internet or in newspapers and magazines.


Image sources: U.S. Navy, NASA, Getty Images.

A hurricane is an organized system of clouds and thunderstorms which rotate in a counterclockwise fashion around a center of low atmospheric pressure (the eye). Hurricanes generally form in the sub-tropical regions of the Earth, given that warm ocean waters are key to providing the warm and humid conditions in this region are key ingredients conducive to thunderstorm development. In the Northern Hemisphere, hurricane and tropical storm formation is most common from late-August through mid-October, the period during which the tropical ocean waters are typically the warmest.


Favorable regions for tropical cyclone formation.


Peak Of Season Frequency of tropical storm formation by date.

The tallest and strongest thunderstorms found within a hurricane make up the hurricane eyewall, which is also the part of the hurricane that experiences that strongest surface winds. In the radar picture (below, right), you can see that the inward spiraling winds associated with the hurricane result in spiraling lines of thunderstorms called rainbands. Between these rainbands, conditions may actually be rain free!


Image sources: Southwest Research Institute and NOAA

While the heavy rains which accompany the spiral rainbands and hurricane eyewall can have a major impact on communities which experience a landfalling hurricane, strong hurricane winds have both direct (damage due to strong winds) and indirect (damage due to wind-added storm surge flooding) effects on impacted communities.

Saffir-Simpson Scale

The strength of tropical cyclones is most commonly categorized by the magnitude of the cyclone’s surface winds, which rotate counter-clockwise at the low-levels in the Northern Hemisphere. These low-level winds are found to spiral inwards toward the center of the storm, before rising and then spiraling outward in a clockwise fashion in the upper atmosphere.

The Saffir-Simpson Scale is used to describe hurricane intensity. Initially formed as tropical depressions (an unorganized cluster of thunderstorms with surface wind speeds from 20 to 34 kt, these storms may strengthen to be reclassified as tropical storms(35 to 64 kt), and potentially hurricane strength (greater than 64kt). In the Saffir-Simpson Scale, five categories have been developed to describe the types of damage due to hurricane winds. These categories are presented below:

Category Sustained Winds Types of Damage Due to Hurricane Winds
1 74-95 mph
64-82 kt
119-153 km/h
Very dangerous winds will produce some damage: Well-constructed frame homes could have damage to roof, shingles, vinyl siding and gutters. Large branches of trees will snap and shallowly rooted trees may be toppled. Extensive damage to power lines and poles likely will result in power outages that could last a few to several days.
2 96-110 mph
83-95 kt
154-177 km/h
Extremely dangerous winds will cause extensive damage: Well-constructed frame homes could sustain major roof and siding damage. Many shallowly rooted trees will be snapped or uprooted and block numerous roads. Near-total power loss is expected with outages that could last from several days to weeks.
3
(major)
111-129 mph
96-112 kt
178-208 km/h
Devastating damage will occur: Well-built framed homes may incur major damage or removal of roof decking and gable ends. Many trees will be snapped or uprooted, blocking numerous roads. Electricity and water will be unavailable for several days to weeks after the storm passes.
4
(major)
130-156 mph
113-136 kt
209-251 km/h
Catastrophic damage will occur: Well-built framed homes can sustain severe damage with loss of most of the roof structure and/or some exterior walls. Most trees will be snapped or uprooted and power poles downed. Fallen trees and power poles will isolate residential areas. Power outages will last weeks to possibly months. Most of the area will be uninhabitable for weeks or months.
5
(major)
157 mph or higher
137 kt or higher
252 km/h or higher
Catastrophic damage will occur: A high percentage of framed homes will be destroyed, with total roof failure and wall collapse. Fallen trees and power poles will isolate residential areas. Power outages will last for weeks to possibly months. Most of the area will be uninhabitable for weeks or months.

While hurricane is located out over the open waters of the oceans, the winds rotating around the hurricane’s center or relatively uniform . However, hurricanes themselves on in motion! To the right of the hurricane’s path, the total speed of the winds represents the speed of the winds rotating around the storm’s center PLUS the forward speed of the hurricane. In the example shown below, the total speed of the winds to the right of the hurricane’s path would be:

Total Wind Speed (knots) = Storm Winds (80 knots) + Storm Motion (10 knots) = 90 knots

To the left of the hurricane’s path, the storm motion would have the opposite effect on the Total Wind Speed, given that the storm motion is in a direction that is opposite to the direction that the Storm Winds are blowing.

Hurricane Storm Surge

While the storm is located out over the open waters of the oceans, the force of the wind acting on the ocean surface will result in the development of waves. Given the strength of hurricane winds, and their persistent force that they apply in a given direction, hurricane winds can help the surface waves build to substantial heights. When these waves move onshore, they can cause considerable damage both due to their considerable force, but also due to the effects of flooding on property, electrical distribution systems, etc. The storm tide represents the height of the landfalling waves, with the the storm surge representing the added height of the waves above the normal high tide for a given time and location.

According to the National Hurricane Center, Hurricane Katrina (2005) was responsible for an incredible storm surge of 27.8 feet at Pass Christian, MS, making it the highest storm surge ever recorded on the U.S. Coast. The storm surge was said to have penetrated inland by as much as six miles along portions of coastal Mississippi and up to 12 feet inland along bays and rivers. The surge severely strained the levee system in the New Orleans area, resulting in levees and floodwalls being overtopped and/or breached, ultimately flooding about 80% of the city of New Orleans with depths up to about 20 feet (http://www.stormsurge.noaa.gov/event_history_2000s.html).