A Homeowner's Guide to Hurricanes by AustinRealEstate.com
Hurricane Fast Facts
What is a Hurricane?
- A "hurricane" is the most severe category of the meteorological phenomenon known as the "tropical
- Tropical cyclones are low pressure systems that have thunderstorm activity and rotate counterclockwise. A tropical cyclone that has winds of 38 mph (33 kt) or less is called a tropical depression. When the tropical cyclone's winds reach 39-73 mph (34-63 kt), it is called a tropical storm. When the winds exceed 74 mph (64 kt), the storm is considered to be a hurricane.
- The Saffir-Simpson Hurricane Scale defines hurricane strength by categories. A Category 1 storm is the weakest hurricane (winds 74-95 mph or 64-82 kt); a Category 5 hurricane is the strongest (winds greater than 155 mph or 135 kt).
- The category of the storm does not necessarily relate directly to the damage it will inflict. Lower category storms (and even tropical storms) can cause substantial damage depending on what other weather features they interact with, where they strike, and how slow they move.
- Hurricane Facts For Kids
- Ten Facts about Hurricanes!
- Hurricanes: Interesting Facts and F.A.Q. (PDF)
Anatomy of a Hurricane
- Typical hurricanes are about 300 miles wide although they can vary considerably in size.
- The eye at a hurricane's center is a relatively calm, clear area approximately 20-40 miles across.
- The eyewall surrounding the eye is composed of dense clouds that contain the highest winds in the storm.
- The storm's outer rainbands (often with hurricane or tropical storm-force winds) are made up of dense bands of thunderstorms ranging from a few miles to tens of miles wide and 50 to 300 miles long.
- Hurricane-force winds can extend outward to about 25 miles in a small hurricane and to more than 150 miles for a large one. Tropical storm-force winds can stretch out as far as 300 miles from the center of a large hurricane.
- Frequently, the right side of a hurricane is the most dangerous in terms of storm surge, winds, and tornadoes.
- A hurricane's speed and path depend on complex ocean and atmospheric interactions, including the presence or absence of other weather patterns. This complexity of the flow makes it very difficult to predict the speed and direction of a hurricane.
- Do not focus on the eye or the track-hurricanes are immense systems that can move in complex patterns that are difficult to predict. Be prepared for changes in size, intensity, speed, and direction.
- What is a Hurricane?
How Tropical Cyclones are Observed
- Direct measurements of tropical storm and hurricane dimensions and wind speeds are taken primarily by reconnaissance aircraft, although ships and buoys also take important measurements. Once a hurricane is near and/or on land, Automated Surface Observation Systems (ASOS) provide surface conditions, and radio sondes take upper air measurements.
- Indirect observational methods include satellite imagery and Doppler radar. In particular, satellites have greatly improved our ability to monitor and understand hurricanes. Radar data are important once the storm comes close to shore and after landfall for forecasting hurricane-related weather
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 ten tropical storms develop over the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. Many of these remain over the ocean. Six of these storms become hurricanes each year.
In an average 3-year period, roughly five hurricanes strike the United States coastline, killing approximately 50 to 100 people anywhere from Texas to Maine. Of these, two are typically major hurricanes (winds greater than 110 mph).
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.
- What Are Hurricanes?
- What is the difference between a hurricane and a typhoon?
- National Ocean Service: What is a hurricane?
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.
When the winds from these storms reach 39 mph (34 kt), the cyclone is given a name. Years ago, an international committee developed six separate lists of names for these storms. Each list alternates between male and female names. The use of these easily remembered names greatly reduces confusion when two or more tropical cyclones occur at the same time. Each list is reused every six years, although hurricane names that have resulted in substantial damage or death are retired.
The Birth of a Tropical Cyclone
Tropical cyclones form over warm waters from pre-existing disturbances. These disturbances typically emerge every three or four days from the coast of Africa as "tropical waves" that consist of areas of unsettled weather. Tropical cyclones can also form from the trailing ends of cold fronts and occasionally from upper-level lows. The process by which a tropical cyclone forms and subsequently strengthens into a hurricane depends on at least three conditions shown below:
1. A pre-existing disturbance with thunderstorms
2.Warm (at least 80ºF) ocean temperatures to a depth of about 150 feet
3. Light upper level winds that do not change much in direction and speed throughout the depth of the atmosphere (low wind shear)
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.
Stages of Hurricane Development:
Growth and Maturity
In these early stages, the system appears on the satellite image as a relatively unorganized cluster of thunderstorms. If weather and ocean conditions continue to be favorable, the system can strengthen and become a tropical depression (winds less than 38 mph or 33 kt). 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. If the storm continues to strengthen to tropical storm status (winds 39-73 mph, 34-63 kt), the bands of thunderstorms contribute additional heat and moisture to the storm. The storm becomes a hurricane when winds reach a minimum of 74 mph (64 kt). At this time, the cloud-free hurricane eye typically forms because rapidly sinking air at the center dries and warms the area. During their life span, hurricanes can last for more than two weeks over the ocean and can travel up the entire Atlantic Coast.
The Storm's End
Just as many factors contribute to the birth of a hurricane, there are many reasons why a hurricane begins to decay. Wind shear can tear the hurricane apart. Moving over cooler water or drier areas can lead to weakening as well. Landfall typically shuts off the hurricane's main moisture source, and the surface circulation can be reduced by friction when it passes over land. Generally, a weakening hurricane or tropical cyclone can reintensify if it moves into a more favorable region or interacts with mid-latitude frontal systems.
The main parts 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.
- How Does a Hurricane Form?
- Stages of development - tropical storm
- Hurricane Facts
- Hurricane Genesis: Birth of a Hurricane
The hurricane's center is a relatively calm, clear area usually 20-40 miles across. 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.
The dense wall of thunderstorms surrounding the 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.
The Spiral Rainbands
The storm's outer rainbands (often with hurricane or tropical storm-force winds) can extend a few hundred miles from the center. Hurricane Andrew's (1992) rainbands reached only 100 miles out from the eye, while those in
Hurricane Gilbert (1988) stretched over 500 miles. These dense bands of thunderstorms, which spiral slowly counterclockwise, range in width from a few miles to tens of miles and are 50 to 300 miles long. Sometimes the
bands and the eye are obscured by higher level clouds, making it difficult for forecasters to use satellite imagery to monitor the storm.
Typical hurricanes are about 300 miles 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 most devastating hurricane of this century, was a relatively small hurricane. Hurricane destructive winds and rains cover a wide swath. Hurricane-force winds can extend outward to about 25 miles from the storm center of a small hurricane and to more than 150 miles for a large one. The area over which tropical storm-force winds occur is even greater, ranging as far out as almost 300 miles from the eye of a large hurricane.
Hurricane Circulation and 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 on the northern edge of the orange area in Hurricane Fran would experience winds from the east, while a boat on the southern edge would have westerly winds.
A hurricane's speed and path depend on complex interactions between the storm with its own internal circulations and the earth's atmosphere. The air in which the hurricane 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 hurricane. In turn, it can modify the environment around the storm. Typically, a hurricane's forward speed averages around 15-20 mph. However, some hurricanes stall, often causing devastatingly heavy rain. Others can accelerate to more than 60 mph. Hurricane Hazel (1954) hit North Carolina on the morning of 15 October; fourteen hours later it reached Toronto, Canada where it caused 80 deaths. Some hurricanes follow a fairly straight course, while others loop and wobble along the path.
- Storm surge is the greatest potential threat to life and property associated with hurricanes.
- A storm surge is a large dome of water, 50 to 100 miles wide, that sweeps across the coastline near where a hurricane makes landfall. It can be more than 15 feet deep at its peak.
- The level of surge in a particular area is primarily related to the intensity of the hurricane and slope of the continental shelf.
- The Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model is used by communities to evaluate storm surge threat from different categories of hurricanes striking from various directions.
- Because storm surge has the greatest potential to kill more people than any of the other hurricane hazards, it is wise to err on the conservative side by planning for a storm that is one category more intense than is forecast.
- Typically, the more intense the storm (in terms of the Saffir-Simpson Hurricane Scale), the more wind damage a community will sustain, particularly if it does not have an effective mitigation program and has not prepared in advance for the storm.
- Tropical storm-force winds (39-73 mph) can also be dangerous, and it is wise to have evacuations completed before they reach your area.
- Hurricanes (and some tropical storms) typically produce widespread rainfall of 6 to 12 inches or more, often resulting in severe flooding.
- Inland flooding has been the primary cause of tropical cyclone-related fatalities over the past 30 years
- Rains are generally heaviest with slower moving storms (less than 10 mph).
- The heaviest rain usually occurs to the right of the cyclone track in the period 6 hours before and 6 hours after landfall. However, storms can last for days, depending on what inland weather features they interact with.
- Large amounts of rain can occur more than 100 miles inland where flash floods and mudslides are typically the major threats.
- Tornadoes are most likely to occur in the right-front quadrant of the hurricane. However, they are also often found elsewhere in the rainbands.
- Typically, the more intense a hurricane is, the greater the tornado threat.
- Tornado production can occur for days after landfall.
- Home Safety Preparedness for Tornadoes by AustinRealEstate.com
- Most tornadoes occur within 150 miles of the coast.
- The National Weather Service's Doppler radar systems can provide indications of tornadoes from a few minutes to about 30 minutes in advance. Consequently, preparedness is critical.
- Hurricanes - Ready.gov
- Hurricane safety tips: Learn what to do before, during and after a hurricane
- Hurricane Safety Tips and Resources
- Hurricanes and Other Tropical Storms
- Tornadoes - ready.gov
- What to Do Before the Tropical Storm or Hurricane
- Hurricane Safety
- Austin Real Estate Search
- NWS Hurricane Safety Brochure - National Weather Service (PDF file)
- Preparing for a Hurricane or Tropical Storm
- Hurricane Preparedness and Response
- Hurricane Preparedness - Be Ready
By: Jim Olenbush