P!N Facts – How Hurricanes Are Born

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This diagram details the conditions needed for hurricanes to form. Image courtesy of Windows2universe.org.

For residents of the United States, Central America, Caribbean nations and even Canada, hurricane and tropical storm season is a six-month-long period when circumstances are prime for the formation of destructive wind, rain and waves. While only a portion of storms that form in the Atlantic from June to November form into hurricanes, and even fewer threaten land, it helps to understand the factors that lead to tropical storm formation in the first place.

Meteorologists from the National Hurricane Center (NHC) forecast storms days ahead of their expected landfall. These warnings, as well as any evacuation orders from local officials, should be heeded, but it also helps to understand exactly what the storm is that is heading your way. Below is information about how hurricanes form, and travel, and how they are tracked by meteorologists. For more information, visit the National Oceanic and Atmospheric Administration’s Hurricane Research Division information page here.

Q: What are tropical cyclones, hurricanes and tropical storms? What is the difference between them?

A: Strong tropical storms go by many names, depending on where they are located. In the Atlantic, Northeast Pacific Ocean east of the International Date Line and South Pacific Ocean, they are “hurricanes.” In the Northwest Pacific Ocean, west of the International Date Line, they are “typhoons.”

A tropical cyclone is the term used for what weather scientists call a “non-frontal synoptic scale low-pressure system” forming over tropical or sub-tropical waters with organized thunderstorm activity and definite circular (or “cyclonic”) surface wind circulation. If maximum sustained surface winds are less than 39 miles per hour, the storm is usually a tropical depression. Between 39 mph and 74 mph, it’s a tropical storm; beyond 74 mph, it is classified as a hurricane or typhoon.

In the Indian Ocean and Southwest Pacific Ocean, these storms are classified as “severe tropical cyclone,” “very severe cyclonic storm,” or “tropical cyclone.”

Q: What are the stages of tropical storm formation?

A: Tropical storms build progressively from a large collection of thunderstorm activity to a more organized storm system.

Tropical Disturbance – A discrete tropical weather system of apparently organized thunderstorms – generally 200 to 600 km (100 to 300 nautical miles) in diameter – originating in the tropics or subtropics, having a “non-frontal migratory character” (meaning it is not related to the general transition area between two large bodies of air), and staying that way for 24 hours or more.

Tropical Depression – A tropical cyclone in which the maximum sustained wind speed (using the U.S. 1 minute average standard) is up to 38 mph. Depressions have a closed circulation (counterclockwise winds blowing around a center of low pressure in the Northern Hemisphere).

Tropical Storm – A tropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1 minute average standard) ranges from 39 mph to 73 mph. The storm formation is usually more concentrated near the center with outer rainfall organizing into distinct bands.

The wind speeds mentioned here are for those measured or estimated as the top speed sustained for one minute at 10 meters above the surface. Peak gusts would be on the order of 10-25 percent higher.

Q: Why is it called a “hurricane?”

A: The name is derived from Hurican, the Carib god of evil in an Arawakan language (adapted from native groups by Spanish and Portuguese explorers to Central and South America and the Caribbean islands). The Carib god is thought to be derived from the Mayan god Hurakan, a creator god who brought about dry land in the Caribbean and later caused a great storm and flood.

Q: How do tropical cyclones form ?

A: Several conditions must be met for a disturbance to form into a cyclone:
1. Warm ocean waters of at least 26.5°C (80°F) throughout a depth of at least 150 feet.

2. An atmosphere which cools fast enough with height such that it is potentially unstable to moist “convection,” or thunderstorm activity. It is the thunderstorm activity which allows the heat stored in the ocean waters to be liberated for the tropical cyclone development.

3. Relatively moist layers near the mid-troposphere (3 miles above sea level). Dry mid levels are not conducive for allowing the continuing development of widespread thunderstorm activity.

4. A minimum distance of at least 300 miles from the equator. For tropical cyclones to form, there is a requirement for non-negligible amounts of the Coriolis force to provide low pressure that maintains the storm.

5. A pre-existing near-surface disturbance with sufficient air rotation and air entering the storm system. Tropical cyclones cannot be generated spontaneously.

6. Low values (less than about 10 meters/second or 23 mph) of vertical wind shear between the surface and the upper troposphere. Vertical wind shear is the magnitude of wind change with height. Large values of vertical wind shear disrupt cyclone formation.

These conditions must be met for a storm to form, but do not guarantee tropical cyclone formation.

Q: What is the relationship between a hurricane’s size and its strength?

A: There is very little correlation between a storm’s intensity (either in wind speed or central pressure) and its radius. Changes in size and strength of a storm are thought to be independent of each other. For example, Hurricane Andrew was relatively small (a radius of about 90 miles) but very powerful Category 5 hurricane (winds of 165 mph at landfall in Florida in 1992).