Hurricanes are one of the most powerful forces on Earth.  These natural disasters have the potential to devastate entire coastal landscapes, putting the lives of millions at risk.  In the summer of 2005, Hurricane Katrina resulted in $108 billion in property damage and the deaths of 1,836 people, while Hurricane Sandy in 2012 cost $71.4 billion and resulted in 285 deaths.  Each year, during hurricane season, a close eye is kept on storms in the tropical regions that could grow to become hurricanes.

How a Hurricane Forms

In order for a hurricane to form, six criteria must be met:

Warm ocean waters of at least 78°F (26°C):   Tropical cyclones thrive off the heat from the warm tropical and subtropical oceans. Warm ocean water evaporates from the surface and is taken in by the developing cyclone. As the water vapor rises, it eventually condenses into liquid water, forming clouds and releasing energy in the form of heat. Tropical cyclones use the released energy to gain organization and strength.

The warmer the water, the more energy can be extracted during condensation, and the stronger the storm can grow. However, tropical cyclones require a large temperature difference between the ocean surface and upper-level air to be most efficient at turning the heat energy into mechanical energy (in the form of strong winds).

At least 5° latitude from the equator:  Tropical cyclones get their spin from the rotation of the earth. At the equator, the influence of Earth’s rotation is zero and it increases moving toward the poles (known as Coriolis force). While there is nothing special about 5º degrees latitude, there is just enough influence from the earth’s spin to be favorable for tropical cyclone development.

wind shearLow vertical wind shear:  Vertical wind shear is the difference in wind speed and direction at two different heights in the atmosphere. Tropical cyclones have trouble developing in high shear environments because it displaces the thunderstorms in the cyclone away from its center of circulation. The thunderstorms are the primary location for the cyclone to gain energy through condensation (thinking back to the first criteria). By moving the thunderstorms away from the center of circulation, the rotation weakens because it is not receiving a steady supply of energy.

Moisture in the mid-troposphere:  Tropical cyclones also require a moist mid-troposphere to help keep their clouds and thunderstorms in tact. Dry air introduced into the mid-levels will begin eating away at the cyclone’s clouds. Furthermore, the dry air can become wrapped into the circulation of the cyclone, disrupting the thunderstorms and causing asymmetries in the structure of the cyclone.

The troposphere is the lowest layer of Earth’s atmosphere. It starts at Earth’s surface and goes up to a height of 7 to 20 km (4 to 12 miles, or 23,000 to 65,000 feet) above sea level.

Unstable Conditions:  Tropical cyclones contain thunderstorms, which ingest moist air near the surface. The most effective way to bring the air near the surface up into the thunderstorms is if that air is unstable. Unstable air is typically very warm, moist air (such as over the tropical oceans) that, when lifted, will continue to rise on its own. The more unstable the air is, the more rapidly it will rise. Rapidly rising air helps to create vigorous thunderstorms which contribute to the overall tropical cyclone.

A pre-existing disturbance:  A pre-existing disturbance can be considered a “seedling” that, if placed under the previous five conditions, may grow to be a tropical cyclone.

Hurricane Formation
Source: Weather Underground


The Anatomy of a Hurricane

Hurricanes are composed of three parts:

The Eye

The eye is the center of the storm. It is the most calm place in a hurricane since conditions are clear with light winds usually less than 15 mph. The eye is typically 20 to 40 miles across; however, they have been known to be a small as a few miles and much greater than 40 miles. It doesn’t usually develop until winds have reached hurricane strength – exceeding 74 mph – and thus is a good indicator of a strong, organized storm.

The Eye Wall

The eye wall surrounds the eye. The strongest winds and heaviest rains are found in the eye wall, making it the most dangerous part of the storm. It is made up of deep convective thunderstorms, which form a complete ring around the eye. Contraction or expansion of the eye wall can cause changes in wind speed and storm strength. As a tropical cyclone grows and changes, it can build concentric eye walls that replace the original eye wall. Changes in eye wall size and eye wall replacement pose the greatest challenges in forecasting the strength of the storm.

Rain Bands

Rain bands are long, arching bands of clouds and thunderstorms that spiral out from the eye wall. Heavy bursts of rain and wind are usually associated with rain bands. These structures form the outer most fringes of the tropical cyclone structure, and the winds contained within the bands decrease outward from the eye wall. With landfalling hurricanes, tornadoes are a common threat associated with rain bands coming onshore. Also, gaps between the bands are often calm with no wind or rain.

Anatomy of a Hurricane
Source: NOAA, USGS, Infographic World/Popular Science

When and Where Hurricanes Strike

The Atlantic hurricane season is from June 1 to November 30, but most hurricanes occur during the fall months. The Eastern Pacific hurricane season is from May 15 to November 30.

Tropical storms occur in several of the world’s oceans, and except for their names, they are essentially the same type of storm. In the Atlantic Ocean, Gulf of Mexico, and the Eastern Pacific Ocean, they are called hurricanes. In the Western Pacific Ocean, they are called typhoons. In the Indian Ocean, the Bay of Bengal, and Australia, these types of storms are called cyclones.

Hurricane Strength and Damage Potential:  The Saffir-Simpson Hurricane Scale

The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 rating based on a hurricane’s sustained wind speed. This scale estimates potential property damage.  Hurricanes reaching Category 3 and higher are considered major hurricanes because of their potential for significant loss of life and damage.

Saffir Simpson Scale for Hurricane Strength

Damage can be done by the wind itself, flying debris, flooding, and a storm surge.  A storm surge is a coastal flood or tsunami-like phenomenon of rising water commonly associated with low pressure weather systems (like tropical cyclones), the severity of which is affected by the shallowness and orientation of the water body relative to storm path, and the timing of tides. Most casualties during tropical cyclones occur as the result of storm surges.

Storm Surge from Hurricane
Source: NOAA, The Comet Program

To learn more about hurricanes, to track their location, and view historical data, visit the National Hurricane Center’s website.