How hurricanes develop and grow

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:

• A pre-existing disturbance with thunderstorms.
• Warm (at least 80ºF) ocean temperatures to a depth of about 150 feet.
• Light upper level winds that do not change much in direction and speed throughout the depths of the atmosphere (low wind shear).

Step 1 - Tropical Disturbance

A hurricane in the Atlantic Ocean usually starts as a “tropical disturbance” — defined as a group of thunderstorms that stretches at least 100 miles across and stays together for more than 24 hours.

During the summer, these disturbances often start as storms moving westward off the coast of Africa in what are known as “tropical waves.”

If meteorologists think a tropical disturbance may develop further, they’ll designate it as an “investigative area,” or invest.

Step 2 - Tropical Depression

In these early stages, the system appears on the satellite image as a relatively disorganized 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).

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.

Step 3 - Tropical Storm

When the pressure in the center of the tropical depression drops, air rushes in, creating strong winds. If the system strengthens and wind speeds rise past 39 mph, the system is dubbed a “tropical storm” and is given a name.

The US National Hurricane Center makes the call for when a tropical depression officially becomes a tropical storm. It relies on data from islands and buoys as well as from reconnaissance aircrafts that fly into the storms to measure wind speed.

About 100 tropical cyclones form each year worldwide. Many of them die out before they can grow stronger, only about half of them eventually strengthen into a hurricane (or typhoon).

Tropical storms can intensify quickly if they pass over a region of especially warm water and don’t face much wind shear. As that happens, the pressure in the center drops even further and the winds really pick up. The system gets rounder and often forms a clearly defined “eye.”

Step 4 - Hurricane

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. As the storm continues to strengthen 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.

Step 5 - Mature Hurricane

A mature hurricane is a well-oiled meteorological machine, but disruption of the processes that drive the storm (i.e. interaction with land or colder air feeding in) will begin to destroy the storm, and the disintegration of a hurricane can often be quick and dramatic.

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.

A Developed Hurricane’s Structure

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.

The Eye

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 Eyewall

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.

Hurricane Size

Contrary to how many weather maps appear, a hurricane is more than a point on a weather map, and its path is more than a line. Typical hurricanes are about 300 miles wide. It is a large system that can affect a wide area, requiring that precautions be taken far from where the eye is predicted to come ashore. Size is not necessarily an indication of hurricane intensity. Hurricane Andrew (1992), the most devastating hurricane of this century before Katrina, was a relatively small hurricane. 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 hurricane 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 These seemingly erratic changes make hurricanes difficult to forecast.

The Right Side of the Storm

As a general rule of thumb, the hurricane’s right side (relative to the direction it is travelling) is the most dangerous part of the storm because of the additive effect of the hurricane wind speed and speed of the larger atmospheric flow (the steering winds). The increased winds on the right side increase the storm surge. Tornadoes are also more common here.

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Background Picture: Waves crash over a seawall at the mouth of the Miami River from Biscayne Bay, Fla., as Hurricane Irma passes by, Sunday, Sept. 10, 2017, in Miami. (AP Photo/Wilfredo Lee)