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The Life & Times of Hurricanes
Even though they can grow into monstrously powerful storms, tropical cyclones are finicky. They need to have all the right conditions before they can form or get stronger. If conditions are right, they can grow from a small group of thunderstorms into a full-fledged powerhouse. But when those same conditions disappear, the storms weaken and die. You'll learn about the different phases in the life of a tropical cyclone on this page.
Nearly all tropical storms and hurricanes start out as tropical disturbances. These are areas of unsettled weather and thunderstorms in the tropics. Sometimes, tropical disturbances are created by cold fronts that sneak down into tropical areas. Sometimes, low pressure centers spinning high in the atmosphere manage to spin themselves down to the surface of a tropical ocean or sea. During hurricane season in the Atlantic Ocean and Gulf of Mexico, tropical disturbances often grow from a pattern of stormy weather, called a tropical wave, that blows onto the Atlantic from Africa.
This satellite picture shows three tropical cyclones (next to arrows 1, 2, and 3) marching across the Atlantic Ocean toward the Caribbean Sea and North America. These storms were probably born from African tropical waves.
Several important ingredients are needed for a tropical disturbance to become a tropical cyclone and later strengthen into a tropical storm or hurricane:
This picture shows the birth of a tropical cyclone. All the right ingredients are in place for the storm to grow.
When the system gets organized and begins to circulate, it is called a tropical depression. It's called a depression because it has low or depressed air pressure in the center. Low air pressure causes winds, as air rushes inward toward the center. The lower the air pressure, the stronger the winds.
If the tropical depression strengthens and its wind speeds reach more than 33 knots, the storm becomes a tropical storm and gets a name. If it keeps getting stronger and the winds reach 64 knots, the storm becomes a hurricane. This table shows the different storm levels.
| Storm Levels | Miles per Hour (mph) | Kilometers per Hour (kph) | Knots (kts) |
|---|---|---|---|
|
Tropical Depression |
0 to 38 |
0 to 62
|
0 to 33 |
|
Tropical Storm |
39 to 73 |
63 to 118 |
34 to 63 |
|
Hurricane |
74+ |
119+ |
64+ |
Try to answer three questions about the following satellite image.
Question 1: Which storm is strongest? (Choose an answer.)
Question 2: Which storm is weakest? (Choose an answer.)
Question 3: How strong are the winds in Tropical Storm Fran? (Choose an answer.)
As the storm gets stronger, it gets rounder and the eye begins to form. Really strong hurricanes have very clear round eyes like the one in this picture. The most intense winds in a hurricane are in the thunderstorms that form the eyewall.
Tropical cyclones strengthen when all of the conditions are right for them. One of the most important conditions is warm ocean water. If a storm moves over a patch of warm water, it can strengthen quickly.
If conditions are just right, the tropical disturbance can grow from a bunch of thunderstorms into an organized system. Scientists can tell this is happening when they see the whole group of thunderstorms starting to spin or circulate. In the Northern Hemisphere, the circulation is counterclockwise; in the Southern Hemisphere, the circulation is clockwise.
Here are pictures of two fully-grown cyclones, one in the Northern Hemisphere and the other in the Southern Hemisphere. Can you tell which is which?
Question: Where is each storm? (Choose the correct answer.)
Hurricanes need to have the right conditions to survive. Some of the most important conditions are warm ocean water and weak winds at high levels in the atmosphere. When hurricanes move over cool water or land, they can weaken very quickly. Strong upper-level winds can also quickly tear a hurricane apart.
One very common way that hurricanes die is by moving over cool water. This picture shows the average sea surface temperatures in the world during the month of November. The warmest seas are colored red. Tropical cyclones can survive in tropical areas with warm seas, but when they move out over cooler waters such as the yellow and green areas of the Atlantic Ocean or along the west coast of North America, they quickly start to lose their power.
When hurricanes move over land, they usually start to weaken quickly because they no longer have their source of fuel: warm moist air above the sea.
Here are two satellite pictures of a tropical storm that moved over land. In the picture on the left, the storm is a hurricane that has just hit the coast of Florida and Alabama. The parts that are colored red and light blue are the strongest parts of the hurricane.
The picture on the right shows the same storm 24 hours after it has become much weaker over land. There are no red-colored areas and the light blue areas are much smaller.
Sometimes, though, hurricanes can get stronger for a little while when they move over land if the land is very warm and swampy. This is because warm swamplands can provide a lot of warm moist air to a hurricane. Some scientists think this happened when Hurricane Andrew moved over the Florida Everglades in 1992.
Hurricanes also need to have light winds at high levels in the atmosphere. If a hurricane runs into strong winds or other systems at high levels, it can be torn apart.
Hurricanes weaken when they lose the right conditions. When winds slow below 119 kilometers per hour, the system becomes a tropical storm. When they slow even more, below 63 kilometers per hour, a tropical storm becomes a tropical depression. These storms might not have strong winds, but they can have a lot of rain and cause dangerous floods. Sometimes, hurricanes move over land or cool water and weaken, then they move back over warm water and begin to strengthen again.
