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How hurricanes form! (a child-friendly version)

The equator, shown by the red line, is the imaginary line that runs through the exact middle of the earth.

Each year, we hear about hurricanes that might be coming near us in Florida. These are very large and powerful storms that form over the ocean waters in the warmer months of the year, all around the world. Here, we call them hurricanes, but they’re called typhoons or cyclones in other places.

But, have you ever wondered how these big storms form?

It starts with warm water, moist air, and an area of low pressure. So, storms that become hurricanes start near the equator, where the water is at least 80 degrees Fahrenheit.

Evaporation and Condensation

That warm, ocean water evaporates into the warm air, and rises into the sky.

Evaporation is the process where liquids change to a gas. For example, if you leave a cup of water out in the sunlight for a few hours, a bit of that water will be gone next time you check on it. That is because the molecules of the water are heated by the sun, which causes them to change from a liquid form to a gas form. The gas form of water is called water vapor.

Evaporation is a normal part of the Earth’s water cycle, also called the hydrologic cycle. Another important part of the water cycle is condensation. Condensation occurs when water vapor—a gas—turns back into liquid water. This is how clouds form.

How hurricanes form

View of a hurricane from space. [CREDIT: NASA, via].

Evaporation and condensation happen on a very large scale when a hurricane forms. The warm, ocean water evaporates, and rises into the sky, creating big clouds.

As the warm air evaporates, that leaves low pressure near the surface of the ocean, or what is known as a low-pressure system. A low-pressure system happens because there are fewer molecules near the surface of the ocean, allowing the molecules that are still there to have more room and spread out.

But, the cooler air above has a lot of molecules who also want some room to spread out. So that cooler air swoops down to the low-pressure area, and heats up as it does. So, it becomes warm air, and then wants to rise.

The process continues, with warm air evaporating and rising, and the cooler air coming down toward the ocean’s surface. This happens over and over again, creating giant clouds we call cumulonimbus clouds.

Now, add in the way tropical winds and the rotation of the Earth push on and twist the storm, causing it to spin, and a tropical storm starts to form.

Because of the way storms get pushed on the Earth, storms that form north of the equator spin in the opposite direction of a clock (which is called counter-clockwise), but storms that start south of the equator spin the same way as a clock.

Once the wind system around the storm reaches at least 74 miles per hour, that storm officially becomes a hurricane!