In summer and fall, tropical bodies of water have been warmed by the sun and store fantastic amounts of energy as heat. Tropical cyclones use that energy as their driving power. But how do they start the engine?

Did You Know?
Cyclone formation is called cyclogenesis, which accounts for all types of cyclones (not just tropical cyclones).

As warm surface water in the tropics evaporates, the moist air mass rises into the atmosphere, expanding and cooling. It stops climbing when it has cooled enough to condense into clouds. When the water vapour forms droplets, it releases energy that warms the air and causes the clouds to rise further.

Did You Know?
As water vapour changes state from gas to liquid, it releases energy called the "latent heat of condensation".

To replace the air that has risen, surface winds sweep along the water, gathering warm, moist air before rising into the atmosphere. The storm system grows, using the heat energy of the water, into giant thunderstorms that rage over the ocean.

The Coriolis Effect causes air currents to deflect to the right in the northern hemisphere due to the rotation of the Earth (and to the left in the southern hemisphere). A low pressure area, or low, is a region where the atmospheric pressure is less than the surrounding area. Wind converges on a low and is deflected to the right of the centre of the low; this causes the wind to rotates counterclockwise around it.

Did You Know?
Counterclockwise rotation in the northern hemisphere and clockwise rotation in the southern hemisphere are called cyclonic circulation.

When a low interacts with the storm system, it can cause the storm system to rotate, adding energy to the low and creating a tropical cyclone. As the low strengthens, water vapour is thrown higher into the atmosphere and the tropical cyclone grows taller. Like a figure skater pulling their arms in to spin faster (conservation of angular momentum), as the cyclone stretches upward and converges it spins faster.

The engine has been started. If the conditions are just right and there is enough fuel, the tropical cyclone can grow until its maximum sustained wind speeds are greater than 119 km/h, making it a hurricane!

Learn more:

Demonstrations of Coriolis Effect

Saffir–Simpson Hurricane Scale: hurricane classification

Necessary conditions for the formation of tropical cyclones

Explanation of the Coriolis Effect

Tropical Cyclones: general

Tropical Cyclones: general

Article first published November 24, 2010.

Photo credit:

CarolAnne Black

As a masters student in the Department of Oceanography at Dalhousie University, I study how ice forms and gets moved by the currents in the Bay of Fundy in Nova Scotia.  I try to spend as much time as possible outside, playing ultimate frisbee, cycling, hiking and camping. My brother and I biked through the Rockies from Vancouver to Calgary last summer.

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