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How do tropical cyclones form
Tuesday, March 17, 2015
Anyone who lives or vacations in the tropics knows that the weather is usually warm with
gentle breezes and occasional thunderstorms.
It seems surprising that these quaint conditions can turn into a ferocious storm that can
potentially disrupt the lives of millions of people. How does this happen?
It all begins with what meteorologists call a "tropical disturbance", or a group of
thunderstorms over warm tropical waters.
As low-level winds flow into the disturbance, they evaporate water from the ocean surface.
This process transfers energy from the ocean into the atmosphere.
When the winds arrive at the disturbance, they rise up and release that energy into the air
as they form clouds and precipitation. This warms the air and makes it buoyant, almost such
as a hot air balloon, and encourages more warm/moist air to flow in from the outside.
As the air moves toward the center of the disturbance, it "curves" or "spirals", rather than
flowing in a straight line.
This spiral effect comes from the rotation of the Earth — as air moves over large distances,
the Earth moves underneath it, producing a spiral effect. Meteorologists call this the
"Coriolis Effect".
The curved-band features that many of you see in the Cyclone Center images are curved
because of this effect. For this reason, tropical cyclones cannot form near the Equator; the
Coriolis Effect is too small there to provide the needed rotation.
If the atmospheric and ocean conditions remain favourable, the energy brought in by the
incoming air accumulates in the center of the disturbance, leading to a drop in atmospheric
pressure. This in turn increases the speed of the wind and the incoming energy, which then
leads to even larger drops in pressure. Once the wind speeds reach a certain threshold, a
tropical cyclone is born.
Interestingly, only about 7 per cent of tropical disturbances form into tropical cyclones; the
rest are destined to be absorbed into the warm tropical breezes, never to be named or
remembered.
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