How do tropical cyclones form?

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 like a hot air balloon, and encourages more warm/moist air to flow in from the outside.

Tropical disturbances such as this one are the precursors to tropical cyclones

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 favorable, 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 winds speeds reach a certain threshold, a tropical cyclone is born.

Interestingly, only about 7% 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.

 

– Chris Hennon is part of the Cyclone Center Science Team and Associate Professor of Atmospheric Sciences at the University of North Carolina at Asheville.  Help us learn more about tropical cyclone intensity by classifying storms at cyclonecenter.org