Why a Hurricane Has Never Traveled Between Hemispheres

Tropical weather season has kicked off in the Northern Hemisphere just as things quieted down in the Southern Hemisphere. What most people do not know is that a hurricane has never crossed the equator, the defining line between the two hemispheres. But why is that? Read on for the answer.

Understanding the Impact of the Coriolis Force on Tropical Weather Tracks

While a few tropical weather systems have come close to crossing the equator in the past, it has never fully happened. An element connected to the Earth's rotation explains why tropical weather events have never crossed the equator. The meteorological principle known as the Coriolis force is the reason for the lack of equator crossings.

The Coriolis force works to deflect winds to the right while in the Northern Hemisphere. Conversely, the force directs winds to the left in the Southern Hemisphere. This force is the atmospheric factor that provides the swirl that distinguishes tropical weather features, moving counter-clockwise while spinning in the Northern Hemisphere and in a clockwise direction when in the Southern Hemisphere.

What is most interesting is that the Coriolis force is basically at zero at the equator. This is what prevents tropical weather makers from moving from one hemisphere to the other.

Hurricane experts explain that most tropical systems remain north of at least 5 degrees north latitude or to the south at 5 degrees south latitude. You have to go back to 2001 to find the closest time that a tropical storm or hurricane has come to moving over the equator. This distinction belongs to Typhoon Vamei, a feature that churned in the western Pacific Ocean in December. Vamei traveled within 100 miles of the equator, coming to life at only 1.4 degrees North latitude.

Back in December of 1973, an unnamed tropical depression formed at 0.5 latitude. According to the China Meteorological Administration, this feature went on to become a tropical storm when it reached about 0.7 degrees latitude. However, this agency's threshold for a tropical storm designation is different than the parameters used by the National Oceanic and Atmospheric Administration (NOAA) in the U.S. NOAA said that the 1973 feature did not pack tropical-storm-force winds by their criteria. The feature did not reach this destination until it traveled north of 10 degrees north latitude.

Where Tropical Weather Activity is Nearly Non-Existent

It is also interesting to note that there has never been a recorded tropical storm in the southeastern corner of the Pacific west of South America. Notably, there have been just a few sub-tropical or tropical storms that have roamed off the east coast of South America.

There has only been one known hurricane to form off the coast of South America. This happened in 2004 when an unnamed storm known by locals as Catarina sprang to life unexpectedly. According to the National Hurricane Center (NHC), there have been only two other unnamed tropical storms to move through the southern portion of the Atlantic basin. These events happened in consecutive years in 2010 and 2011.

The colder water that circulates off the west coast of the continent of South America is largely to blame for the lack of tropical action. The Peru Current sends cold water to the north along the coast, mitigating the chances of tropical weather in this part of the world's ocean. Higher amounts of wind shear in this region also work to break apart tropical storms before they have the chance to strengthen.

While the waters located just off the coast of Brazil are warmer, the prevalence of wind shear is also a prohibitive factor in this development.

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