So far this season, most of the hurricanes that formed in the Atlantic  Ocean, the Caribbean Sea and the Gulf of Mexico have steered away from the  US. Of the 11 tropical storms that have formed, only 6 developed into  hurricanes, and only 1 of these has connected with the mainland.  Officially, the hurricane season extends until November 30, but most of the  activity generally occurs in August and September, and with nothing  currently brewing, perhaps we'll get off easy this year.

Last year, we had a question about whether or not a hurricane had ever hit  Europe. It was concluded that it's highly unlikely that a hurricane could  remain intact very long once it hit the cooler waters of the eastern  Atlantic and was subjected to wind shear from extra-tropical weather  systems. Europe has a higher average latitude than does North America. Even  the southernmost parts of Europe (southern Greece and southern Spain) are  at about the same latitude as North Carolina.

In contrast, South America is more tropical than North America. The  center of South America is near 15 degrees south latitude (in southern  Brazil, east of Bolivia), while the center of North America is in the state  of North Dakota, about 48 degrees north latitude. At first glance, then, it  would seem that our neighbors to the south would have to deal with  hurricanes on a regular basis.

Hurricanes need to have water temperatures, which are no less than about 79  degrees F in order for development to occur. In the late summer and early  autumn, water temperatures almost always exceed 80 degrees F in tropical  areas of the north Atlantic Ocean and in the Gulf of Mexico. Once a  hurricane forms, any cooler water it encounters on its travels will quickly  sap its energy. Along the western flank of South America, in the eastern  south Pacific Ocean, the Peru Current brings cold polar waters northward  along the coasts of Chile, Peru and Ecuador. Thus, the equatorial waters to  the west of the South American coast are just too cool for hurricanes to  get going. Similarly, off the west coast of southern Africa, the Benguela  Current, also moving northward, ushers in cold water off shore of South  Africa, Namibia and Angola. Again, the waters here are too cool for the  liking of hurricanes.

Off the east coast of South America, in the southern Atlantic Ocean, the  Brazil Current moves southwestward bringing warm waters to the Brazilian  coast. The waters here are sufficiently warm for hurricanes to develop,  however, while the water is warm enough, its proximity to the equator  diminishes the likelihood that tropical systems will form. Hurricanes  rarely form within 10 degrees of the equator and have never been observed  to cross the equator. At these low latitudes, air will not spiral inward  toward the center of a forming low pressure system. Compared to polar  areas, the Coriolis force (an apparent force caused by the Earth's  rotation) is negligible in the area on either side of the equator, so  there's simply too little spin available to initiate hurricane development  near the equator. The Coriolis force varies with the sine of the latitude.  The latitude of the equator is 0 degrees, and the sine of 0 is 0. For the  north pole, the latitude is 90 degrees, and the sine of 90 is 1.

North or south of about 10 degrees latitude, air will more easily begin to  spin or spiral inward toward lower pressure. However, because northernmost  South America (Venezuela and Colombia) is only at a latitude of about 12  degrees north, hurricane development off its northern coast occurs  infrequently. When a "cane" forms in the southern Caribbean Sea, the ocean  currents and trade winds work to steer it northwest, toward Central America  or the Gulf of Mexico. Additionally, land masses inhibit the formation of  tropical depressions and typically prevent them from intensifying into  hurricanes.

This being said, a few hurricanes have likely hit Venezuela or Colombia  sometime in the past. Although, most of the recorded systems that have  impacted these two nations have been tropical storms. For example, an  unnamed tropical storm in 1933 was known to have killed about 25 people in  Venezuela. In 1988, tropical storm Joan skirted the coast of Venezuela and  Colombia as did tropical storm Bret in 1993. In 1996 Hurricane Cesar formed  just off of Venezuela, and just a couple of weeks ago, tropical storm Joyce  flirted with the Venezuelan coast.

For more about this see the web site for the National Hurricane Center at  http://www.nhc.noaa.gov/