Chris W. Landsea is a researcher at the Atlantic Oceanographic and Meteorological Laboratory/Hurricane Research Division of the National Oceanic and Atmospheric Administration (NOAA), located in Miami, Fla. He replies:

"Hurricanes form both in the Atlantic basin, to the east of the continental U.S. (that is, in the Atlantic Ocean, the Gulf of Mexico and the Caribbean Sea), and in the Northeast Pacific basin, to the west of the U.S. The hurricanes in the Northeast Pacific almost never hit the U.S., however, whereas the ones in the Atlantic basin strike the U.S. mainland just less than twice a year on average.

"There are two main reasons for this disparity. The first is that hurricanes in the northern hemisphere form at tropical and subtropical latitudes and then tend to move toward the west-northwest. In the Atlantic, such a motion often brings the hurricane into the vicinity of the East Coast of the U.S. In the Northeast Pacific, the same west-northwest track carries hurricanes farther offshore, well away from the U.S. West Coast.

"The second factor is the difference in water temperatures along the U.S. East and West coasts. Along the East Coast, the Gulf Stream provides a source of warm (above 80 degrees Fahrenheit, or 26.5 degrees Celsius) waters, which helps to maintain the hurricane. Along the West Coast, however, ocean-surface temperatures rarely rise above the lower 70s F. (the low 20s C.), even in the middle of summer. Such relatively cool temperatures do not provide enough thermal energy to sustain a hurricane's strength. So the occasional Northeast Pacific hurricane that does track back toward the U.S. encounters the cooler waters of the Pacific, which can quickly reduce the storm's strength."

Kerry Emanuel in the Center for Meteorology and Physical Oceanography at the Massachusetts Institute of Technology expands on the details of hurricane behavior:

"Hurricanes almost always form over ocean water warmer than about 80 degrees F. in a belt of generally east-to-west flow called the trade winds. They move westward with the trade winds and also drift slowly poleward. Eventually, if they last long enough, they will drift poleward far enough to enter the belt of westerly winds that prevails in middle latitudes. When this happens, the hurricanes 'recurve' toward the east and thereafter follow paths that are generally both eastward and poleward.

"In the Atlantic region, hurricanes form anywhere from the tropical central Atlantic to the Gulf of Mexico. Those that form in the central Atlantic and Caribbean region usually start off moving westward; when they recurve, they may strike the North American mainland. Some of the storms that begin in the Gulf of Mexico may move poleward and eastward right from their inception.

"In the eastern Pacific region, one has to go all the way down to the central Mexico coastline to find water warm enough to sustain hurricanes. This warm water lies well within the belt of easterly winds, so almost all the storms that form there move away from the coast, toward the west. By the time those storms recurve, they are usually many thousands of kilometers west of the coast of North America.

"A few storms recurve right next to the coast. Some of these make it as far north as Baja California and can strike land with hurricane-force winds. But to make it all the way to the U.S. West Coast, the storms have to traverse a long stretch of ocean water that is far too cold to sustain hurricanes. Occasionally, tropical storms do strike coastal southern California. By the time they do, they have lost their hurricane-force winds, although they may still bring with them very heavy rainfall that can cause extensive flooding.

"Essentially, the very cold water that upwells off the California coast and gives coastal California such a cool, benign climate also protects it from hurricanes. Real-time maps showing the distribution of the potential intensity of hurricanes clearly show the various regions worldwide that can sustain hurricanes.