It is mid-September and hurricane season is in full swing in both the Atlantic and Pacific. With these mighty oceans bordering both sides of the US, have you ever wondered why hurricanes only make landfall on the east coast?

The answer is two-fold, involving the direction of prevailing winds in the tropics and the difference in water temperature in the two basins.

Hurricanes develop at tropical and sub-tropical latitudes in both the Atlantic and Pacific, where water temperatures are at least 80°F. This part of the globe is also where the Trade Winds prevail, flowing from east to west.

In the Atlantic, storms traveling west-northwest often run into the east coast or Gulf Coast of the US. There, the warm waters of the Gulf Stream that flow along the eastern seaboard sustain them as they move northward.

In the Pacific, storms tend to be pushed out to sea by the Trade Winds.  Any hurricanes that manage to move north quickly dissipate when they encounter the cooler waters of the California current that flows southward along the west coast from Canada.

Only two tropical systems have ever made landfall on the west coast of the US. A hurricane slammed San Diego, CA in 1858 and a tropical storm battered Long Beach, CA in 1939. That said, hurricanes and tropical storms generally have indirect impacts on the western states. When a named storm makes it as far north as Baja California, remnants of it can travel across the border and cause heavy rain and flooding in parts of the American southwest.

All North Atlantic and Eastern North Pacific hurricanes, through 2013. Credit: NOAA/NWS