The east coast of the United States is affected mainly by the Gulf Stream current, which brings warm water from the tropics into higher latitudes. The warm current flows north along the coast towards Maine, before veering northeast out toward England. Because warmer water evaporates faster, putting more moisture into the air, the current serves to increase the humidity of the air along the coast. This moisture occasionally gets sucked into storm systems and rained (or snowed) out over the mid- Atlantic states and New England. Similarly, because water temperature changes much more slowly than air temperature, when a cold air mass moves toward the coast from from Canada, the coastal areas will stay much warmer than farther inland. Thus, the warm ocean current tends to moderate the temperatures along the coast.

Oceans circulate in a clockwise fashion in the northern hemisphere, which means that the current flows north along the east coast of North America, then south along the west coast of Europe, before crossing back west along the equator. This means that the water on the east coast of the U.S. has recently been in places like the Caribbean and Central America - in other words, tropical, and warm. This has a warming effect on the east coast of the U.S., which is much warmer than Europe at the same latitude. It also means that the east coast is subject to tropical-type weather - i.e. they get hurricanes, when Europe does not.