<P> Together with the trade winds, the westerlies enabled a round - trip trade route for sailing ships crossing the Atlantic and Pacific oceans, as the westerlies lead to the development of strong ocean currents in both hemispheres . The westerlies can be particularly strong, especially in the southern hemisphere, where there is less land in the middle latitudes to cause the flow pattern to amplify, which slows the winds down . The strongest westerly winds in the middle latitudes are called the Roaring Forties, between 40 and 50 degrees south latitude, within the Southern Hemisphere . The westerlies play an important role in carrying the warm, equatorial waters and winds to the western coasts of continents, especially in the southern hemisphere because of its vast oceanic expanse . </P> <P> The westerlies explain why coastal North America tends to be wet, especially from Northern California to Alaska, during the winter . Differential heating from the sun between the land which is quite cool and the ocean which is relatively warm causes areas of low pressure to develop over land . This results in moisture rich air from the Pacific Ocean to flow from the west, resulting in frequent rainstorms and wind on the coast . This moisture continues to flow eastward until orographic lift caused by the Coast, Cascade, Columbia and Rocky Mountains cause a rain shadow effect which limits further penetration of these systems and associated rainfall eastward . This trend reverses in the summer when strong heating of the land causes high pressure and tends to block moisture - rich air from the Pacific from reaching land . This explains why most of coastal North America in the middle latitudes experiences dry summers, despite abundant rainfall in the winter . </P> <P> The polar easterlies (also known as Polar Hadley cells) are the dry, cold prevailing winds that blow from the high - pressure areas of the polar highs at the North and South Poles towards the low - pressure areas within the westerlies at high latitudes . Like trade winds and unlike the westerlies, these prevailing winds blow from the east to the west, and are often weak and irregular . Due to the low sun angle, cold air builds up and subsides at the pole creating surface high - pressure areas, forcing an outflow of air toward the equator; that outflow is deflected westward by the Coriolis effect . </P> <P> In areas where the wind flow is light, sea breezes and land breezes are important factors in a location's prevailing winds . The sea is warmed by the sun to a greater depth than the land due to its greater specific heat . The sea therefore has a greater capacity for absorbing heat than the land, so the surface of the sea warms up more slowly than the land's surface . As the temperature of the surface of the land rises, the land heats the air above it . The warm air is less dense and so it rises . This rising air over the land lowers the sea level pressure by about 0.2% . The cooler air above the sea, now with higher sea level pressure, flows towards the land into the lower pressure, creating a cooler breeze near the coast . </P>

Most of the u.s. is situated in which zone of prevailing surface winds