<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> <P> The strength of the sea breeze is directly proportional to the temperature difference between the land mass and the sea . If an off - shore wind of 8 knots (15 km / h) exists, the sea breeze is not likely to develop . At night, the land cools off more quickly than the ocean due to differences in their specific heat values, which forces the daytime sea breeze to dissipate . If the temperature onshore cools below the temperature offshore, the pressure over the water will be lower than that of the land, establishing a land breeze, as long as an onshore wind is not strong enough to oppose it . </P>

Where are the strongest winds in the united states