![]() The prong of higher precipitation that occurs in western Mexico is related to a northward flow of moisture as part of the “ Arizona monsoon.” Near the mountains, intense surface heating and orographic lifting cause summer thunderstorms. The clockwise circulation around this feature supplies warm, moist air into interior North America.ġ3. In summer the Bermuda-Azores high-pressure area reaches its northernmost extent. This weak environmental lapse rate (or even an inversion) stabilizes the atmosphere, the opposite of winter conditions, limiting the amount of precipitation.ġ2. The Great Lakes are cool relative to the overlying air in July. Also, the region is much closer to the cold-warm boundary in summer than in winter, facilitating frontal lifting.ġ1. The warmer air has more energy to evaporate water, producing water vapor that can then precipitate. July totals are much higher than those in January in the far north. This is because of the drying effects of sinking air on the eastern side of the Hawaiian high-pressure feature, which makes its closest approach to North America in summer.ġ0. Summer precipitation totals are low in western North America and over the adjacent Pacific Ocean, even in places that are relatively wet in winter. In most areas, local surface heating destabilizes the atmosphere, but the cold California Current off the West Coast negates this effect.ĩ. July totals are higher than in January in most of North America, except along the West Coast. Note the regional patterns and then compare this map with the one for January.Ĩ. This map shows precipitation totals for July, the middle of summer. Precipitation in both the Atlantic and Pacific oceans at this time of year is focused near the boundary between cold air and warm air, in part because these regions experience frequent passage of fronts. Thus, the lakes destabilize the atmosphere, increasing local “lake effect snow” totals downwind.Ħ. This generates a steep environmental lapse rate, which makes an adiabatically moving air parcel likely to be warmer than its surrounding environment. ![]() The Great Lakes are much warmer than the overlying air in winter. Cold air has lower water-vapor capacity and therefore will not produce clouds with a sufficient amount of moisture to allow for much precipitation.ĥ. ![]() Precipitation decreases inland away from moisture sources and northward with lower temperatures. These areas are blocked from oceanic moisture sources by mountains both to the west and to the east.Ĥ. Inland from the western mountains, precipitation totals decline dramatically. Heavy rainfall along the coast of Canada and the Pacific Northwest is due to proximity to the ocean combined with orographic effects that cause moisture-laden Pacific air to move upslope under the influence of the mid-latitude westerlies.ģ. Note any regional patterns and the precipitation that is typical for where you live.Ģ. In northern and high-elevation regions, much of this precipitation is as snow. ![]() Examine this map, which shows average amounts of precipitation for January, averaged over several decades. How Does Precipitation Vary Across North America? Januaryġ. We explore some of these by examining regional variations in precipitation over North America and the world. The daily variations are related to short-term changes in weather, but variations between regions and seasons reflect differences in the overall climatic setting, such as differences in latitude, prevailing wind directions, proximity to large water bodies, any nearby ocean currents, elevation of the land, and countless other factors. THE AMOUNT OF PRECIPITATION varies from region to region, season to season, and day to day. ![]()
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