Factors that Influence Rainfall in the United States
A type of precipitation is rainfall and it develops when water vapor condenses into droplets in the atmosphere thus becoming too heavy to stay suspended in the air so gravity causes them to fall. The pattern or occurrence of rainfall in the United States or basically anywhere in the world depends on several factors such as ground elevation, wind directions, location within a continental mass, areas of low pressure, cool fronts, jet streams and even mountain ranges. For instance mountains have an influence on wind and wind is an important feature in rainfall, “The windward sides of mountainous islands in the trade wind regime are among the rainiest places on earth, where rains exceed the regional oceanic precipitation by several fold. Over Hawaii, where nearby oceanic rains are of order 50 cm per annum, favored island locations receive rainfall in excess 500 cm yr-1”(Carbone 2847). This means that a part of Hawaii gets significantly more rainfall than the other part and it is because of geographical factors; its mountains push the air mass upward, condensing water vapor into droplets thus creating rainfall. The distribution of rainfall in the United States is somewhat like the distribution of rainfall in Hawaii, there exist dry and wet sides but it all depends on the factors that interfere with the precipitation development of rainfall. As mentioned before, mountains are one of the factors that cause rain. This is so because in order for the air to pass over mountains it has to rise. So, in mountains such as the Washington Cascades the air is forced to rise to pass over them. This rise causes the air to cool, condensing and creating rainfall on one side, like Seattle, and leaving the other side with the dry air receiving very little or no rainfall. This sinking, dry air is the result of what is called the rain shadow effect, an area in the lee of a mountain with less rain and cloud cover (Brinch 1). Another place that is affected by the rain shadow effect is the Death Valley National Park located on the eastern border of south central California which also includes a small area of Nevada and it is considered one of the hottest and driest places in the United States. Death Valley has three significant barriers which are Sierra Nevada, Argus Range and the Panamint Rage. This is where the rain shadow effect comes in place because with such barriers air masses lose their moister as they are forced up the mountains. According to the New World Encyclopedia, the rain shadow effect is what makes Death Valley the driest spot in North America, receiving 1.7 inches of rainfall annually. “Annual average precipitation varies from 1.9 inches overall in the areas below sea level to over 15 inches in the higher mountains that surround the Valley” (New World Encyclopedia). So, for the wind to continue moving east it must lift up to go over the Death Valley, by the time this happens there is not enough moisture in the air to release rainfall. Now, distinct from the rain shadow effect is the orographic effect, which is basically the rainfall that occurs either on the wind-ward slope, upstream of a plain area or over the lee slope of a mountain (Chen and Lin 1). Some places where the orographic effect takes place is in the Northwestern United States such as Oregon and Washington and also the ski country region of New York and Pennsylvania. A study done of the Department of Geology and Geophysics in Yale University investigated the orographic precipitation and Oregon’s Climate Transition. It was concluded that wet and dry regions orient themselves across the wind vector. “The dry lee slopes arise from strong wave-induced descent and parameterized evaporation of condensed water. On the larger scales, the precipitation falls upstream of the crest” (Smith, Barstad, and Bonneau 190). With all this said, mountains have a great influence on where rainfall occurs or does not....
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