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. Another factor that causes the air to rise, cool and condense is low pressure. Atmospheric pressure at the earth’s surface is a really important aspect when it comes to weather.
When the pressure is ‘high’ it means that the air’s pressure is higher than the pressure of the surrounding air in the area. A ‘low’ is where it is lower. This means that where the air is slowly descending the pressure is high and vice versa. So, as air descends, it warms, which inhibits the formation of clouds. This means that the air that descends in high pressure area has to get to high altitudes in some way which is why air actually rises in low pressure areas.
Winds tend to blow into low pressure areas because are moves from higher pressure areas into lower pressure areas. When the wind moves into a low area the air moves up and eventually cools and rainfall comes about. Also the low pressure that causes the air to rise has to do with the Criolis Effect, which is the force caused by the Earth’s spin that redirects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
According to meteorologist Tony Pann, who works with Coriolis Effect stated, “With the Coriolis effect, air circulates in patterns that depend on latitude. At the mid-latitudes of the USA, most weather systems move from west to east. However, systems tend to move from east to west near the poles and in the tropic” (Pann). The upslope flow is also related low pressure areas which is basically the result of air rising and cooling while being forced to ascend to higher terrain.
According to a study, The Statistical Relationship between Upslope Flow and Rainfall in California’s Coastal Mountains: Observations during CALJET, in a winter storm track situation where states such as Washington and Oregon receive the majority of the winter precipitation, if there is a strong enough low pressure it will cause upsloping winds to accompany the storm and it will fall in the Cascade Mountain range as well as California with the Sierra Nevada Mountains (Neiman and Ralph 1469).
Another place that often experiences upslope conditions during the winter is the eastern Rockies; Denver, Colorado has some wintry precipitation as well and it is also common along the eastern slopes of the Appalachian Mountains. Another factor that can influence rainfall is cold fronts, large areas of moving cool air that push up the warmer air it meets forcing the warm air to cool and finally condense. An example of a cold front tends to occur in Texas, which is called the Texas Norther and is also known as a blue norther.
According to the Glossary of Meteorology the Texas Norther is “…a cold air outbreak associated with the southward movement of a cold anticyclone.. (which comes as)…a rushing blast and brings a sudden drop of temperature of as much as 25 degrees Fahrenheit in one hour”(Yang and Webster 16). Now, among the many features of upper air circulations of relatively strong winds there is what is called jet streams. They are located above areas with strong temperature gradients such as frontal zones.
Jet streams usually affect rainfall in the tropic areas because the intense heat lifts air in the equatorial region which cools and condenses the air. An example of rainfall caused by jet streams would be El Nino and La Nina. The Department of Meteorology from the Penn State University, a study was conducted where they researched the effect of summer tropical heating on certain locations and intensities of extra tropical westerly jet streams. The research led to the conclusion that similar associations between the northern hemisphere heating pattern and the location and variations of the certain jet streams exist, they tated “.. interannual changes in the location and magnitude of the westerly jet streams are closely related to the El Nino/Southern Oscillation (ENSO) phenomenon” (Yang and Webster 27).
Temperature has a big influence in precipitation. There is a possibility that there is a correlation between the summertime U. S. precipitation variability and the Pacific Sea surface temperature. A study done from the Department of Atmospheric Sciences at the University of Illinois at Urbana-Champaign analyzed the linkage between the Great Plains precipitation fluctuations and the North Pacific SST anomalies during summer season.
The results from this study were that on average, the amount of rainfall varies from about 6mm day ? 1 over the southeast corner of the United States to less than 0. 25 mm day-1 along the California coast (Ting and Wang 1855). It has been considered that above normal precipitation over the United States is often associated with excessive low-level moisture transport and frequent storm activities passing through the United States, however in this study it was found that the linkage between the North Pacific SST and the U. S. precipitation could not be caused by direct moisture transport from the Pacific toward the U. S. land region because of the presence of the Rockies and other mountain ranges in the western United States.
So, since there is no linkage between the North Pacific SST and the U. S precipitation it means the North Pacific SST can only be linked to the central and eastern U. S. precipitation through the change of atmospheric circulation over the United States which results in changes in moisture transport (Ting and Wang 1867).
The results of this study showed that in the Great Plains area the amount of precipitation varied greatly, concluding that there is a maximum fluctuation over the states of Kansas and Missouri. The various mechanisms for causing air to rise are the main result of rain. Now the various mechanisms that exist for causing air to rise are also influenced or affected by other environmental factors such as where it is. The United States is composed of many different geographic features which contribute in the precipitation around the country.
There exists many environmental factors that contribute to the rain, “That range from orographic-uplifting as air flows over mountain ranges, to a host of instabilities in the atmosphere that arise from unequal heating of the atmosphere, to potential vorticity dynamics”(Trenberth, Dai, Rasmussen and Parsons 205). Rainfall varies around the United States and it is because of the mixed mechanisms that produce the parcel of air that rises such as mountains, low pressure areas, cold fronts, jet streams and temperature.