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I I I I I I I I I I I I I I I I I I I - The Effects of Large-scale, Low Pressure Lifting In the Northern Hemisphere, air rotates counter-clockwise and toward the center of low pressure air masses. As air converges in the low pressure center, it is forced upward and as the air is lifted it cools, clouds form and precipitation may occur. Large-scale low pressure systems are sometimes known as "closed lows," "cold lows," and when elongated, as "troughs" or "troughs aloft." These systems are not stationary, but travel in a general easterly direction, being guided by large-scale, hemispheric flow patterns and jet stream winds. These low pressure systems may occur at any time, but are most frequent during the weather transition periods (spring or fall). The low pressure systems are large enough in scale to cover most of the state with clouds and precipitation. Storms resulting from large-scale air masses tend to decrease in intensity as they travel from the north and northwest toward the southeast. Storms travelling from the southwest and south toward the northeast tend to intensify. As the storms intensify, more air mass lifting takes place and this lifting process results in more precipitation. When these low pressure systems pass over Utah, they bring wide-scale lifting, cloudiness and precipitation. Low pressure systems of this nature which cross the Wasatch Mountains from the northwest lose some of their intensity as they move into the Uinta Basin. However, the low pressure systems that approach from the south or southwest generally intensify and produce more precipitation and storminess as they move over the Basin. Storm fronts may be imbedded in a low pressure system and may add some local intensity to the large-scale storm. Topography may also influence the storm and result in local differences in precipitation as noted above. The Effects of Subsidence Subsidence is the opposite of lifting. When air subsides, it descends. As it descends, it warms and the warming makes it possible to hold more moisture. Although significant amounts of moisture may still be in the air, clouds evaporate and revert to the invisible water vapor form. Without the clouds there is no precipitation. Subsidence occurs in high pressure air masses. A high pressure air mass or ridge condition dominates large sections of Utah most of the time. This condition brings dry, sunny days, clear skies and fair weather. Snow and Precipitation Correlations Correlations Among Snow Courses Snow courses have been established in several strategically located mountainous areas to provide assistance in forecasting available water supply for the irrigation season. Accumulated snow is measured monthly in the late winter and early spring of each year and is reported as inches of water equivalent. 8