| OCR Text |
Show while summer precipitation is characterized by thunderstorm activity, resulting from the northward flow of warm moist air masses from the Gulf of Mexico. An area of extensive thunderstorm activity extends throughout this part of Utah. Figure 7 shows values of potential evapotranspiration in inches according to the Thornthwaite formula. Potential evapotranspiration is the amount of evaporation and transpiration that would occur if there were no shortage of moisture. It is an index of the heat energy available to vaporize water. Since temperature decreases with increasing altitude higher elevations have a lower potential for evapotranspiration than do areas at lower elevations. Under natural conditions the available moisture is less for lower elevations because they receive less precipitation than higher regions. Therefore, the actual amounts of evapotranspiration that occur at lower elevations will usually not even approach the potential amounts. At higher elevations, moisture is generally abundant and both evaporation and transpiration are largely determined by the supply of available heat energy. If the potential evapotranspiration map is compared to the normal annual precipitation map, it can be noted that over much of the West Colorado Hydrolbgic Area potential evapotranspiration exceeds actual precipitation. The reverse is true only in mountainous area above about 7,000 feet. The potential evapotranspiration map can be used to give a rough indication of climatic type. The general boundary between dry and humid climates can be determined by connecting all points where the potential evapotranspiration and actual precipitation are equal. If precipitation exceeds potential evapotranspiration, a humid climate exists. If the reverse is true, a dry climate exists. In a dry climate, permanent streams cannot originate because no surplus water exists to maintain a constant ground- water level. Figure 8, the frost- free season map, shows the average number of days per year between the last freeze in the spring and the first freeze in the fall. The table below can be used to obtain the mean dates of occurrence of the last spring freeze and the first fall freeze. Values obtained from the left portion table are mean dates of occurrence; that is 50 percent of the years the freeze will occur earlier, and 50 percent later than shown. The right portion of the table is used to obtain dates that correspond to other probability levels. The average freeze date is selected from the map and the factor corresponding to the desired probability level is obtained from the table. |
