Prevailing Wind Variation. Prevailing surface winds in the region surrounding the Hypothetical Site blow from the south through west quadrant at speeds varying from 4 to 15 miles per hour throughout the year. There are no large daily variations in wind speed or direction. Observations of wind velocities at altitude indicate a gradual increase in mean speed and a gradual shift in prevailing wind direction from southwest near the surface to westerly aloft.

Frequency of Temperature Inversions. Surface-based atmospheric inversions occur fre­quently during summer and early fall nights with clear skies and low wind speeds. These inversions are destroyed quickly by solar heating. Inversions occurring during winter or spring are more likely to extend into the daytime. Inversions occur most frequently when the winds blow from the south. Unstable weather conditions usually occur with winds from the north or west.

Stagnation periods with steady light winds and a high frequency of inversions are most probable from August to October. A persistent inversion with its base between 1000-4000 feet; wind speeds less than 5 miles per hour below 5000 feet; and clear skies which permit the formation of surface based inversions at night are characteristic of these periods. The annual average percentage of time with inversions is 48 percent. A survey of United States climatology records indicates that 50 percent of total annual time with inversions is a representative national average.

Frequency and Severity of Disturbances. A maximum wind velocity of 100 mph has been recorded at the site.

Snow Load. 30 psf shall be used for snow loading.