The primary source of moisture is the Gulf of Mexico with a secondary contribution by the Atlantic Ocean. Due to the long overland trajectory, the cold Canadian air masses are somewhat modified by the time they reach central Pennsylvania. The annual precipitation of 38.71 inches reflects the drying effect of the Appalachian chain to the west. As a frontal system moves eastward across the Appalachians, the air mass is lifted, clouds form, and moisture is lost in the form of rain or snow. By the time this air mass reaches the Mifflin County area some of the original moisture content of the air has already precipitated out, resulting in a lesser, but adequate, rainfall total for this area.
Summers are warm and at times humid.
Daytime highs usually reach the mid-80s with nighttime lows in the upper 50s. Extended periods of warm humid weather occur occasionally, and on the average 20 days, primarily during June-August, reach temperatures of 90 degrees or higher. Seldom does the mercury drop below 45 degrees during these three months. The all time record high of 103 degrees was observed in August 1948 and early September 1953. Cloud cover is at a minimum during the summer as better than 60 percent of available sunshine is received while nights are generally clear.