Here I give a brief overview of these factors, and the hydrology of the Lehigh Valley area. The Lehigh Valley has a humid continental climate typical of the mid-Atlantic region. Mean annual temperature is approx. 53 F. Annual precipitation is approx. 45 inches, and is evenly distributed throughout the year.
Annual evaporation is roughly one-half of precipitation.
The normal succession of high and low pressure systems moving eastward across the U.S. produce cyclic weather changes every few days from late fall through spring. In summer and early fall, these changes are less frequent due to a slowing of the atmospheric circulation with warmer air temperatures. High pressure systems bring cooler, fair weather and wind from the west or northwest. Further climatic data for the area can be found at the PA State Climatologist website. Large precipitation events of 1-2 inches occur each year, with larger totals much more infrequently.