The general topography is rolling, the North Valley Hills rise to about 600 feet msl a mile north of central Coatsville and the South Valley Hills peak at slightly over 500 feet a half mile south of the downtown area. The West Branch of Brandywine Creek flows southward through the town finally emptying into the Delaware River. Typical of the Southeastern Piedmont, the climate is classified as Modified Humid Continental due to the nearness of the Atlantic Ocean. Most weather systems that affect the Coatesville area either develop in the Midwest and are steered eastward by the prevailingly westerly flow aloft or they form in the southeastern United States and track northward along along the Atlantic Coast. The long overland trajectory of the Midwest weather systems considerably modifies the accompanying cold and dry Canadian air masses so that upon reaching southeastern Pennsylvania they have warmed and gained moisture either from the Gulf of Mexico or the Atlantic Ocean.
Summers are warm and rather humid.
Usually highs reach into the mid-80s with nighttime lows near 60 degrees. The maritime influence tends to produce a fairly uniform weather pattern, resulting in occasional prolonged humid conditions. During the period June-August, daytime highs exceed 90 degrees on the average of 20 days with nighttime lows rarely lower than 45 degrees. The record high was 107 degrees set back in August 1918. Prevailing winds flow from the south-southwest averaging 8 mph.