1 The annual mean temperature in Philadelphia, Pennsylvania, Usa is quite mild at 12.4 degrees Celsius (54.3 degrees Fahrenheit).2  The variation of average monthly temperatures is 25.7 C (46.3 F) which is a moderate range. 3  The variation/ range of diurnal mean temperatures is 10.1 C (18.3 F).hiladelphia, Pennsylvania Average Temperatures Chart. The average temperature charts use a fixed scale so that you can easily compare temperatures between two or more locations. Simply line up the charts in separate tabs in your browser and toggle between tabs to visualise the differences.