1 The average annual temperature in Detroit, Michigan, Usa is a little cool at 9.2 degrees Celsius (48.6 degrees Fahrenheit). 2  The range of average monthly temperatures is 27.5 C (49.5 F) which is a moderate range.3  The average diurnal temperature range/ variation is 10.6 C (19.1 F). The average diurnal temperature range/ variation is 10.6 C (19.1 F). 2  July is the hottest month (really warm) having a mean temperature of 22.4 degrees Celsius (72.32 degrees Fahrenheit). 3  The coolest month (January) is cold having a mean temperature of -5.1 degrees Celsius (22.82 degrees Fahrenheit).