In December, O'Fallon typically sees daytime highs of 6.3°C and overnight lows around -4.0°C. Compared to the annual average high of 18.9°C, December runs -12.6°C.
December in O'Fallon averages a daytime high of 6.3°C and an overnight low of -4.0°C.
December runs -12.6°C compared to O'Fallon's annual average high of 18.9°C, ranking #11 of 12 months by warmth.
Plan for daytime highs near 6.3°C and cooler evenings around -4.0°C.