Because of the city's low elevation and southerly location, St. George is the hottest part of the state, with maximum daily July temperatures averaging about 102°F (39°C). In winter, temperatures frequently drop below freezing overnight (due to radiational cooling resulting from low humidity), but temperatures warm into the 50s°F (low 10s°C) during the day.
http://en.wikipedia.org/wiki/St._George,_Utah#Climate - Thu Jul 10 2008, 09:57