St. Louis lies in the transition between a humid subtropical and humid continental climate (Köppen climate classification Cfa/Dfa), with neither large mountains nor large bodies of water to moderate its temperature. It is subject to both cold Arctic air and hot, humid tropical air from the Gulf of Mexico. Spring is the wettest season and produces erratic severe weather ranging from tornadoes to winter storms. Summers are hot and humid. Winters are cold with periodic snow and temperatures often below freezing. Winter storm systems, can bring days of heavy freezing rain, ice pellets, and snowfall.