Concord, as with much of New England, is within the humid continental climate zone (Köppen "Dfb"), with long, cold, snowy winters, very warm (and at times humid) summers, and relatively brief autumns and springs. In winter, successive storms deliver light to moderate snowfall amounts, contributing to the relatively reliable snow cover. In addition, lows reach at least on an average 15 nights per year, and the city straddles the border between USDA Hardiness Zone 5b and 6a. However, thaws are frequent, with one to three days per month with + highs from December to February. Summer can bring stretches of humid conditions as well as thunderstorms, and there is an average of 12 days of + highs annually. The window for freezing temperatures on average begins on September 27 and expires on May 14.