Boston has a continental climate with some maritime influence, and using the −3 °C (27 °F) coldest month (January) isotherm, the city lies within the transition zone from a humid subtropical climate (Köppen Cfa) to a humid continental climate (Köppen Dfa), although the suburbs north and west of the city are significantly colder in winter and solidly fall under the latter categorization; the city lies at the transition between USDA plant hardiness zones 6b (most of the city) and 7a (Downtown, South Boston, and East Boston neighborhoods).