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). Most of the city lies in which USDA plant hardiness zone