Generally, Massachusetts' hilly central interior (e.g. Worcester) and its western Berkshire Mountain region (e.g. Pittsfield) have colder winters than its coastal and Connecticut River Valley regions. Stockbridge, in the Berkshires, has a January average temperature of 21.6 °F (-5.8 °C). In Eastern Massachusetts, Boston, on the coast of Massachusetts Bay, has an average January temperature of 29.0 °F (-1.7 °C). The island of Martha's Vineyard has the state's highest average temperature—31.8 °F (-0.1 C), due to the warming effect of the Atlantic Ocean. Summer temperatures are highest in the state's urban centers, due to the heat island effect. Average July temperatures in Massachusetts' three most populous urban centers are: Boston (coastal) - 81.7 °F (27.6 °C); Worcester (central) - 79.2 °F (26.2 °C); and Springfield (Connecticut River Valley) - 85.0 °F (29.44 °C). By contrast, the coolest average summer temperatures occur in the Berkshires and on the state's offshore islands. The average temperature in August, the warmest month on Nantucket Island, is 68.7 °F (20.4 °C). The average in July in Stockbridge is 68.9 °F (20.5 °C). Both daily and seasonal variation in temperature are greatest in the Berkshires and lowest along the coast.