Precipitation is spread fairly evenly throughout the year in Massachusetts. Boston averages 43 in (1091 mm) of precipitation annually, with a maximum monthly average of 4.3 in (109.2 mm) in November and a minimum monthly average of 2.9 in (73.7 mm) in July. Springfield, in the Pioneer Valley, averages 45.8 in (1163.9 mm) of annual precipitation, with a 4.6 in (116.8 mm) maximum monthly average in June and a 2.7 in (68.6 mm) minimum monthly average in February. Interior Massachusetts tends to have a summer precipitation maximum due to convection in air masses heated over the interior, which gives rise to frequent thunderstorms. These occur less frequently over the coast, due to the relative lack of convection over the cooler ocean waters. On the other hand, cold, dry air masses over the interior of the state tend to suppress winter precipitation.