Boston weather is fickle. Mark Twain said, "If you don't like the weather in New England, just wait a few minutes." This is particularly true during the spring and fall seasons, when it can change from being warm to cool and windy to wet and rainy all in the same day.
Summer tends to be mild and not humid, with the hottest months being July and August, when the average temperature is 72F (22C). However, there can be days when the temperature exceeds 90F (32C) but that is often tempered by the sea breezes.
Winter can be harsh. The coldest months are January and February, with an average temperature of 29F (-2C), but expect snow from November through March, and sometimes in October and April, too.