Occasionally the latter part of June is mild and dry, but July and August offer more consistently mild weather with highs in the 70s F and low 80s. Lows vary from the high 60s to low 70s.
Unfortunately July and August are also when fog is a common occurrence, due to the cooler sea temperatures and the southwest wind coming off the mainland. It can come in very quickly and is often accompanied by strong gusty winds. It can also be highly localized and burn off as quickly as it came—or sock in an entire portion of the bay for several days.
Navigation can become a serious challenge in the fog, and it is not the time to succumb to “get home- itis”. Patience is truly a virtue among sailors.
By late August or early September, fog becomes less of an issue as winds turn northerly and the air temperature begins to drop. These clear, cool late summer days can be the finest sailing days of all, with sparkling seas and cobalt skies.
All summer long the prevailing winds are southwesterly, building to 15 - 20 kts in the afternoon. Mornings are usually calm. Like most places in the northeast, winds from the northwest will mean a cool gusty breeze; easterlies are a sure call for some fog, drizzle, and generally unpleasant weather. Sea conditions depend on the wind, but be aware that steep swells can develop in any conditions the farther you venture offshore.
Although moderate gales do occur, hurricanes are rare because of the cool water temperature. Gales can occasionally accompany the southwesterlies, but expect only a handful of these during the course of the season.
As any prudent mariner knows, weather forecasts are never cast in stone. Conditions on the bay can be highly localized. Take advantage of whatever local knowledge is available (lobstermen are a great resource), especially when seeking the shelter of a good harbor or hurricane hole. Also, keep in mind that storm surges and tidal variations are serious concerns when anchoring, so always put out more scope than you think you need.