What can I say? Sunny clear days, a sea breeze in the afternoon, and cool nights perfect for sleeping. It’s weather fine enough to charm every visitor to our fair state (although the lack of rain is worrisome to local gardeners and farmers). 

But, as anyone who spends time on the water knows, all this sunshine gives a false view of what’s taking place out on the Gulf of Maine. Out at sea the sun quickly can become hidden by a bank of cold, dark fog, obscuring land, sea and sky for hours or days at a time. Maine and Canada’s Maritime provinces have the dubious reputation as the summertime home of fog. Where does all this fog come from?

“Fog is the condensation of water particles in the air,” explained John Jensen, a National Weather Service meteorologist in Gray. “Humid air will move over a colder surface. As the air temperature drops to the dew point, you start to get fog.” 

Air contains water vapor. As air cools, it loses the ability to hold all that dissolved moisture. The dew point is the temperature at which air can no longer hold water vapor; that moisture precipitates out as fog or rain. On land, fog often occurs overnight in valleys, as cooling air sinks into the valley, reaches the dew point, and its water vapor is released as fog. 

Despite reports of its warming, the Gulf of Maine remains a cold sea. For example, data from the Eastern Maine Shelf buoy recorded by NERACOOS (Northeastern Regional Association of Coastal Ocean Observing Systems) in 2015 show that the sea surface temperature in that area of the Gulf stayed in the 49°F to 53°F range for most of July and August. On a few occasions the surface water reached 56°F and 58°F. The air temperature, however, hit 65°F several times during those months. That contrast leads to fog.

There are lots of different types of fog. “Advective fog means fog that is moving,” Jensen said. “If you have a sea breeze in the afternoon, that fog will move in to land.” Radiation fog happens when large air masses begin to cool, typically in the fall, yet the ground is still quite warm. The moisture evaporating from the land hits the cool air and makes fog. In the winter, fishermen may experience sea smoke. In this situation, it’s the water that is warmer than the air. Extremely cold, dry air moves in over the ocean, whose temperature is significantly warmer than the frigid air. “Moisture from the warmer water moves up into the air. That’s sea smoke,” Jensen said. 

It’s not surprising that fishermen from Newfoundland to Maine refer to the thick banks of fog they encounter during the summer as a “dungeon of fog.” You can’t see a thing in a fog bank. Sound refracts oddly; straining eyes see things that aren’t there. Given that the temperature of the Gulf of Maine has gone up by 0.4°F annually during the past decade, one might think that a time would come when the traditional fogs of summer would disappear from the Maine coast. Don’t get your hopes up. “It’s hard to project into the future. But even if the atmosphere and the water are warming, there still will always be a sharp difference,” Jensen said. And that difference equals fog.