As appeared in the
June 2016 Edition of Up Portland

Hello everyone!

This month I’ll be talking about a fairly simple phenomenon but one that impacts the Portland area daily in the Summer months: fog. Fog is as much a staple of life on the coast of Maine as the salty air and the rocky coast. Seemingly clear nights can turn into an ever shifting wall of grey by morning. Likewise, warm morning sun can instantly give way to a cold misty afternoon.

Fog forms off the coast of Maine when warm air moves over cold water. The cold water lowers the temperature of the air which acts to condense the moisture in the air. The condensation of the moisture in the air results in thick fog. Fog banks move at the mercy of the wind which accounts for their sudden arrival and departure. A subtle shift in wind from the South to the South-East can mean the difference between sunny with 80 degree temps and thick mist/drizzle with 60F temps.

These wind shifts can be quite subtle due to the geography of the Gulf of Maine. Southerly or South-Westerly winds such as those seen during the Summer months will travel over either the baking hot Southeast U.S. and Mid-Atlantic or the toasty Gulf Stream waters before they arrive here. This ensures a constant supply of warm moist air. Once you get to around Cape Cod and the cold Labrador current takes over the ocean so the warm, moist air cools, condenses, and turns quickly to fog. Even the slightest easterly component to the wind will drive this fog bank right into the coast.

How does morning fog burn off to a beautiful afternoon? 

As the sun comes up and its warm rays beat down on the city, the temperature of the fog bank begins to rise. Eventually, layer by layer, the air comprising the fog bank warms above the dew point and the condensation vaporizes into invisible water vapour leaving a wonderful Summer’s day to be enjoyed.

​Enjoy the coming start to Summer and be sure to think about condensation and how all this happens next time you wake up to a foggy morning!