As appeared in the
May 2016 Edition of Up Portland

Hello everyone!

Aside from a surprise 2.2 inches of snow on Tuesday the 26th, which just happened to set a record for the greatest amount of snow for the date, a stretch of calm weather has graced Portland for most of April and that looks to continue more or less into May with no significantly above average or below average temps forecast. 

The one neat weather phenomenon we’ve seen a bit of during April (aside from that snow) is that of the sea breeze. The phenomenon is simple but the effects can be drastic. 

A classic sea breeze developed during the Boston Marathon on Patriot’s Day just a few weeks ago, dropping the temperature in the final miles from the downright hot (if you are a runner) low 70’s to the nearly perfect (for marathon running) mid 50’s. 

How does the atmospheric switch flip so drastically and so quickly? It all has to do with density.

As the sun beats down on coastal Maine (or Massachusetts or anywhere on the coast), the ocean warms much more slowly than the land. This leads to a difference in temperature between the water and the land. 

The air over the land is warmer and thus rises because it is less dense than the cold air that surrounds it. The rising warm air over the land creates an area of low pressure. 

That means the cold, dense air over the ocean then rushes inland to replace the ‘void’ of sorts left by the warm air. The cold air moving inland to fill that ‘void’ is the sea breeze we all know and love.

The sea breeze is even visible on radar on some days. This happens when a variety of particulates gets concentrated along the boundary between the warm and cold fronts. These particulates include dust, pollen, sea salt and, yes, bugs. They all get swept out of the way by the incoming cold air and get clustered along the leading edge of the sea breeze. The radar beam bounces off these particulates in the same way it bounces off rain and snow. 

Look at the radar screen shot which I’ve included for a good example of this from last Spring. There’s the main (no, not Maine) sea breeze front and a smaller one from Sebago Lake. Yes, this phenomenon even happens on lakes, just on a smaller scale. Anyone who has ever visited Chicago in the Spring and Summertime is familiar with the latter as that city’s forecasts often include that infamous line “cooler near the lake.”

Enjoy the weather this month and remember the difference in density between warm and cold air next time you feel the sea breeze kick in... and if you are living or visiting near the shore, don’t forget a hoodie or light jacket.

—Jack