Run Like the Sea Breeze

The Chief described in detail everything you could possibly want to know about the weather for today for whatever your plans are in his blog yesterday.  If you want to know more about how this Patriots’ Day compares to others weather-wise, I talked about that in my blog yesterday.  But just in case you are wondering, "what is this sea breeze they are talking about," let me explain…    

April sun in very strong.  In fact, the sun angle in mid-late April is similar to that of mid-late August.  So use sunscreen, just like you would in August, even if it might not be so warm.  But that’s a side note.  The point is that the strong April sun will add a lot of heat to Earth’s surface in the morning.  However, no matter how strong the sun is, land warms up much more quickly than water.  You have probably figured that out if you have ever tried to boil a pot of water…it can seem like a painstakingly slow process…or if you’ve ever walked barefoot on the beach on a sunny, summer day…ouch!   

So, the air over land gets warm very quickly each April morning.  Warm air rises.  The rising air creates a weak low pressure area, or small vacuum, due to the decrease in air mass at the surface.  As the saying goes, "nature abhors a vacuum."  Cool, dense air from offshore rushes onshore as the atmosphere seeks to equalize the pressure.  That cool air is the sea breeze.  The sea breeze acts like a small, local cold front in the sense that it drops temperatures and changes the wind direction as it moves forward.  If there is enough humidity in the air, like say in summertime, the sea breeze can also generate clouds, showers and even spark thunderstorms much like a cold front would.  This fancy graphic from the National Weather Service illustrates it nicely. 

The first trick is figuring out exactly when the sea breeze will form.  That determines how much it will warm up before it cools down dramatically.  The second trick is figuring out how far inland it will penetrate and exactly who will see the dramatic cool down.  The sea breeze is such a small, localized phenomenon that its tough for even our most detailed weather computer models to predict.  At this moment, Boston has already warmed into the low 60s.  But very shortly those numbers should drop to the 50s.  In general, areas east of 128/95 should drop to the 50s this afternoon while areas further west will continue warming into the 70s.

Everyone cools down dramatically on Tuesday thanks to an actual (normal scale) cold front.  Temperatures rebound to the 70s once again for Thursday and Friday.  Will there be another sea breeze?!?  Well, generally, if you have a prevailing offshore wind that is sustained at 10-15 mph or greater, you won’t get a sea breeze.  Winds will be from the west or southwest for most of Thursday and right in that 10-15 mph range.  So, its a bit tough to make that call at this point.  Stay tuned!

Yet another actual (normal scale) cold front arrives late Friday bringing everyone cooler weather for next weekend.

Happy Patriots’ Day and good luck marathon runners!!!