Why is it smoky in Boston?

BOSTON – If you’re wondering why it smelled like smoke from northeastern Massachusetts all the way down to the Boston area Monday, it was the brush fires in Salem and something known as an “inversion.”
Believe it or not, the sunny, dry fall weather was to blame as brush fires erupted across southern New England over the weekend.
The greater Boston area has gone two weeks without any measurable rain. The ground has become extremely dry and covered with kindling in the form of leaves and twigs. Add to that some very breezy and dry (low relative humidity) days and you have the perfect recipe for an increased brush fire danger. In conditions like these, all it takes is a small spark to ignite a fire which can then spread extremely fast.
But, why are we smelling the fires several miles away? That has to do with a temperature “inversion” in the atmosphere.
Typically, smoke from a ground fire would simply continue to rise vertically and eventually get swept away by winds at upper levels of the atmosphere.
The smoky air is warmer than the air around it and, therefore, would continue to rise (warm air is lighter than cold air). 
However, in the case of Monday morning, there was a lid on the atmosphere right around 3,000 to 5,000 feet up. This is called an inversion.
On clear and cool nights with very little wind, the heat trapped near the ground “radiates” upward. This is called radiational cooling and it is why we get so cold on clear, calm nights. Many times, like Sunday night, this creates a temperature inversion in the lower parts of the atmosphere.
So, instead of the air getting colder and colder with height, this milder air that escaped from the ground settles a few thousand feet above our heads. This essentially creates a lid on the atmosphere.   
This lid keeps things like smog and smoke from escaping. The smoke from the North Shore fires was trapped by this inversion and was stuck in the bottom few thousand feet.
The winds near the ground were light overnight and by the morning, blowing between 3 and 8 miles per hour out of the north-northwest. This directed the smoke (and its odor) from places like Middleton and Salem southward into Boston and surrounding towns.
There is good news in the short term. First of all, we expect the inversion to lift during the day Monday, allowing most of the smoke to escape upwards into the atmosphere.
Also, the wind direction is forecast to change from northerly to easterly. Any remaining smoke/smell will be pushed westward towards the Route 495 area between Marlboro and Lowell, but again, likely at much lower concentrations.
What we really need is some rain.
Unfortunately, there is very little in the forecast this week with just a slight risk of a few sprinkles Tuesday night and again on Friday. Certainly not enough to alleviate the current drought that exists across the region.
Therefore, we are likely to continue to see brush fires popping up here and there. It is imperative that everyone avoids any activities that could spark a fire. This includes any outdoor burning. You may think you have it under control in your backyard, but the smallest of sparks could easily lead to an uncontrollable brush fire.