The cooling trend will continue through the Labor Day weekend, although it is unlikely we will see anything more than a few weekend clouds. High temperatures will drop to the mid-80s Friday, and continue to fall to the mid-70s by Sunday and Monday. Mostly clear skies by the end of the weekend will also allow low temperatures to drop enough so that some of the very cold microclimates could see their first frosts of the season Monday and Tuesday mornings.

So why would it get colder all of a sudden at sunrise on a clear calm morning? Here’s how it works. Because of radiational cooling, on clear nights the ground gets much colder than the air just a few feet above it. Since thermometers are placed about five feet above the ground, it will show a warmer temperature than the air touching the ground. Once the sun comes up, the sunlight excites the cold air in the first foot or so above the ground (which can be 10 or more degrees colder), which causes it to move around and mix into the next several feet of air. That “mixing upward” drops the temperature of the air at thermometer level.