The average high temperature through the first eight days of January was 60.3 degrees. The forecast high temperature today January 9, 2019 is a dismal 47 degrees. Seems like a great way to open a blog. I guess it’s something we had to face at some point that Winter would eventually arrive in the Tennessee Valley. High temperatures will be below average and low temperatures will give way to hard freezes. Today’s blog will discuss the specific atmospheric setup that is leading to such a dramatic change in temperatures.
I use the above image and then think to myself I saw The Shining for the first time in the year 2017. I know I know I’ve herd it all before. A cold front moved through the Tennessee Valley Monday night and very early Tuesday. This was noticed through the day Tuesday as clouds increased in coverage and winds picked up as well. Behind the cold front is an area of high pressure that will have a large influence on the forecast through the rest of the week. The influence means clear skies and cool temperatures through Friday.
All that said and the real story wont be the lack of warming during the day, but the extreme cooling at night. While we’ll be breezy this afternoon, those winds will calm down tonight allowing temperatures to plummet.
Radiational cooling is the process by which the surface experiences a net loss in heat. At least that’s how I define it. There’s probably a more complex definition. What this means is that after sunset when we lose the solar heating from the sun the ground continues to radiate heat, but it’s not being replenished. Under clear skies and calm winds we will cool even quicker once the sun goes down.
Think of clouds as a barrier. Under cloudy skies when the surface radiates the heat it sort of bounces off the clouds and back to the surface. I think ricochet would be a good word here. Under clear skies that doesn’t happen and the heat continues to rise further and further.
Under breezy or windy conditions the air is able to mix which would prevent us from cooling as quickly. The air at the surface cools much quicker than the air above. With a bit of wind we can mix that air back down to the surface. With calm winds that mixing doesn’t happen and the cooling at the surface continues.
Putting this all into numbers, lows tonight and tomorrow will easily dip down into the mid and upper 20s.
The good news is that thanks to an area of low pressure of the NE and the aforementioned high, the coldest air will stay well to our north.