How many times in the last 3 months have we talked about below average temperatures and arctic blasts?? A lot right? Well, how about we look at this logistics behind our current cold spell in a totally different way. The cold we are experiencing this weekend isn't record breaking cold… not even close if you can believe that… but it certainly makes headlines when temperatures are 25 degrees below average. But, this cold blast may actually be setting some records in other parts of the atmosphere.
Almost every aspect of our atmosphere that can be measured, is getting measured. Anything from temperatures to dew point to cloud height, pressure, and wind speed… it's all getting ingested to try and predict what's going to happen next. One of these variables that you may not even think about is thickness. Wait… thickness of what? The atmosphere? Yes, you can measure the thickness of the atmosphere. It is just another way we can determine the potential temperature at the surface. Let me explain…
If we go back to basic physics, we know that cold air is more dense than warm air because as an air parcel warms, it expands, kind of like a balloon would if you were blowing it up. This adds volume to the air parcel which means there is more space for those little molecules to fly around. The more space you have, the less dense those molecules are clustered and the faster they can move is seen as a rise in temperature. Well the atmosphere works in the exact same way except at a much larger scale. So when the atmosphere heats up, it expands which actually makes it thicker than if it were colder. This thickness is measured in decameters from 1000 mb (roughly the surface) to 500 mb (roughly 18000 feet). So the smaller the number, the more compact the atmosphere is, and therefore the colder and drier the atmosphere is.
In the meteorological world, we have 3 thresholds that roughly divide air masses . Anything under 5100 is low and considered to be some sort of arctic air mass. The 5400 number is used as a rough dividing line between snow and rain as temperatures underneath this line are around the freeze mark give or take. And lastly the 5700 line is seen as the tropical barrier with very warm temperatures associated with the air above it.
So let's check out just how compact the atmosphere is expected to get during this cold snap. First, I will show you a numerical representation of the forecast for certain time periods in the coming days.
When these thicknesses are displayed, they are just a 3 digit number because the assumption is there that these will all be around 5000, so the last zero is just left off. This means that the first number, 527 is actually 5270, but the zero is just thrown away. Look how low these numbers get. Remember that low threshold of 5100, or in this case 510? These numbers drop below this tonight and early tomorrow indicating some truly polar air. I bring your attention to these because these are the lowest numbers I have seen this late in the season since I can remember, so we may not be breaking any actual records, but it is extremely cold nonetheless.
Just for comparison, with the arctic blasts we had in January, our thickness values got as low as 4900 which was one of the lowest numbers I have ever seen enter the state of Minnesota. So what do these numbers look like on our computer forecasting model graphics? Well, here you go…
I have circled the number and highlighted the lines that represent the thickness values nationwide. See… there is A LOT more to weather than meets the eye. Stay warm!