Ever since I’ve been working in Charlotte there has been a running joke among my fellow meteorologists about the “official” temperature readings taken out at the airport. For years I’ve noticed there is a sharp dip in temperatures in-between hours in the morning. We’d be at 30° at 6am and 29° at 7am but some how in the hour in-between the low would come in at 25°.
Now this can be somewhat explained by evaporative cooling of the dew on the grass surface right after the sun comes up, but this is a little extreme.
The other oddity is that on nights when we have clear skies, calm winds and dry air in place for ideal radiational cooling. We get odd jumps in temperatures between 9-11pm. Last night was a perfect example of this. You can see to the right that the temperature was falling steadily from sunset which was at 5:12pm(17:12). Then we started cooling down fast as we dropped from 43° at 8pm down to 35° at 9pm. Then for some odd reason the temperature jumped back up 6° to 41° at 10pm, then it kept rising to 43° at 11pm. Then at midnight back down to 35°.
So what gives? Well I have a few ideas and the first one has an impact on the morning lows directly and contributes to the evening jumps. The ASOS(Automated Surface Observing System) as we call them is sited in a low spot between a runway and a wooded area. In fact it lies in a lowering that also serves as drainage for water on the airport grounds. The problem with this is cold air at night drains just like water to the lowest spot it can find. Due to cold air being denser than warmer air. Below is an illustration of this cold air drainage.
Now take at look at the location of Charlotte-Douglas’s thermometer. Below are 2 views of the ASOS site notice the low spot it sits in. Then in the USGS terrain map you actually see the water drainage areas marked by the blue arrows.
So this location clearly gets cold air drainage and this may explain the morning lows dropping around sunrise so much. The coldest of air would just be settling in right after sunset into the morning when we have the ideal radiational cooling set-up. I should also note that Charlotte(CLT) was one of the few locations to actually have their 1980-2010 climate averages go down (see previous wxbrad post). I think this cold air drainage might have contributed to that for sure.
So what’s going on in the evenings?
Since I have established we have a cold air drainage location what could causes a sudden rise in temperatures in the evenings? Well here’s a great example of a cold air drainage temperature profile, slight exaggeration but it makes the point.
So what would happened if you mixed some of the warmer air from the higher spots down into the lower spots? Likely a temperature rise would occur. We’ll at the airport in the evenings we have lots of planes taking off and landing very close by. Notice that big runway to the right. Think maybe a huge U.S. Airways Airbus taking off on a night when we have cold air drainage might mix things up a bit?
If one runway doesn’t convince you how about an airport expansion with a new taxiway on the south side of the ASOS? Check out the changes.
2007 Pre-Airport Expansion:
Lots of wooded areas to the West and Southwest.
2010 Post Airport Expansion:
Now look West and Southwest.
Notice not only are all the trees cut down between the airport and 485 but there is a new taxiway just to the south of the ASOS. So now there are planes traveling on 2 sides of the ASOS instrumentation. There’s no doubt that these changes to the airport and the increased airport traffic are causing the inversion on cold nights to temporarily break down. Causing a warm up in the evenings when many flights are taking off or landing. I also think that any light south breeze which is now coming from a concrete taxiway to the south could also cause these brief warms ups. Remember this from last night notice the wind direction change when the temperature went up. West and Southwest light wind which is blowing right from the new taxiway and the airport expansion area.