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Charlotte-Douglas’ odd temperature readings

coldairdrainage_thumb.jpg

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 2011-12-02_0015this. 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.

coldairdrainage

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.

ASOS elevation

2011-12-02_1648

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.

coldairdrainage

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? Smile

ASOS

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.

CLT2007

2010 Post Airport Expansion:

Now look West and Southwest.

CLT2010

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.

2011-12-02_0015

  • http://twitter.com/SolakNC Tom M

    Sounds reasonable, Brad. I once asked Stu Ostro from NWS RAH about a similar temperature drop at KJNX which is just 2 miles down the road from me. My question was in regards to the wintertime differential between here and RDU, 30 miles to the NW as the crow flys. Another thing he pointed out was that KJNX is approx. 200′+/- lower in elevation than RDU, and also mentioned the ‘lay of the land’ around the airport, although it’s nowhere as busy as CLT. It’s nice to see your similar theory in writing. As it stands, on these clear, calm, cold nights, we can almost always be assured of being, as RAH so fondly refers to us, one of those “…temperatures lower in the usual rural locations” — Tom

    • Anonymous

      Tom, yeah the ASOS at CLT is 4′ lower than that runway and almost 10″ lower than the ridge just to the west. Makes a big difference.

  • Anonymous

    So the question is; Is this really an accurate reading of the official temperature in Charlotte? Seems like the ASOS should be relocated to an area of the airport that provides an better representation of the the true air temperature in Charlotte.

    • Anonymous

      This a valid question & I think yes maybe just to another spot on the the airport grounds. I think the day time highs are fine it’s really the night time lows that maybe a cooler representation of Charlotte as a whole. I will also say the airport is way better than the previous location prior in uptown.

      • Anonymous

        Where was the exact location previously Uptown?

        • Anonymous

          Prior to 1939 is was actually on top of the old uptown post office. LOL

  • http://twitter.com/memphisweather1 MemphisWeather.net

    Brad, Nice write-up and makes sense. One additional question that would help prove your point with the evening temp rise would be to see if you could verify the ground movement of aircraft during those hours. Not sure how US Airways works their “banks” of flights, but is there a scheduled bank of departures during the 9-10pm timeframe and is their preferred departure configuration to the south when wind is light/calm?

    –Erik

    • Anonymous

      Already checking with my neighbor who’s a pilot for them. Problem with public searches so far is you only get the “schedule” departure times. Would love to know which way they are coming in and taking off on that runway.

  • http://twitter.com/bdibar Brian DiBartolo

    nice write-up, Brad. I was wondering where the ASOS was located @ CLT. I believe with the new runway, there should be a discussion of relocating the instruments.

    I have a question in regards to the official ASOS at the Gastonia Municipal airport, has there been any discussion of moving it to be “inside” the city limits? The airport was way out in the county, a good 15-20 minute drive from downtown Gastonia (6 miles away)

    • Anonymous

      Thanks Brian, I agree they need to move Gastonia closer to at least the middle of the county. Heck it’s almost too close to CLT which is why I know something fishy is going on at CLT. I haven’t heard of any plans to move it but more likely would be an additional station in the county closer to downtown.

  • http://twitter.com/Cole_Regan Cole Regan

    As a meteorology student at UNC Asheville, we focus on ASOS quite a bit as it serves as a major part of getting current conditions. I thought this article was very interesting and many of the things you brought up were very well laid out. When I finally found the ASOS at KCLT, I was actually surprised at how close to Runway 18C it is. Having an east wind could send some warm air from the terminal area towards the ASOS, and having two large runways and two taxiways doesn’t provide the best readings. I’m sure many would say we should move it to a different location (like downtown Charlotte) but then you get the effect of the concrete/pavement and other warm factors relative to downtown that don’t help temperature readings. I’d keep it at KCLT, but I’d suggest to the WFO at GSP that they should consider moving it to a different location on the airport property. Looking at the 2010 Google Maps shot you provided, it doesn’t seem that there’s a definite location to put the equipment.

    • Anonymous

      Cole, I agree it should be moved but to a location somewhere on the airport property. That airport expansion sure does make it harder though.

      • http://twitter.com/Cole_Regan Cole Regan

        We may not have the best placement of our ASOS, but of course there’s the famous USC ASOS location. This was the talk of our weather analysis class when we discussed poor locations. I’m not sure if you’ve seen it, but this is terrible! http://wattsupwiththat.files.wordpress.com/2008/03/usc_aerial_asos-520.jpg?w=640

      • http://www.thebetaversion.org Greg Simmons

        And the pending new runway and terminal expansion at KCLT don’t make things any easier. All the extra concrete, jet wash and loss of vegetation just add to wild swings in the temperature.

  • http://twitter.com/MikeOnPipes Mike Lynch

    Brad, I’m a layman when it comes to meteorology, but I really enjoy your posts. This is very interesting AND educational! Thanks for sharing.

    • Anonymous

      Thanks Mike!

  • Anonymous

    Last night was another odd night, though we did have a dry cold front come through and complicate things. The winds were pretty breezy last night but some how we dipped to 31°. check the obs below.

  • Kwazie Wabbit

    Impressive….

  • Anonymous

    so geeky! i love it.

  • Anonymous

    I have never understood how climatological data that was taken in 1900 in uptown/downtown Charlotte can be projected to an airport miles away.

    • Anonymous

      Cannoneer, yeah it’s an issue when you compare downtown temps prior to 1939 to temps after at the airport. I sometimes separate climate numbers from the airport and downtown for this reason.

  • Anonymous

    And wxbrad…thanks!

  • Anonymous

    I knew something was different at the airport. There are businesses located on Eastway Drive and Sugarcreek Road which post temperatures that are typically higher than it is at the airport. Thank you for this article.

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