I got involved in a discussion at WUWT about recent unusually early high temperatures in S Kansas, and particularly a reading May 5 of 102°F at Wichita. The WUWT contention was that this and other high temperatures could be ignored because they were measured at airports. But there were other NWS Coop readings not at airports, and I drew attention to one, WWXK1, that reported 101°F, and was on the airport land, but a mile from the Wichita ASOS site, and away from airport activity in grassy land. It also turns out to be a well-equipped NWS site.
At WUWT there were objections to a path, parking not too far away. But still, it's open land around, with a not major road and extensive parkland opposite, and I do tend to think the NWS knows what they are doing. It seems a modern enough site (photo below). So I thought the corroboration of the Airport measure was significant.
Anyway, aside from the immediate issue, I thought this conjunction of sites gave a good opportunity to test in local detail how airport siting might affect temperature measurement. They are only a mile apart on flat grassy land, so if it were not for airport perturbations, one might expect temperatures to be very close.
So I downloaded the GHCN daily unadjusted TMAX and TMIN data. The airport (KICT) code is USW00003928 and the NWS is USC00148847.
My first disappointment was that the NWS record starts in May 2010. If anyone knows where to find more, I'd be glad of a pointer, because the site doesn't look that new. So I can't get a trend difference, but mainly I'm interested in just what kind of day to day differences are observed.
I'll show below plots of those differences. The upshot is that the airport site averages about 0.4°C higher on both TMAX and TMIN. Of course any two apparently similar sites separated by a mile might show such a difference. But it is interesting to see how they track.
Here is a Google Map of the airport site overall:
The official airport ASOS thermometer is in the orange ring, and the NWS site is in the light green. The ASOS site is where the usually quoted Wichita temperatures come from. The airport is on the western edge of the city, with just one recent suburb further west from the km wide park across from the NWS.
Anthony Watts at the WUWT post showed a closeup of the ASOS site:
I have to say it doesn't look too bad to me. But let's see.
Here is a closer view of the NWS site. I think the MMTS instrument is about in the middle of the green ring.
Anthony posted this very good shot
which shows the various instrumentation nicely. you can read criticism of the site here and below.
It turns out that they track each other well, so for a general view of the yearly cycle, I'll show just the ASOS site for 2012-3. I show Oct-Sep so you can see a whole summer and a winter. Months are marked in colors to distinguish.
OK, now here is the plot of differences (ASOS-NWS) for TMAX for each day of the five part years. Light grey lines show 1°C intervals - I've put temperatures on the y axis to quantify the scale, but each year is offset.
Red means the airport ASOS is higher, and you'll see that it is indeed. I've marked large deviations in a transparent color when they overlap the adjacent year. The values are quantised, reflecting readings in whole °F.
There are some large spikes, which I think are some kind of error. For example, on March 12, the ASOS showed 15.0°C, and the NWS site 21.7. Both agree the adjacent days were 21-22°C, and the minima were similar, at about 0°C. So I first thought the ASOS might be wrong, although the NWS max was a repeat of the previous day, which is a little suspicious. To add to the confusion, Tutiempo gives rather different readings, with a dip to 14.4°C on 12th. But Tutiempo also has a similar dip at another Wichita airport (JABARA).So I think on balance the error if any is at NWS. One would expect fewer errors at ASOS. Some other such deviations I looked at also looked like glitches at NWS.
Other than spikes, there isn't much interesting seasonally in the TMAX data. The ASOS site is usually about 1°F warmer; the annual average differences are below, in °C:
Here is the corresponding plot of daily minima. There are similar spikes, again I think mainly glitches at NWS. But there is now also some seasonal variation, with the ASOS often colder in winter.
I wondered if the inversion might happen on very cold nights, but I can't find such a pattern
The airport ASOS site is consistently warmer than its grassy neighbour, but only by about 0.4°C. With just one case, one can't definitely attribute that to airport effect. The winter reversal of minima is interesting, but aside from that there doesn't seem to be any pattern.