A Tucson climate scientist commenting on the location of the University of Arizona weather station in parking lot acknowledged that:
It’s true that situating a weather station in a parking lot is not the best location.
An understatement to day the least. He argued against locating the weather station on a nearby lawn:
The emissivity of the asphalt is higher than that of grass. That’s why it’s hotter when you stand over the asphalt compared to a grassy area. However, the native rocky terrain of the Southwest also has a high emissivity. [I tried to find a plot of the spectral emissivity of dry soil, but couldn’t locate one quickly.] I would posit that the dry soil has spectral characteristics more closely related to the asphalt than the grass.
He absurdly proposed that the parking lot wasn’t used very often, suggesting that the cars had not been moved in over 3 years. Given that he could see the parking lot from his window, this shows some inattentiveness to observational detail. His point that the nearby lawn would not be representative of Southwest terrain was fair enough, but he was wrong to merely say that the parking lot is “not the best location”. The location did not meet minimal WMO standards – a lapse thast is inexcusable in a university department.
But let’s turn to the infrared properties of asphalt – is it really like dry soil? I’ve collated a few searches below, though obviously not a full survey.
Aseada ert al 1996 in ” Heat storage of pavement and its effect on the lower atmosphere” stated:
Heat flux at the air/ground interface was observed and analyzed for various pavement materials on summer days. The surface temperature, heat storage and its subsequent emission to the atmosphere were significantly greater for asphalt than for concrete or bare soil. At the maximum, asphalt pavement emitted an additional 150 W m[-2] in infrared radiation and 200 W m[-2] in sensible transport compared to a bare soil surface. Analyses based on a parallel layers model of the atmosphere indicated that most of the infrared radiation from the ground was absorbed within 200 m of the lower atmosphere, affecting air temperature near the ground. With large difference between air and ground surface temperature at noon, the rate of infrared absorption by the lower atmosphere over asphalt pavement was greater by 60 W m[-2] than that over the soil surface or concrete pavement, a figure comparable to the absorption by turbulent transport.
These are obviously not small numbers and point to a substantial differences between asphalt and bare soil. Toudert 2005 , which has much interesting material on heat flux in an urban context, stated:
With respect to urban surfaces, Aseada et al. (1996) pointed out the importance of the pavement materials in the resulting heat fluxes and air-ground interface on summer days. They reported that an asphalt pavement emits an additional 150 Wm-2 infrared radiation and 200 Wm-2 sensible transport compared to a bare soil surface. The water content in a bare soil and thus the evaporation from it produces much lower surface temperatures. By contrast, waterproof soils such as asphalt, increasing thickness of the covering material increase the temperature and heat stored under the surfaces (Asaeda and Ca 1993). Urban surfaces with high albedos typical of light colours reduce the storage in the materials (Doll et al. 1985, Akbari et al. 1995, Taha 1997, Taha et al. 1997).
Some simple measurements were reported in a chatline here:
Old asphalt street: soles feel warm after 50 sec. Measured 141 deg F
New asphalt parking lot: soles feel hot after 20 sec. Measured 162 deg F
Steel manhole cover: soles feel very hot after 8 sec: Measured 140 deg F
By the way, I recently verified my theory about surface heat absorption on asphalt. I took my infrared thermometer out with me and measured a spot on the parking lot asphalt at 158 degrees F. Then I placed the ball of my bare foot over that spot and stood for a couple of seconds then measured the spot again. It read only 136 degrees! That means if your eyes could see infrared, you would see cool footprints left on the asphalt as you walk across it. I tried the same thing on the steel manhole cover and it only dropped about 2 or 3 degrees!
UCAR in a teaching kit for high school students (which USHCN researchers might well apply) proposed the following experiment:
Proceed to specified parking lot and cars. Take surface measurements with an infrared heat detector of each car’s roof, hood, trunk, windshield, dash board, and tire. Lastly take a measurement of the asphalt surface, parking lot line markings, and the nearest sidewalk.
Maybe Atmoz could try the experiment in Tucson and report back on whether he still considers the Tucson asphalt parking lot an adequate location for a weather station.
Fazia Ali Toudert, 2005. Dependence of Outdoor Thermal Comfort on Street Design in Hot and Dry Climate, http://www.meteo.uni-freiburg.de/forschung/publikationen/berichte/bericht_nr._15.pdf
ASAEDA T.) ; CA V. T. ; WAKE A. ; TSUTSUMI Jun-ichiro ; BORNSTEIN R. , 1996. Heat storage of pavement and its effect on the lower atmosphere, CUTEST ’92 : conference on the urban thermal environmental studies in Tohwa, Fukuoka , JAPON http://cat.inist.fr/?aModele=afficheN&cpsidt=2964741