WMO guidelines state that weather stations should be at least 100 feet from paved areas. As we see the USHCN pictures unfold, we’re obviously seeing one site after another in non-compliance with this requirement, a point notably made in connection with Tucson (Univesity of Arizona) site, where the location was particularly gross, but the point is seemingly pervasive. While many of these pictures also show air conditioners, my guess is that the asphalt pavement may prove to be a more substantial problem than the air conditioners.
I notice that GISS apologist Eli Rabett has another post arguing that traditional quality control doesn’t matter – this time arguing that heat rises and thus, for example, nearby air conditioners don’t matter. Perhaps so, perhaps not. Eli’s implication is that WMO policies don’t “matter”, that, in effect, the practical WMO people are just fuddy-duddies, making pointless QC demands that are unnecessary when Hansen’s on the scene with magic adjustment software. While Eli’s implied criticism of WMO policies may be borne out, my own guess is that the WMO guidelines were created for a reason and that they embody useful practical knowledge – that there’s a reason why, for example, WMO guidelines require that weather stations be 100 feet from pavement and perhaps there are even reasons not to locate them near air conditioners.
But today a little more on pavement and specifically asphalt pavement and why it’s not a good idea to locate weather stations within 100 feet of pavement. The radius is relevant since the pavement strongly re-radiates IR and will affect weather stations that are not directly above it.
Asheville NC – ASOS and CRN
Eli has posited the CRN program as essentially a complete answer to the defects of the USHCN record. While I endorse the creation of a valid record going forward, I must say that this by itself does not negate the need to carefully scrutinize the historical record. Having said that, the CRN information can shed light on issues in the historical record. A recent conference paper compares ASOS and CRN instrumentation at an identical site in Sterling VA and between an ASOS station (not a USHCN station) and the Asheville CRN station, only 1.5 miles away. (This paper was cited by a poster at Eli Rabett here.)
The Sterling, VA test showed slight biases between ASOS and CRN measurements under varying conditions of wind, solar radiation etc.
The Asheville ASOS station information here including photo is a site that really looks pretty good in the scheme of things – the sensor is not located directly above pavement, no air conditioners, barbecues or basketball nets.
Asheville ASOS station.
Yet even this very good site had a local warming bias of of about 0.25 deg C, which the authors attributed to “airport runway and parking lots next to the ASOS site” as follows:
At the Asheville site, the effect of siting difference between the ASOS and CRN led to a Î”T_local effect of about 0.25o C, much larger than the Î”Tshield effect (about -0.1o C). This local warming effect, caused by the heat from the airport runway and parking lots next to the ASOS site, was found to be strongly modulated by wind direction, solar radiation, and cloud type and height.
The existence of an effect as large as this in what appears to be an exemplary site should give a little pause to GISS apologists for really bad sites, such as Tucson. Indeed, it creates an issue for essentially every weather station which is non-compliant with the WMO pavement policy.
Infrared Pavement Images
Some extremely interesting images and analysis of the infrared properties of different pavements has been carried out by advocates of “cool pavement”. Much of the work has been carried out in Arizona – so the U of Arizona climate scientist who was stumped by the infrared properties of asphalt really didnt have to go very far to find information. Here are two interesting online links pavements4life EPA Cool Pavement Report The images and analysis below are taken from pavements4life.
Here is a remarkable infrared photograph showing parking lots in Phoenix, in which the infrared coloring has a temperature code attached to it.
The first graphic shows the substantial differences between asphalt, concrete and vegetated surfaces in Phoenix. In this picture, there is about a 50 degree difference between the temperature of the asphalt and the temperature of the lawn. In addition to this contrast, note the very sharp difference between the temperature of asphalt and the temperature of concrete, which can be very distinctly seen in the left photograph. Even scrub vegetation lowers the surface temperature. The bare earth has a distinctly lower temperature than asphalt, though not nearly as low as grassy surfaces. As noted in my earlier post on asphalt, the IR emissions (in wm-2) from asphalt surfaces can be extremely high and can hardly be neglected.
The pavements4life website reported that “asphalt pavement temperatures in Phoenix have reached as high as 172 ⹆.” They showed a marked diurnal cycle in the differential between asphalt and other surfaces in the figure below – a phenomenon that is presumably well enough known in Arizona to have been considered by the University of Arizona meteorology department in locating their weather station not only in a parking lot, but closer to surface than normal.
Another figure at the website shows the effect of wind speed on the daily cycle – in this case, calm is associated with a higher site effect, but the effect is by far the most pronounced on the maximum temperature and NOT on the nocturnal minimum. (Oke is from Vancouver and other effects may well be more important in Phoenix and Tucson.)
Given these known problems with paved areas in Arizona, here, one more time, is the Tucson USHCN station, the site with the greatest temperature trend in the United States. I’ve observed that the temperature history from the Tucson station is very different from the Grand Canyon temperature history and questioned the GISS adjustments.
But the issue here is a little different: I’m contesting Eli Rabett’s implication that WMO policies are, in effect, old-fashioned and that adherence to WMO standards (or not) doesn’t “matter”. And perhaps the WMO rules are not at all that necessary (but this has not been established yet), but there’s not a shred of evidence so far that their rules on pavement are invalid. The Asheville NC measurements show an impact in a very decent site that far exceeds the Jones 0.1 deg C upper limit – the Tucson effect is going to be much much higher.
It should also be noted that the proportion of pavement can vary by city. The EPA study shows the following table, in which Sacramento has a larger proportion of paved (and roofed) area than Houston. So there’s not necessarily one simple formula that fits all cities – population is merely a proxy.