Steve S gets right to the point;

“Does this represent an effect on surface measurements which would convey the impression of a significant alteration in climate?”

Land stations by definition can only occupy ~26% of the Planet’s surface (the remaining ~74% being ocean areas. Subtracting out from that ~26% an additional (and considerable) percentage of uninhabited (and uninstrumented) land areas, then it would seem obvious that on the planetary scale these improperly named “global” surface station networks are clearly overweighted with urban areas.

To answer my own question; “…[W]hat percentage of the planet is urbanized?” The following appears in an article which can be found here.

According to the United Nations Population Division, urban regions only cover 0.2 percent of Earth’s land surface, but contain nearly half of the world’s population. By 2025, 60 percent of the world’s population will live in cities and urban landscapes will likely expand well beyond 0.2 percent. However, to date, global climate models (GCMs) and regional climate models (RCMs) do not reflect urban landscapes, according to the paper’s authors.

Fascinating. I’m off to a meeting but will come back to this.

On another thread I believe someone provided figures on urbanization.. Something like 2%

Check the Parker thread ( parker 2006 and urban myth )
I think it was there. Also, you’ll find my crude analysis of UHI
there. Please eviserate it, Do it on this thread if you like, as the parker 2006 is
winding down at bit.

My question would be different. “Does this represent an effect on surface measurements which would convey the impression of a significant alteration in climate?”

In my experience with temperature data…mostly from CRS instrumentation …the princpal delta T,s between urban and rural areas occur on clear and calm nights where radiationally sponsored heat loss at the ground and other surfaces near and around the instruments is most effective. Urban exposures are almost universally less cold under these conditions as a much more complex system of absorption and reradiation from concrete, brick and other artifical structures prevails.

The most profound examples of radiational cooling can be found over snow cover on clear, calm nights. Snow is (ironically) an almost perfect black-body radiator with low thermal conductivity. The effect of its low conductivity makes it an excellent thermal blanket as it inhibits the capability of the snow surface having its radiational heat loss restored by heat conducted up from the ground. If dewpoints in the air above the snow surface are low enough, the nocturnal cooling process will continue right into sunrise. On such cloudless, calm nights s, the actual minima are usually reached shortly after sunrise. Why then? Because as the sun starts to heat the surface, some convective movement of air develops generating light winds, and this allows a portion of the very cold surface temperatures to rise to the level of the sheltered instruments (typically at 1.5m elevation).

During the daytime, the snowcover reflects a significant portion of incoming radiational energy skyward (or spaceward). Thus, even daytime temperatures are colder than their urban counterparts when there is snow. Over sod, the day/night radiational properties are quite different, preferentially absorbing energy in the red end of the spectrum and in the UV as well (the reason that grass is green). Grass is also capable of replacing radiational heat loss at night by drawing heat from the ground. Even so, they cool down better than concrete or other compounds with large thermal mass.

Finally, on windy nights, even in rural areas, radiational heat loss at the ground surface is mixed more deeply through the boundary layer and the sensible heat loss through radiational cooling is distributed through a much larger volume of air.

BTW, an UHI is by definition an anthropogenically caused temperature enhancement and no one has argued this in my lifetime. The real question is that on the planetary scale of things does this represent a significant alteration of climate. Can any geographer out there with a facility for pushing numbers, or finding data sources, tell me what percentage of the planet is urbanized?