I frequently see references to so called “urban heating” which try to explain the phenomenon by handwaving references to anecodatal causes such as direct heating from houses or changes in solar absorption of concrete. Most of these are so obviously wrong that I felt like giving a quick debunk of some of them.

Direct heating

London was the first place where urban heating was recognised and it is usually quoted as being raised by a degree centigrade or more:

The Mean Temperature of the Climate … is strictly about 48.50° Fahr.: but in the denser parts of the metropolis, the heat is raised, by the effect of the population and fires, to 50.50°; and it must be proportionately affected in the suburban parts.

Based on the size of London (609square miles) and population (1.3 million households), thus the average area per household is about 1000m². When I last looked average electricity consumption is 400w and something like half of all energy usage, so let’s say 1000w over 1000m2 = 1w/m². The change in global temperature is usually quoted at about 4w/m² per degree C suggesting around 0.25C warming. But the figure will be smaller for a small region as there are more ways for heat energy to escape,  thus this direct heat accounts for only a small fraction of the warming.

Albedo

I searched for papers giving the relative change in albedo between urban and rural environments. And it quickly became apparent that they are difficult to find (This is often an indication that the subject doesn’t fit in with the global warming alarm). However, despite the lack of research, this subject is relatively easy to check, because most energy from the sun arrives in the visible region, a change in albedo will be shown as a difference in lightness/darkness of the ground. To check this I had a look at aerial photos. I chose relatively small areas where i could be certain the images were from the same run of aerial photos.First an area close to where I live in Scotland where there is a distinct change from urban to rural:

A village in Scotland where an area of urban and rural image have been rendered black and white and then blurred.

The original aim of blurring the square was to produce a uniform shade. I think the difference at the centre was 33 & 38 percent?) but perhaps the fact that the square are not uniform shade, but still retains some of the change within the area, also shows that the variation between urban and rural is far less than the variation within urban and rural.

However, this is Scotland where it is damp and moss grows on roofs. So I wanted to check this in a the drier parts of the UK and chose London.

South London showing areas of greenery amongst the housing in colour (left) black and white (right). When shown as B&W, the green largely disappears except for areas of woodland. Likewise, the urban area is relatively uniform except for modern warehouse style buildings which show white.
I was going to repeat the exercise, but as soon as I removed the colour, I was unable to spot the areas of green as they were virtually the same colour as the areas of housing. The easiest way to show this was to reproduce the image with and without colour. Again, the variation within areas of urban and rural is more significant than the variation of the average between them. Importantly, the really dark areas are woodland and the very light areas are warehouses. And of the two, in all but a very few places, the area of woodland is much greater and therefore more important in terms of temperature than the area of warehouse, which although they stick out like a soar thumb, their size is small.
Thus contrary to many assertions that albedo dramatically changes, there is in fact very little difference in albedo between (UK) housing and open fields. Indeed, as the following shows even in Central London the difference is small
.
Image of central London shown in colour & B&W showing that even in central London, there is little difference in shade between built up areas and parkland.
In contrast there is very considerable differences between types of farm land. As such it is difficult to explain “urban heating” by a change in albedo.

The real causes of urban heating

This leave two other related effects:
1. A reduction in average wind speed over urban environments
2. A reduction in evaporation
The reason they are related is because around 50% of all heat lost via air movement from the surface is through evaporation, and this is in turn affected by wind speed.
The reason the wind speed is reduced is because the presence of housing roughens the surface and increases drag on wind. This also happens in woodland, where the wind speed is dramatically reduced near the ground. But unlike woodland, houses do not evaporate from their roofs. So the key here is that air speed is dramatically reduced at the level at which evaporation from plants is important.
Anyone who has ever gone into a wood on a hot day in a dry summer, knows that a wood is much cooler than the dry open fields whereas the wind speed reduces and the humidity increases. Thus, if it weren’t for the active cooling by the trees (with deep roots which still tap into water), then it would feel much warmer in the woodland.

The flip side: urban runoff

Urban areas are characterised by 75-100 percent impermeable material which is much higher than rural areas, which may have only 10% impermeable material. As water that runs off is therefore unavailable for evaporative cooling, this difference contributes to higher surface area temperatures. It is complicated because water also penetrates into the ground, so not all the precipitation contributes to cooling but with about 50% of the heat leaving the ground leaving through convection, and with evaporation being one of the key factors driving convection, even a small change in evaporative cooling in the urban environment will have a significant effect on temperature.

The flip flip side: urban cooling

The bizarre thing is that in some areas such as Australia, urbanisation has led to cooling not warming. The likely explanation is simple: in dry regions, people bring in water to grow plants. The result is that the evaporation from the plants cools the environment.

Conclusion

Whilst direct heating is the most often cause cited for urban heating, the actual scale of direct heating is relatively small. Likewise whilst changes in albedo can be significant particularly between woodland and farmland or between housing and warehouses, the average difference in albedo between urban and rural (farmland/parkland) environments is relatively small. Therefore the main reasons for urban warming are the effect buildings have in reducing average wind speeds at the “growing level” (i.e. where most plant leaves occur), and the reduction in evaporative cooling due to a reduction in plants and an increase in hard surfaces with runoff (which is then not available for cooling).