Saturday, 8 March 2014

Are trees good for you?

Of course they are.  I firmly believe that.  Obviously, so do advocates of urban forests. Oh, yes, I am one of those.

Advocacy for increased attention to tree scale planting in cities, for the establishment of viable urban forests, is at least a generation old.  And even though rarely adopted as a coherent policy by cities during that time, there has nevertheless been a distinct trend for the public and private domains to acquire significantly more green fuzz.

Paddington, Sydney 2012
It is quite enlightening to compare old aerial photographs of 'inner ring' suburbs of a city like Sydney, with recent images.  It becomes clear how little greenery featured in these places when they were a dense matrix of unregulated industry and predominantly worker housing, strung together by tramways and above ground electricity and telephone wiring.  In contrast, gentrification also seems to bring a commitment to transforming bare streets to treed, and small gardens to intensely planted havens.  In particular, not so many people these days seem to share the former suburban phobias about gutters blocked by leaf litter, so they don't maintain personal vendettas against trees taller than three meters.
Paddington, Sydney, 1947

But lately there also seems to be a more concerted push towards urban forests, driven by a loose conception that greening the city will help do away with the so-called 'urban heat island effect'.

The latest such initiative I have come across is from Melbourne, Australia, already one of the worlds most livable cities.  The article is on, and may very well have some of the usual baggage of journalistic oversimplification, but I suspect that the point I am about to tease apart is not one of them.

Thermal image of inner Melbourne
As almost always, the key image in the argument for the putative benefits of a green canopy is an aerial view as a 'false colour' thermal photograph.  The image supporting the Melbourne press releases is the prettily coloured one I reproduce in this post.  What you are meant to see is that all the parklands show up as reassuringly blue-green cool, in contrast to the warm yellow-reds of all the concrete and asphalt of buildings and roads.
But now look more carefully.  Look for all the coolest surfaces, the ones in the most intense blues.  I am not really familiar with Melbourne, but I can tell by the shapes that the coolest surfaces are in fact the metal roofs of the sporting venues, and indeed of the large industrial and warehouse sheds.  Now, I don't know too much more about the total heat balance over time, of these lightweight roofs as opposed to the grass and canopies of the areas of vegetation.  But I am sure my reading of the pretty picture is not going to be popular with the people who would like it to be self-evident, that literally greening the city will by itself make the urban heat islands go away.

Most readers of my blog would be familiar with my obsession with looking more carefully at data and imagery, particularly when looking for evidence of technical performance.  Well, this is one more example.  In fact, US research suggests that simply extending green surfaces by way of street planting and vegetated roofs makes far less impact on the city's average albedo than we would hope.

Which is not to say that a lot of the other benefits of trees in cities are not worth it for their own sakes.  I'd rather a shaded street sidewalk to walk along than to bake in the merciless sun of a Melbourne summer, I'd rather the leaves overhead cooling and humidifying by transpiration the microclimate under them.  But to capture that correct depiction of what happens, you need to show thermal images of the street at pedestrian level.  It would be just as compelling as the misinformation of the thoughtlessly chosen and misleading aerial.

Read the article here.

To see another similar aerial thermal image, with exactly the same issues, see the ones of Sydney, here.  But if the reader is interested, this page also links to other microclimate monitoring by the City of Sydney.  If really, truly interested, take the trouble to go to the UNSW link and download the full report of the pilot project
Micro-Urban-ClimaticThermal Emissions: in a Medium-Density Residential Precinct.  Don't be put off by the poorly written report itself, and spend your time instead on the comprehensive literature and methodology review in Appendix 1.
Micro-Urban-Climatic Thermal Emissions: in a Medium-Density Residential Precinct - See more at:
Thermal Impact of the Designed Environment on the Urban Heat Island - See more at:


Anonymous said...

It is interesting to see that the metal roofs are in fact cooler. I would have thought metal heats up more quickly since it is such a good conductor of heat. Afterall, I thought that was the exact reason why not many people use metal roofs. Pardon me, I am from Singapore, and there is currently a frenzy over green roof. So, I did a little bit of research on metal roofing and it surprised me.

There are two important properties of a roof that affects the temperature of a building, namely the solar reflectance and infrared emittance of the roof surface. The reflectance determines how much heat incident on the material gets reflected, whereas the infrared emittance determines how far the heat energy got deflected away. Therefore, since metal is more reflective than dark asphalt or terra-cotta, it reflects and deflects heat energy more effectively, hence resulting in a cooler surface.

Uncoated metal roof would be sufficient, but in warmer temperate, granular coated or painted metal roof would be preferable. I am not sure if the roof profiles would make a difference, though?



Metal Roofing Alliance

Steve King said...

Metal is light weight, and therefore stores very little heat. When you look at a night-time thermal image, like the one illustrated, you see all the thermally massive surfaces, like concrete and roads, which retain and continue to emit heat.

BTW. 'Deflects heat' is not a technical term that anyone would use, because it doesn't describe what is happening.

Anonymous said...

Singapore have been obsessed with this notion of a green city for a couple of decades now. Since independence in the 1960s, the then prime minister Mr Lee Kuan Yew put forth a plan to use greenery to differentiate itself from competing neighbors. Razing slums and cleaning up the river to make way for more green spaces.

This is embedded in it's urban planning. In Singapore, a building site outside the core area have to have a 2m "Green buffer" on all sides. No physical structure must encroach into this zone. This green verge must be planted, and is non negotiable. And if a development provides a publicly accessible sky terrace, the development is allowed to built an extra floor. Also, existing matured trees are to be preserved; the build structure have to be built around it or the tree have to be transplanted to another spot if need be. This creates a lot of constrains for architects in Singapore. However, these constraints helped generate many innovative designs in the city state. For example The School of the Arts by WOHA architects in downtown Singapore. The massive tree on the site was preserved, splitting the building mass into two. (

In land scarce Singapore, Buildings are being built taller and taller. These sky terraces give breathing space to the city's inhabitants. Although the green buffer result in more unusable turfed space due to the tropical climate, it actually helps break the urban landscape, so that the city don't feel as dense as it is.

In a sense, Singapore's "City in a Garden" is less due to its desire to reduce the "Urban Heat island" effect but more on sustainable urban design. Designing a more livable city on a land scarce tropical island of 5 million.

On a side note, look into WOHA architects, they are an innovating Singapore practice and creates beautiful green building using green walls and sky terraces, effectively exploiting the council into giving them more FSR.

Anonymous said...

The misinterpretation of data from one of the many standardized measuring tools is a result of oversimplification which leads to generalisation of what is best practise. Hence, a reassessment of position is to be considered, as not to stray from accurate data provided by the several accessible tools at the industry’s disposal.

Essentially, this post highlights the target issue attempting to be solved- the urban heat island effect, and how we best should go about the problem. What needs attention is the dealing with heat transfers between the built and landscaped environment, and what are effective ways of climate changing agents of cooling within these urban environments.

Although we can see that the consideration of urban forests is of great significance in the provision of shaded and cooling by convection, there are other options available in regards to the raised issue, dealing with material and colour surface painting of roofs (which are also mentioned in later blog posts “Rooves in black and white” and “fiddling with the roof, again”). To be more specific, along with urban forests, the implementation of green(vegetated) roofs and white painted roofs into upcoming design, is a strategy in tacking the heat island issue.

Overall, I think what is important to take from this post, is that a close analysis of data will result in the implementation of holistic design solutions to problems, such as the urban heat island effect.

American Society of Landscape Architects, 2011,
United States Environmental Protection Agency,
Zinzi, M., and S. Agnoli. "Cool and green roofs. An energy and comfort comparison between passive cooling and mitigation urban heat island techniques for residential buildings in the Mediterranean region." Energy and Buildings. 55. (2012): 66-76. Print.