Thermal absorption – a black and green issue

How some surfaces affect average temperature and rate of cooling

[Update – Broken links fixed 28/10/10 – please advise if any further problems]

Figure 1. Sensor results from Queens Botanical Gardens experimental weather station

Anthony Watts investigation (here and here) of how the poor siting of the electronic temperature sensor at Carefree AZ Skypark airport likely contributed to a record temperature again threw up the issue of thermal absorption of (black) asphalt and dark surfaces generally near to temperature sensors. A graphic illustration of this comes from research by NASA/GISS into cooling cities and reducing the urban heat island. The background to this explains:

About 14 percent of the New York City’s impervious surface area consists of rooftops, most of them dark, heat-generating surfaces, typically tar, and sometimes overlain with gravel. “The tar beach roof—ubiquitous in cities—is an oven in the summer, reaching 160 degrees Fahrenheit,” [….] Two options exist for changing a tar beach roof into something cooler: a light-reflecting surface and vegetation.

And so an experiment was born. NY Columbia University Center for Climate Systems Research website now links to three temperature logging ‘green roof weather stations’ at the university, Ethical Culture Fieldston School (Fig. 2) in the Bronx and Queens Botanical Gardens (Fig. 1; Fig 3).

Fieldston School and the university have “Sedum roofs” planted with spreading succulents in a shallow  growth medium (4″ and 2″ respectively for the two locations); the Queens site is a more deeply planted garden (6″).  All three are monitoring the temperature over the green roof vs a conventional black roof, the sub surface and ambient air temperatures and, in the case of Queens , a white painted roof.

Figure 2. Sedum roof at Fieldston School

Figure 3. Roof garden weather station in Queens Botanical Garden


In addition to the static graph (Figure 1) at the head of the posting, updating links for the three sites are here: Queens BG, Columbia University, Fieldston School. Each website also has a page with some sensor detail on it; the sensor on each roof type is 1′ above the surface. Taking a look at the maximum temperature for each of the roofs (Fig. 4) there are only small differences on an overcast day, but on a sunny day the black roofs are up to twice the temperature of the green roofs. At night all three green roofs retain heat (Fig. 5).

Figure 4. Differences between sites and roof types on a sunny and overcast day

Figure 5. Differences between sites and roof types minimum temperature and sub surface temperature (under each green roof)

Figure 6. Summary of local weather conditions for New York on relevant dates (source Weather Underground - Astoria NY)

Although the three locations are within ~10km of each other, there are variations in the air temperature recorded at each site – on 11th July: Queens recorded (Tmax) 32.8°C; Fieldston 29.4°C, University 34.4°C (data from Weather Underground (Fig. 6) (weekly graph here).  Gaffin et al., 2009 discussing the work note this, particularly the difference between the University (downtown Manhattan) and Fieldston (leafy site in the Bronx).

“There is still much to be learned about urban climatology too. For example, are there significant persistent micro-variations in UHI intensity within an urban landscape. In principle, one would assume yes, but convincing data on this is still not elementary to come by, without a good network of observation stations.
There are well-known challenges to locating weather stations in urban areas. Instrument security at ground level is one issue. Also ground level siting, even if secure will present problems with respect to nearby building obstructions and biases from extraneous heat sources and wind distortions (air conditioning, vehicular effects, building facades that affect winds, etc).”

Somebody please tell James Hansen (his office shouldn’t be that far away).

Looking again at the data there are several things to note: the black roof is not equivalent to asphalt and each green roof is different from lawn grass (although perhaps the thicker QBG roof would come closest), nonetheless, this does give an insight into the potential for a significant Airport Heat Island effect at some airports under certain weather conditions.

To examine this more closely I calculated average temperatures from daily maximum and minimum readings over the course of the last week (Fig. 7).

Figure 7. Average temperatures for the three sites over the week 10-16th July 2010 (Hot Days: 4; Cool Days 3)

Taking the Queens site first, there is very little difference between average temperatures for the black and green roof. Although the green roof does not achieve the same maximum temperature the heat retained overnight by the 6″sub surface means the minimum temperature is much higher and the overall average is similar.  For the other two sites there are huge differences, most notable on hot, sunny days.  But to extrapolate this to a conventional ground-based MMTS weather station we have to consider a few issues:

  • The sensor would should be 1.5M above the surface
  • The depth of the surface is not limited as in these sites therefore, depending on the conduction of heat into the under-surface, it may not heat up as much and may cool much more slowly.
  • Differences in vegetation – grass should be short, or where grass will not grow there should be bare earth.

This last point is important.  Looking in detail at the cooling of the three sites (Fig. 8)….

Figure 8. Close up of rates of cooling and minimum temperatures at the three stations (Colour key as in Fig. 1).

The first thing to note is that the ‘retained heat’ is proportional to the depth of the subsurface (University – 2″ < Fieldston – 4″ < Queens – 6″).  In fact the temperature of the green roofs at University and at Fieldston is very close or sometimes lower than that of the black roof; these green roofs also cool as rapidly, if not more so than the black roofs.  This can be explained by the properties of the plants at this site.  The metabolism of succulents like Sedums is different from normal plants in that they can keep most of their stomata (pores) closed during the day to conserve moisture; conversely they then open them at night for O2/CO2 exchange, so we’d expect more moisture loss (evapotranspiration) and therefore cooling at night at these sites.  The Queens roof might be closer to a conventional weather station site, except for one thing – it has lush vegetation which I suppose would result in increased evaporative cooling.  The white roof at Queens (Fig. 1; Fig. 8b) does not reach as high a maximum temperature as the black roof, but mostly cools to the same minimum, albeit cooling more slowly. It also has lower average temperatures (Fig. 7).

So really none of these roofs is analogous to the (short) grass prescribed under weather stations, nonetheless this does illustrate the potential magnitude of the issues of surface effects on temperature measurement.

The original point of this NASA research was to look at ways of using vegetation to mitigate the heat within cities, but the final sentence in the abstract of paper by Gaffin et al suggests an additional purpose:

“These roofs should be superior to other urban rooftops as sites for meteorological stations.”

My first thought was God forbid! This crude comparison of a small portion of their data shows that choice of subsurface depth and even plant type could result in a station that, like the Queens green roof, is no different in terms of average temperature than a black roof.  On the other hand the other two stations do show a reduction in average temperature and could be viewed as an improvement. Clearly strict adherence to ‘green roof weather station’ design guidelines would be important, but isn’t that the case anyway with set up of weather stations?

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4 Responses to Thermal absorption – a black and green issue

  1. Rob uk says:

    It does not need a rocket scientist to conclude that a thermometer placed on a grassed area will be cooler than one placed on tarmac, why do these idiots take offical temperature readings at airports, average global temperature, utter rubbish.

    [Reply – I couldn’t agree more! – vjones]

  2. Ian Middleton says:

    Thankyou for posting this study. I found it most facinating.
    It will be interesting to see what data this study produces over all the seasons.

    Ian

    [Reply – you’re most welcome. I’ve always liked the idea of traditional green roofs, especially the Scandinavian ones, and this study made me think hard about just what I would do (you know sometime…. when I build my own ‘Grand Design’…..)]

  3. E.M.Smith says:

    Hmmm….. I’d been thinking someone ought to do a study like this.

    The point that the temperature depended on the thickness of the substrate gives me some concern for the stations WUWT http://www.surfacestations.org/ has documented as being located on various roofs.

    So we know some stations are on urban roofs, and here we know that the type and thickness of the roof can move the temperatures reported by several degrees. Doesn’t that imply all the roof mounted stations are ‘suspect’ at best? Especially when compared with the 1700’s and 1800’s historical records…

  4. Pingback: Examining Urban Heat Islands – Part 2 | Digging in the Clay

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