How Shiny Is Los Angeles?
How can a city be shiny? Cities reflect and absorb light and heat just as plants, human skin and snow does. But in urban areas this can create higher temperatures and smog. With that in mind, Taha set about measuring the albedo (reflectivity) of surfaces in these built up areas.
Now, you might think this is a bit of fun, but no, it’s all a very serious business. He got in a small aeroplane and flew low (about 200 - 300 metres) at about noon (to minimise the shadows from the buildings) all over the Santa Monica mountains, over the Pacific coast, across to the San Fernando valley and finally south to downtown Los Angeles. During the flight, the temperature in the cabin reached 35°C and there was a lot of turbulence - not the best place to be while measuring the reflection of the sun.
The land-use in the observed areas was compared with the albedo and both were overlaid onto a map. The results show that the albedo is higher in urban areas than in the suburbs, mainly due to the fact that the suburbs have more trees, grass and greenery - and vegetation has a lower albedo than buildings. But, because the buildings and green areas are often close together in a mosaic, the variation in albedo was not all that large. The average albedo was found to be less than 0.2, with the coastal zones about 0.08 and the grey commercial areas measured at 0.17.
So, a bit pointless perhaps? Well, it does have an application; areas with a high albedo that may overheat and cause smog could profit from an urban makeover, planting more green in among the buildings to reflect less light and make the area look a little more suburban…
[Taha, H. (1994). How Shiny Is L.A.? Recent Research in the Building Energy Analysis Group, 8.]
Other albedo values:
A perfectly reflecting white surface: 1.0
A perfectly absorbing black surface: 0.0
The Earth: 0.39
Mars: 0.16
Fresh snow: up to 0.98
Old snow: up to 0.6
Sand: 0.20-0.30
Grass: 0.20-0.25
Forest: 0.05-0.10
The back of your hand: about 0.22
Cornish pasty: 0.27
An iced bun: 0.65
Tabby cat: 0.12 (0.19 with extra glossy fur)
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