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Global Bioweather

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Albedo - painting the town white

The Oxford dictionary defines albedo as “the proportion of the incident light or radiation that is reflected by a surface…” and it originates from the Latin word albus meaning white or whiteness. The reflectivity has a value and number 1 represents 100% reflectivity, or a very bright surface, and any fraction of 1 tells us that the surface is less reflective, up to the value of 0 which is complete darkness.

Everyone knows that a dark-coloured car left in the sun heats up more then e.g. a white car, or a dark shirt in the sun is less comfortable than a light-coloured shirt of the same material. As a matter of fact, black surfaces can become up to 400C (700F) hotter than the most reflective white colour when exposed to the sun’s radiation.

Everyone knows? Well, I’m quite amazed then, why so many people who live in hot regions opt for fashionable dark-coloured and even black roofs and walls. I guess, the emphasis is on ‘fashionable.’

This tendency and the loss of vegetation plays a major part in the rapid develoment of the heat island effect in urban areas. So, should we paint our towns and cities white? There are many arguments for it, but might seem over-the-top. All right then, if not white, we should at least consider lighter colours.

In the US, around half the population lives in heat islands. The worldwide trend of urbanisation means that the populations of other countries experience the same or even worse adverse heat effects. Los Angeles is a typical example of this trend. Local authorities and government agencies are now evaluating possibilities to, at least, limit the adverse heat island effects.

“Painting the town white” and “cool communities” have become slogans and the bases for intensive sudies. According to one study, a combination of planting trees and lightening the roof and pavement colours could lower the average maximum summer temperatures in Los Angeles by 30C (50F), and reduce the need for mechanical cooling by 18%., saving around US$170 million p.a. in energy costs.

Since excessive heat also triggers ozone production, the result of lower temperature would also see around 10% less days with smog. The same study goes even one step further, I quote: “With white roofs, concrete-colored pavements, and about 10 million new shade trees, Los Angeles could be cooler than the semidesert that surrounds it, instead of hotter.”

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Global Bioweather

Albedo - painting the town white

Heat Island

Urban