Urban Heatisland Effect

The urban heat-island effect was first documented in the 1960s when scientists noticed that cities were noticeably hotter than the surrounding countryside. Sometime during the winter, during your morning commute, listen to the weather reports, and you'll see that the urban areas are typically 5°F to 7°F warmer than the more outlying areas. This is pretty natural, if you think about it. We pump huge amounts of energy into cities, and as every high-school physics student knows, all energy eventually becomes waste heat. In addition we pave over our cities, remove vegetation and put up concrete structures that retain the sunlight well into the night. We could not have designed a more perfect way to heat up the cities.

Partly as a result, about $40 billion is spent every year by property managers and building owners to air condition buildings.154

In winter this might sound good, just as global warming might sound good to some people in Alaska (until they consider all the consequences). But in summer it creates a much hotter local microclimate in cities, with less cooling from transpiration by vegetation and more air conditioning (which cools buildings but puts all the heat into the street or the air). In Phoenix, reportedly, by the year 2000 nighttime low temperatures in summer were 10°F hotter than in the 1970s. It other words, it never cools off for months on end. This means electricity demand is much higher in summer, which in turn drives the surging demand for new power plants.

What can be done? LEED suggests several simple measures to reduce solar heating in the summer and create cooler microclimates. First, reduce the amount of hardscape or impervious surface areas that can absorb heat. Second, put shading around all absorptive hardscape surfaces (parking lots, sidewalks, patios and plazas) so that at least 50% is shaded at noon on a typical summer day. In some areas such as the South, vegetation around parking lots can get pretty tall within five years! Third, place 50% or more of parking underground or under the building so there is less surface area to heat up from parking lots. Fourth, use highly reflective paving materials (gray or white concrete instead of asphalt) for a parking lot so that more incoming solar radiation will be reflected back into space and not be absorbed. Fifth, use an open-grid pavement system that would have vegetation growing inside the pavers so that there is less area to absorb heat from the sun. Obviously these measures can be used in combination: you can see there are many options for reducing the urban heat-island effect from hard surfaces.

When it comes to the building itself, there are two major options. Elsewhere we have profiled cool roofs and green roofs. LEED requires a green roof to cover at least 50% of the roof surface and a cool roof to cover at least 75%, or some combination of the two approaches. Either will reduce not only the urban heat-island effect, but also a building's demand for air conditioning in summer by providing cooler attic or subroof temperatures.

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