Large city buildings raise temperatures

Q What effect does the indoor cooling of the large buildings downtown have on the air temperature and turbulence outside at the pedestrian level? In other words, does enough cool air escape from seams and other openings in the buildings to noticeably lower the air temperature and/or increase the turbulence of the air at street level?

A It's an interesting question one might consider while standing on a sunny, hot sidewalk and feeling a cool breeze from a revolving door. But here's how Phil Smith at the Minnesota Office of Energy Security, which deals with energy efficiencies and leakage, answered:

Generally, large buildings contribute to higher air temperatures in urban areas. The siding of office buildings is usually a mix of masonry, stone, metal and glass and can serve as a solar collector and radiator, absorbing solar energy over daylight hours and re-radiating the heat to cooler surfaces.

Meanwhile, large, tall buildings alter wind flow and can create increased turbulence. Airflow in urban areas is a science in itself as there are so many effects possible. Oftentimes, plans for a new skyscraper include modeling airflow to see how the building might affect adjoining structures and streets.

Tall buildings also provide multiple surfaces for the reflection and absorption of sunlight, increasing the efficiency with which urban areas are heated; it's known as the "canyon effect."

The cold air flowing through an open door may seem like a lot, but it's really a very small amount and has very limited capacity for cooling the outdoor environment.

Insulation levels can seem quite low in commercial buildings, but if you were to track all the energy flow in an urban commercial building, only a small fraction would be flowing outward through windows, doors and walls.

All of the energy used in buildings -- for lighting, telephones, computers, air conditioning, etc. -- ends up as thermal energy that contributes to the urban heat island. Paved streets, sidewalks, automobiles, buses and light rail also contribute to this island of heat.

According to the U.S. Environmental Protection Agency, "Heat islands form as vegetation is replaced by asphalt and concrete for roads, buildings, and other structures necessary to accommodate growing populations. These surfaces absorb -- rather than reflect -- the sun's heat, causing surface temperatures and overall ambient temperatures to rise."

Buildings also block wind, inhibiting cooling by convection. Air pollution in urban areas increases the heat island effect, because many pollutants change the radiative properties of the atmosphere.

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