Cities Are Heat Islands


Seen on a clear night from a satellite using a high-resolution radiometer, the northeastern US looks mottled. The cold cloud tops are dazzling white, the cool ocean is light gray, warm fields and forests are darker gray. Cities, islands of heat, are black.

The Urban Heat Island concept is attributed to meteorologist Luke Howard (1818). On a summer day, the average temperature in a typical American city is about 3 to 5 F hotter than the surrounding area. Up to 30% of land in cities is covered by energy-absorbing artificial structures. This concrete and asphalt jungle absorbs heat during the day and releases it into the atmosphere at night, heating the night sky. Adding to the urban heat island effect is heat released into the urban atmosphere by combustive processes from vehicles, industrial activity and the heat that escapes from commercial and domestic air conditioning. Just prior to sunrise, urban areas are on  average 7F warmer than adjacent rural areas in summer and 6F. in winter.

This disparity in heat conditions between cities and the countryside is a source of concern in our construction of heat index charts. The SAMSON database contains meteorological readings from airports across the US. At each airport (and almost all of the airports are suburban or rural), an array of meteorological measuring instruments is housed in a small louvered shed. There is airflow through the shed, but the instruments are sheltered, out of direct sunlight, in the countryside. Few athletic events are held in   such conditions.

In order to reconcile airport readings with urban unshaded sports venues, we have utilized the Olympic Stadium site in Atlanta as one of our main modeling tools. Microclimate data obtained onsite during the run-up to the Games and during the games themselves was used to establish the relationship between Wet Bulb Globe Temperature and Heat Index. Simultaneous heat and humidity readings from the Stadium and the Atlanta airport weather station were compared. Although there was usually a very close correlation (on average, the stadium was 2.8F. warmer), at times the Stadium's heat index could be as much as 16F higher than the airport's. This tendency toward higher temperatures at athletic competition venues, plus the fact that our charts reflect all recordings (including rainy days, when competitions would probably   have been cancelled; such days are usually much cooler than competition days), leads to the inevitable conclusion that our Heat Index charts must be considered to underestimate heat stress. As bad as our charts may make some places seem on some days, the reality is probably even worse.

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Heat Islands, More

Atlanta Urban Heat Island Research


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