Warming up while cooling down
Summer in South Louisiana-marred by high heat in the afternoons along with oppressive humidity that persists right through the nights. We seek relief indoors; air conditioning is not just a luxury but a necessity in the south. However, for large metropolitan areas, keeping cool at night may be part of a vicious cycle that only makes it warmer.
Arizona State University researchers have found that the heat output generated by air conditioners is actually enough to raise the environmental temperature at night-especially in highly populated areas.
While the "waste-heat" is actually at a maximum during the day, when temperatures are much higher, the effect is negligible. But, for locations included in the study, nighttime temperatures were raised as much as 2°F. The result is an exacerbated urban heat island effect.
From the Environmental Protection Agency: "The term "heat island" describes built up areas that are hotter than nearby rural areas. The annual mean air temperature of a city with 1 million people or more can be 1.8-5.4°F (1-3°C) warmer than its surroundings. In the evening, the difference can be as high as 22°F (12°C)."
The greater effects at night are most likely attributed to nocturnal decoupling of the atmosphere, or the shallower boundary layer. During the day, hot sun warming the earth's surface and radiating back into the atmosphere creates a relatively thick layer from the ground upward of similar temperature and pressure. At night, much more warmth escapes the surface than what comes in thus producing a shallower layer of uniform temperature. As a result, the air conditioners "waste-heat" has much less air to warm and the effect is more noticeable.
Researchers noted the "positive feedback" that occurs with such an issue. On hotter nights, more air conditioning is run, releasing more heat, making it hotter, causing the need for more air conditioning and so on.
In the Journal of Geophysical Research, the ASU team expresses their recommendation for cities to optimize energy output based on the "positive feedback" scenario created by air conditioners on warm nights. A/C units in urban settings can chew up more than 50% of the outgoing electrical supply, putting a strain on resources. Scientists from this study hope that the data they've complied can kick start a method of recycling "waste-heat" into reusable energy.
This study was completed in and around Phoenix, Arizona during a hot and dry stretch of weather. There has yet to be any research done as to whether or not the same effects occur where nights are hot and humid, like in South Louisiana.
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