We’ve been experiencing hot weather for sometime now. High temperatures have been in the mid an upper 90s in the Southern Willamette Valley around 100 elsewhere toward southern Oregon. For those with home air conditioning and air conditioned cars it has been sweltering when they go outside of either one. For those of us without home or vehicle air conditioning just getting through each day has been a challenge.
Have you noticed that the more distance you put between you and the downtown areas of the city the cooler it seems? That is not your imagination. It is noticeably cooler especially when you change elevation. The higher you go into the wooded hills the cooler it feels. On the flip side of that all you have to do is get out of your vehicle, air conditioned or not, downtown and the heat becomes oppressive. That is not your imagination either. There is a meteorological term for that it is called the Urban Heat Island Effect. Here is the National Weather Service definition of the Urban Heat Island (UHI): “The increased air temperatures in urban areas in contrast to cooler surrounding rural areas.”
According to the US Environmental Protection Agency (EPA) “The annual mean air temperature of a city with 1 million people or more can be 1.8-5.0 degrees F (1-3 degrees C) warmer than its surroundings. In the evening, the difference can be as high as 22 degrees F (12 degrees C). Heat islands can affect communities by increasing summertime peak energy demand, air conditioning costs, air pollution and greenhouse gas emissions, heat related illnesses and mortality, and water quality.”
The actual cause of heat islands is that all of the concrete buildings, streets, and sidewalks of a city absorb solar radiation and then the glass windows reflect the suns rays onto all of that concrete keeping it warmer that the surrounding areas outside the city. That’s the down side of Urban Heat Islands, but did you know there is an upside?
The Princeton University School of Engineering and Applied Sciences published an article July 23, 2018 that was written by Molly Sharlach and titled “Cold wave reveals potential benefits of urban heat islands.” In the article she explains who the researchers were and summed up what they found. “Jiachuan Yang, a post-doctoral researcher, and Elle Bou-Zeld, a professor of civil and environmental engineering, analyzed urban temperatures in 12 U.S. cities in the Northeast and Midwest during a 2014 cold wave. They found that urban areas stayed warmer than the surrounding suburbs and country. The difference in temperature was greatest during the cold wave, which set more than 49 low-temperature records. The temperature differences were more pronounced at night than during the day.”
I read the research paper in the American Meteorological Society Journals Online. The research paper was titled “Should Cities Embrace Their Urban Heat Islands as Shields from Extreme Cold?” In the discussion and conclusion of the paper it is explained that research should continue into combating the effects of the urban heat island during the hot summer months. Examples of heat abatement measures are cool roofs that reflect more of the sun’s rays back into the atmosphere and green roofs which utilize vegetation to absorb the radiation and keep it from heating up the buildings so much.
Quoting the study “This study showed that the cold-wave thermal dynamics are complex and that measures such as cool or green roofs, which usually have comparable summertime benefits, result in very distinct disbenefits during extreme cold events.” They also say “a paradigm shift is urgently needed give equal weight to the benefits of UHIs during wintertime and cold waves in the sustainability and resilience blueprints of cities.” They do make some recommendations which include roof covers that reflect back more solar radiation and can be adjusted for sun angle and green roofs that use a soil that has a larger thermal inertia.
Simply put ways have to be found to keep the urban areas cooler in the summer heat yet warmer during the cold months of winter.
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