Urban warming impacts are usually evaluated by comparing temperature gradients in urban areas with temperature gradients in adjacent rural areas. This post approaches the question from a different perspective. Instead of looking at urban warming per se it looks at the urban heat islands that cause it. How common are they? How variable are they? How large are the areas they cover? What are their amplitudes? Knowing the answers to these questions could help improve our understanding of how much of the increase in global surface air temperature over the period of instrumental record might have been caused by urban warming. Alternatively it might confuse us even further, but that’s a form of progress too.
Unfortunately very little quantitative information is available on urban heat islands, or at least it wasn’t some years ago when I did the work reported here. The available data consisted – and as far as I can see still consist – of temperature profiles across idealized metro areas, infrared satellite images, maps showing metro area temperatures on a hot summer afternoon or on a cold winter morning, temperature profiles measured by car thermometers in cross-city vehicle trips and localized studies such as the temperature difference between parks and office blocks in Singapore. None of this was of any use in quantifying heat island extents and amplitudes, which requires long-term temperature data from weather stations both inside and outside the urban area.
So I developed a data set from scratch. It quantifies at variable levels of precision and detail the UHI signatures of 43 different metro areas (28 in the USA, 15 in other countries) based on annual mean temperature data from 930 stations, and in this post I summarize the results.
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