The featured chart is an attempt to illustrate differences in over night cooling rates by showing a composite profile of temperature for a three hour period prior to reaching a freezing temperature. The airport weather sensor in Des Moines is in a very urban area, while the Waterloo sensor is on the edge of town. The slope of these two lines is slightly different showing a faster cooling rate for Waterloo. This is an illustration of the "heat island" effect that is caused by having lots of heat retaining buildings and pavement nearby the weather sensor.

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Tags:   heatisland