IEM Daily Feature
Thursday, 19 July 2012

Ames v Des Moines

Posted: 19 Jul 2012 05:46 AM

While the thermometer hit 103 degrees at the official observation site in Des Moines (the airport ASOS), it was 6 degrees cooler a handful of miles north in Ames as it only reported a high temperature of 97. The featured chart shows the daily maximum temperature from these two sites for July along with the maximum reported dew point for the day. Why is Des Moines always warmer than Ames and Ames not having reached 100 degrees yet? Location is everything. The Des Moines airport is surrounded by an urban landscape on most sides, while the Ames site has mostly corn and soybean fields nearby. Agricultural areas promote higher humidities due to crop transpiration, while urban areas are comparatively drier. This difference is clear from the difference in maximum dew points. The micro-climate around the Ames site is much more humid, which means it takes more energy to heat the air. So for the same incoming radiation, it will take more energy to raise the temperature one degree at Ames versus Des Moines. There is also a general urban heat island effect that happens moreso in Des Moines than Ames, due to intensity of the urban landscape promoting warmer overnight temperatures (all of the buildings and concrete hold heat better than vegetation and topsoil).

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Tags:   heatisland