In the why we can't have nice things department, the weather world was abuzz yesterday about a number of airport weather stations in Iowa that reported upper 80s and even low 90 degree dew point temperatures. Sadly, the reporting mechanisms lump most of the airport data into one data stream that mixes federally maintained (FAA/NWS) ASOS airport weather stations in with the state maintained (Iowa DOT) AWOS stations. Folks rightly assume that all airport data is created equal, but it is not. Unfortunately, siting, maintenance, sensor packages, and aspiration differences can yield big differences in reported weather variables. The featured chart compares a temperature and dew point time series for three sites in the state. The Denison AWOS (lower panel) was one of the aforementioned sites that hit 90+ for a dew point and the immediate hand-waving explanation for the data being valid is "corn sweat". The other two sites were chosen as sites also in close proximity to agricultural fields, but yielded a different depiction of peak afternoon dew points. Two important times of the day are highlighted to point out physical processes happening. To start off the morning hours, we are at 100% humidity with likely lots of dew and even some fog. The sun rises and heating of the ground and plants ensue, this produces a demand from the air for water that may be available and also the plant is wishing to start transpiring with its activity, dew is also being evaporated. So the temp and dew point separate a bit, but both are rising. By the afternoon, only transpiration is usually left and temperatures are quite warm with good mixing of the lower atmosphere, it is difficult to increase humidity levels during this time. By early evening (blue area), mixing decreases and humidity levels can make one subtle push higher before stabilization happens and air temperature and dew point starts to both slowly work downward. Having 100% relative humidity like Denison does throughout the day is a tell-tale sign of a sensor problem and not depicting what is happening in/near agricultural fields in the state. Yes, "corn sweat" (and soybeans too) contributes a number of degrees to the dew point temperature this time of year, but is not able to push it much higher than the lower 80s. As temperatures warm into the 90s and absolute humidity increases, plants will cut back to conserve water and dew points can't simply keep increasing with the temperature.

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Tags:   awos