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Unreliable Dew Points

21 Jul 2011 12:33 AM
Our recent stretch of extremely hot and muggy weather has gotten Iowa a bit of publicity as some of our automated stations have calculated heat indexes that exceeded 130 degrees! Typically, this is blamed on the abundant transpiring corn crop in the state, but that is not the entire story. The primary source of automated weather data in the state are located at airports, but there are two classes of instruments. The federally administered ASOS sites have higher quality and sometimes different sensors than their state run AWOS counterparts. Unfortunately, their data is typically thrown together in the same pot and reported as such. While the ASOS sites did indicate gaudy heat indexes exceeding 110 at times, they were not as high as the AWOS sites which got all the headlines. The featured chart presents a comparison of an ASOS and AWOS site separated by 30 some miles and both surrounded by farm fields. The bottom plot raises the most doubt as even while visibilities were at 2-4 miles, relative humidity was just 75% at the AWOS site and near 100% at the ASOS site. This is probably an indication that the AWOS air temperature has a warm bias. The second plot shows another issue of having the AWOS dew point rise dramatically during the morning hours at a much higher rate than the ASOS site. It is hard to imagine a natural process that could be putting that much water into the atmosphere in the early morning hours. The first plot of the ASOS site shows a reasonable depiction of transpiring corn (slowly rising dew point throughout the day peaking late afternoon coinciding with plant transpiration), but the dew point temperature barely gets to 80 degrees which keeps the calculated heat index at reasonable values. The moral of the story is that not all automated sensors are alike and while heat indexes over 110 surely were felt in Iowa these past days, the high end values over 120 are questionable.

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Tags:   awos   heatindex   dewpoint  


7 Jun 2004 data traces for MPZ and OTM.

Not Physical

08 Jun 2004 08:47 AM
This plot shows temperature and dew point traces from Mount Pleasant (MPZ) and Ottumwa (OTM) Monday morning. The MPZ plot depicts an early morning scenario that is hard to believe. It is nearly impossible to have 100% humidity while the temperature warms 9 degrees in the early morning from solar insolation. If it was truely that humid in the lower boundary layer, the sun would have had a tough time mixing down the moisture quick enough to maintain saturation with an increasing air temperature. This is not physical and indicates bad data.

Tags:   awos