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.

Voting:
Good = 54
Bad = 5

Tags:   awos   heatindex   dewpoint  

Heat index values soared to well over 100 degrees this past weekend thanks to highs in the 90s and dew point readings in the upper 70s and lower 80s. The featured chart presents frequency of having a 100+ heat index by hour and week of the year based on observations from the Des Moines sensor. The bullseye is right about now, the third week of July which is also the warmest week of the year. The very hot weather looks to continue this week.

Voting:
Good = 14
Bad = 6

Tags:   heatindex  

The big story is the forecasted upcoming hot weather to begin this weekend and last into at least next week. The featured chart presents the number of hours per year that the heat index was observed to be at or above 100 degrees at the Des Moines sensor. The 2011 total is already above four of the past seven years. This weather will be right on time for the climatological warmest week of the year as well.

Voting:
Good = 13
Bad = 15

Tags:   heatindex