National Weather Service Weather Forecast Office

Was Moorhead Minnesota one of the Hottest Places on Earth Tuesday afternoon? Based on data from the Moorhead Automated Weather Observing System (AWOS), there was a period of time when the dew-point temperature reached 88 degrees Fahrenheit! Values this high are usually reserved for locations such as the Mexican Gulf Coast, Saudi Arabia or other extremely hot and humid places. But, was the dew-point actually that high? 

Going back and reviewing the data from the Moorhead Airport, it would appear at first blush the data is accurate. Accurate, but not representative. Verifying the data will take some time however. There are several reasons to question the precision of the dew-point sensor.

First: The AWOS is surrounded by Sugar Beets and Soy Beans - two of the most prodigious transpiring plants. Second, there was very heavy rainfall Tuesday morning across the region. This rain served to saturate the local soils and encourage plant growth. Plus, under the sensor is 4-6" high clover in flower (clover you would find in your yard, not the crop), with much ponding water within a few feet of the sensor as well. Third, when compared to the Automated Surface Observation System (ASOS) at Fargo's Hector Field the maximum dew-point was 5 degrees lower, peaking at 83 degrees Fahrenheit for one hour only. (Below is a table comparing the Fargo ASOS and Moorhead AWOS for part of the day)

In looking at the data from the surrounding stations,  several of the North Dakota Agricultural Network Stations (NDAWN) had similar readings. At face value, this supports the Moorhead dew-point of 88F. However, the NDAWN stations are located in such a way as to measure the moisture of the crop canopy environment, not the atmosphere. So, on the one hand if the dew-point did hit 88 degrees Fahrenheit, resulting in a Heat Index of 130 degrees Fahrenheit, it was not because of true meteorological effects but because of an agricultural bias. This makes the information, relative to official climatic sources, less representative, and should be used with caution. 

While it is possible the Moorhead dew-point did reach 88 degrees Fahrenheit, it did so because the weather station is located in an agricultural field surrounded by water, or very wet soils, and crops that release a great deal of water vapor into the atmosphere. The sensor while measuring the moisture of a very local place, did not represent the free atmosphere as a whole. There are very specific rules and regulations dictating the location of weather equipment, the type of vegetation and distance from agricultural crops

Whatever the cause, a 130F Heat Index would be intense. Is it accurate? It is impossible to determine the accuracy of Moorhead AWOS at this time, so that 130F Heat Index is questionable. Is it a record? That will take time to ascertain. We will be looking at the data and making a determination later.

Below is a table comparing the Fargo ASOS with the Moorhead AWOS during the hours of Noon through Midnight, Tuesday July 19 2011. During the period when the Moorhead AWOS was reporting an 88 degree dew-point, the Fargo ASOS was 5 to 9 degrees lower.