134 Reasons Why Sticking A Weather Station In A Soy Field Might Not Be A Good Idea

With the recent heat event across the middle of the country, there have been countless stories about heat index values of over 120 degrees. I've mused in twitter about "corn effect" heat index values of 128, 130, or even higher. One of the most aggressive examples of this occurred in Moorhead, MN on Tuesday where the heat index topped out at 134 degrees during the peak heat of the day, thanks to a dew point of 88 combined with temperatures in the mid 90's.

Few probably realize that Moorhead's observation site is in the middle of a farm field ten miles outside of Moorhead proper.   The map below shows the twin cities of Fargo (left of graphic) and Moorhead (where the Google map "A" label is).   Residing about ten miles outside of the city is Moorhead's community airport (a municipal aviation airport such as the ones in Coatesville, Pottstown, Millville, etc.), sitting smack dab in the middle of a field of sugar beets, soy, and perhaps some corn for good measure.   I've been through this area a couple of times in my life -- the area is flat as a pancake, lacking trees, and heavily farmed.

The National Weather Service in Fargo provided a really good write up explaining the factors that lead to some suspicion in the high dew point values -- farming areas where heat index values are substantially higher than surrounding larger towns thanks to the transpiration of crops and wet ground.   Transpiration is the part of the water cycle on the plant side of the fence and essentially is the plant's way to cool itself as it releases water vapor through its cells.  Essentially, think a less stinky form of sweat.   I lifted the temperature comparison between Fargo's airport (a bit farther removed from agricultural influences) and Moorhead's airport data for Tuesday from the NWS in Fargo to show the comparison in data for the two sites.   The peak heat index for Fargo was 116 -- still oppressive -- but the top dew point for Fargo was five degrees lower than the top dew point in Moorhead (83 versus 88), with Fargo's dew point consistently several degrees lower than Moorhead's.

There are countless other examples of "corn effect" heat in these heat waves -- parts of Iowa and other parts of Minnesota had heat index values over 120 thanks to observation sites in the midst or near a farm field.   While some of the dew point values reached in the heat wave in the Midwest were legitimate -- the 82 in Minneapolis is a definite "legit" record since their airport is just about smack dab in the middle of the metropolitan area, the mid and upper 80's dew point values in some of the rural areas are likely enhanced thanks to what may be on someone's dinner plate, feeding trough, or gas tank (ethanol) in a few weeks.   There's no denying the heat or even the humidity in many locations but understanding should be taken when analyzing some of the more extreme data due to location and what surrounds it.

All I can ask is that tomorrow we don't get a dew point in the low 80's with any temperatures over 100...I don't think the region needs a repeat of July 1995 around here.

Depressed Dew Points in Vegas

Folks in the US Southwest jest about the 'dry heat' factor when temperatures are over 100 degrees during June and July. When dew point values are low, evaporation is sufficient enough to allow temperatures to feel a few degrees cooler than the actual air temperature. In Las Vegas Monday, the dew point-temperature differential was the largest it had been on record for them....129 degrees (see the last line of the photo below).

The temperature was 107 late Monday afternoon in Vegas while the dew point was a thirst-inducing -22.    Even in the most arctic of airmasses locally our dew point levels will rarely get below -15 in the dead of winter...a -22 dew point is even rare for the high desert in the heat of summer.    The culprit?   My guess is that gusty winds that downsloped off of mountains to Las Vegas' south and southwest yesterday played a role in the huge dew point-temp differential, allowing the air to dry out further and warm up more.

With the dew point factored in, the heat index in Vegas yesterday afternoon was in the mid to upper 90's at the height of the dew point drop.   Hardly comfortable but certainly better than 107!  

Heat Index 101

I rambled in a post here last July about the difference in using dew point versus using humidity in describing how humid it feels outside, with me trying to make a case that dew point is quantifiable as opposed to a relative measurement of atmospheric moisture.   Most folks wouldn't know that 78% humidity on a 60 degree morning isn't humid whereas they would know that a dew point of 53 on a 60 degree morning in summer is rather comfortable.

We're asking in a poll here if you feel heat or humidity is worse -- and so far the results aren't surprising as humidity wins handily.   Taking it to another level, I figured that I would help explain the tie between dew point (and humidity) and temperature when it is hot out.   Most know what a heat index is -- the "feels like" temperature that combines the impact of heat and humidity together.   Some call it the summer equivalent of wind chill although that's a bit of a malaprop and misnomer since heat index does not account for wind nor does it feel chilly on a hot and humid day.   Regardless, the heat index is really the tell tale sign of how it really feels outside.   The higher the dew point (or humidity level) at a constant temperature, the higher the heat index.

Heat index is a critical measurement used to determine excessive heat warnings and heat advisories locally.   Typically, 100 degree heat index values are the standard to issue heat warnings in the Delaware Valley but in the early parts of the summer the criteria is lower (96 before June 15th, 98 in the latter half of June).

When we have a 90 degree day, we generally feel that it is very warm if not hot outside.   Just how hot it feels depends greatly on the dew point outside.   If we have a comfortable atmosphere with dew points in the 50's, the apparent temperature (heat index) is lower than the air temperature as evaporation off of our skin from sweat and regular ol' moisture transport around the body will cool us off somewhat, not accounting for any additional wind that may help keep us comfortable.  As dew points reach around 60, that evaporation benefit is negated and temperatures feel as warm as advertised.   It's hot but most everyone can deal with it feeling like 90 degrees out.   The higher the dew point gets, the higher the apparent temperature becomes on a hot day.   If the actual temperature is 90 and the dew point is 75, the heat index is 100 on that given day and we're all sweltering.

The heat index is a sliding scale, meaning the higher the dewpoint and/or higher the temperature, the progressively worse it will feel.   If we keep a constant 75 degree dew point but increase the temperature from 90 to 95, the heat index is now 107 (seven degrees worse).  Increasing the temperature to 100 with a dew point of 75 brings us an ungodly heat index of 114, whereas a dew point of 50 at 100 degrees yields a heat index of "just" 97.   At some point, even the "dry heat" folks will begin to complain when the air temperature gets too hot, despite a lack of atmospheric moisture.