We are in the middle of a crazy heat/humidity wave this week. Right now at 6:30 pm the heat index is 113°. It’s one of those days when you open the door and can’t breathe because of the humidity in the air. You stand outside for 2 minutes and are completely covered with sweat. (Especially those of us who already “sweat like a …..”).
In Iowa we seldom get much over 90° and 100° would be a record on just about any given day so why does it feel so miserable here? You can have 95° in Wyoming and it’s hot, but it’s bearable. You have 95° here and it’s completely unbearable.
The main reason is the Dew Point, which basically is a measure of how saturated the atmosphere is with moisture (http://en.wikipedia.org/wiki/Dew_point) . Here is a map of the forecasted dew points for Tuesday (from www.weather.com). A dew point of 75-80° is considered “oppressive” and dew points over 80° are rare in this area, although it looks like it’s going to be that high tomorrow.
The Dew Point gives the best calculation of the Heat Index. The Heat Index is the “feel like” temperature, or how hot it feels outside (much like wind in the winter gives you a Wind Chill that makes it feel colder than the actual temperature). I found a Heat Index Calculator here: http://www.hpc.ncep.noaa.gov/html/heatindex.shtml, then looked up Wyoming & Iowa to compare since today we had comparable temps.
Wyo: temp 97°, humidity: 21%, dew point: 52°, Heat Index 94°
Iowa: temp 95°, humidity: 60%, dew point: 79°, Heat Index: 113°
How can that be? The temperature in Wyoming is higher, but it’s unbearable in Iowa.
I found an article today that explains it. Basically when you have low humidity and high temps you sweat but it evaporates and cools you down. Your body temp doesn’t raise that much. With high heat & high humidity you sweat and sweat but since the air is also wet, your body can’t evaporate the sweat, and subsequently can’t cool down which can lead to heat stroke or heat exhaustion very quickly.
But why is it more humid here? You expect it to be humid on the Gulf Coast, Florida and the southern East Coast, but why in the Midwest? Simply put: it’s all the corn! The corn (along with all the other plants) release water vapor into the air. We don’t have the ocean, but we do have PLENTY of corn.
From www.weather.com: “Evapotranspiration is the total amount of water that is transferred from the earth's surface to the atmosphere. It is made up of the evaporation of free water surfaces (lakes, rivers, soil moisture, etc.) plus the transpiration from plants. Evaporation of water from soil with high moisture content (due to passing thunderstorm complexes) and the transpiration of water from tens of thousands of acres of a mature corn crop contribute to an atmosphere that is sopping wet “
If you are interested, here is the full article: http://www.weather.com/outlook/weather-news/news/articles/evapotranspiration-corn-belt-humidity_2011-07-13
Did you learn anything new?
Great lesson! I just learned about this during our Summer Funshine for work yesterday. It made complete sense!
ReplyDeleteIt does make sense after you hear it. It makes me wonder how humid it was in the 1800's before widespread farming? Or did the prairie grasses still make it humid?
ReplyDeleteDoesn't matter how much sense it makes...it's still too hot!!! LOL~I hope it makes it's way away from Iowa soon and doesn't ruin too much of the crops. It's pretty much like that here most summers although it's been hotter much earlier this year and you add that humidity and it's just brutal. Guess you should join Al in that "pooL" of his :)
ReplyDeleteI just want to live somewhere where it's 70-80 all the time. :)
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