Aug 19, 2010

Did you know that a place like Des Moines, IA is fairly frequently more humid in the heart of the summer than areas along the Gulf Coast? I bet you didn’t, but it’s true. The major reason: the vastness and density of corn and bean plants over the Midwest.

You are probably now asking how and why. It is a “simple” matter of the “zillions” of plants spewing tons and tons of water vapor into the air over the Midwest. This process is called evapotranspiration. It is the amount of water vapor in the air that determines how humid it will be and how uncomfortable humans and animals will feel due to the high humidity.

Dew Point

The best measure of humidity is the dew point. When dew points are in the 50s in the summer, it is rather comfortable. With dew points in the 60s, you start to feel uncomfortable and if the dew points are 70 or higher it gets very uncomfortable. A place like Houston, TX typically has dew points in the low to mid 70s in the summer months. But the amazing thing is that a place like Des Moines, IA can have dew points of 75-82 on quite a few days during July and August. With these dew points one sweats standing still. And of course the reason for these extreme dew points is the so called “Sea of Green Effect” described above.

Water Vapor

Believe it or not the “Sea of Green Effect” has changed the Midwest Climate over the past 30-40 years to one that features cooler temperatures and wetter (more rains) conditions in the summer. I know, I know. You are probably confused by now. How could it have changed the climate to cooler and wetter when the globe as a whole has been warming up as a general rule during this period? The answer is simple. The increased water vapor keeps the day time high temperatures cooler and the night time lows warmer, but the day time highs are significantly cooler than are the night time lows warmer. And of course the greater the amount of water vapor in the air, the easier it is to rain with any little meteorological excuse. Thus, it is a self perpetuating phenomenon.

Did you know that there has not been a record high temperature in Des Moines in the summer months (June through August) since 1988? There have been numerous record highs in every other month of the year. There have been quite a few record warm minimum temperatures (night time lows), but no record warm maximum temperatures. What does this tell you? You guessed it, the “Sea of Green Effect.”

Past Changes

The only other remaining question one might have on this subject is the following: Why was there no “Sea of Green Effect” prior to the past 30-40 years. Well, the reason for this is that the aerial size of the “Sea of Green” has expanded tremendously in the past 30 years or so and the plant population density has likewise astronomically increased. There was a “Sea of Green Effect” prior to the past 30-40 years, but it was much less pronounced.

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