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    Jesse Ferrell

    Dew Points Near Record Levels in Midwest

    By Jesse Ferrell, Meteorologist/Community Director
    7/19/2010, 7:05:00 AM
    UPDATE 7/16/10: Chris Burt says:

    "The July/August issue of Weatherwise Magazine reported the following instances of 90°+ dew points in the USA. Thomas Schlatter of NOAA provided this data to Weatherwise.

    90° at Appleton, Wisconsin at 5pm, July 13, 1995 (dry bulb temp 101) 90° at Melbourne, Florida at 2pm, July 12, 1987 (dry bulb temp of 95) 91° at New Orleans Naval Station at 5pm July 30, 1987 (dry bulb temp of 95)"

    UPDATE: Links were broken due to a notepad word wrap; now fixed. Here's another blog by Facebook Friend Daryl H., (assistant scientist in agronomy at Iowa State University and coordinator of the Iowa Mesonet project) who says "In general, the AWOS sites [like Newton & Fairfield] are located literally in the middle of crop fields and use a temperature/humidity sensor that struggles at extreme values." However, this doesn't mean yesterday wasn't notable. He goes on to say "12 [more reliable] ASOS sites hit 80+ today, this is rather unprecedented. 29 July 1999 is the most recent comparable day to today."

    Facebook friend James H. has pointed out that both stations are reading above their brethren this afternoon (shown in red here), which could mean their sensors are malfunctioning or "corny"... however there are multiple stations (green) that are even higher today, but did not report higher readings yesterday.

    ORIGINAL REPORT: Dew points reached 88 degrees yesterday in Newton, Iowa according to official NWS stations (matching the highest I've ever seen in the U.S.), and 92 was reported at an unofficial station. This means that extreme heat and humidity was present in the area yesterday (which helped lead to dozens of tornado reports to their north).


    dew715annot


    Of course, dew point sensors (even official ones) are historically unreliable, especially at high temperatures (I don't believe the 92 and the 88 is questionable. Since another station also reported 86, that may be closer to what happened). This may be why official U.S. records of high dew points don't exist (Chris Burt: Extreme Weather says the world record is 95 F). If I had to guess at a U.S. record I would say it was in the upper 80s. Even above 80 is rare here. The National Weather Service in Des Moines, Iowa said: "FAIRFIELD IA HAD A DEW IN THE INCREDIBLE MID 80S. THESE ARE THE HIGHEST DEWPOINTS SEEN SINCE 6 YEARS AGO AND THE HIGHEST YOU WILL SEE AROUND HERE."


    meso_table_chart-KTNU714a


    This translated to a Heat Index (and AccuWeather.com RealFeel temperature) of 129 F (123) at Newton and 125 (121) at Fairfield, steamy to be sure but a far cry from the 145 F (134) reached during the East Coast heat wave of 2006 (linked at top) because temperatures weren't as hot in Iowa yesterday.

    Is corn to blame for near-record dew points happen in the Midwest yesterday? As I've illustrated in the link at the top, they also occur in the Southeast U.S. Many meteorologists will tell you the high dewpoints in the Midwest are due to the corn crop giving off excess moisture, but a NWS study to determine that was inconclusive.

    The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com

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    Jesse Ferrell