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Why do you get dewpoints so much higher than here? Are cornfields really the reason?
Yes. When Gulf Coast/South winds bring humid air to the Midwest in July, it gets super charged by the corn through evapotranspiration. So instead of just 70 degree dew points we get 5-10 degrees added to it. You can see this even locally. MDW and ORD may hit a dew point of say 74 or 75F, (which are both in an urban setting) but the outer suburbs like Joliet, Aurora, and where I live almost always have a few degrees more since we are closer to the agriculture.
This is a recent phenomenon though.
Quote:
I would say with near certainty that surface dew points have increased in the upper-mid Mississippi and lower Ohio Valleys over the last 30 years,” Corfidi told me by e-mail. “In particular, it seems as though broad swaths (say, 300- to 50-mile–wide corridors) with surface dew points averaging in the mid 70s Fahrenheit are no longer uncommon.”
He points out that average corn yields in Illinois have increased from about 50 bushels per acre in 1950 to more than 130 bushels per acre in 2000. Planting densities climbed dramatically as well, from about 18,000 seeds per acre to nearly 30,000 seeds per acre during the 1970s, when farmers started planting crop rows closer together.
Quote:
Changnon and his students analyzed hourly dew-point readings recorded from 1959 to 2000 at Chicago's O'Hare Airport and Rockford Airport. The top four high dew-point frequency years were 1983, 1987, 1995 and 1999.
Analysis of the 10 most extreme heat waves in the region further showed that the number of high dew-point hours was much greater after 1980. Additionally, the researchers discovered that over the 42-year period they studied, three different dew-point indices at both airports showed general increases over time. Those indices include:
Hours per summer with high dew points reaching or exceeding 75 degrees Fahrenheit.
Summer days with at least one hour of high dew point.
Summer days with 12 or more hours of high dew points.
It's also a trade off though. Nowadays we are seeing fewer 90 and 100 degree days in July because of all that added moisture in the air. Only during drought years where widespread lack of moisture like 2012, 2005, and 1988 are we going to see temps like the "old" Midwest
Quote:
In the last 80 years, those lazy, late summer days in July and August have been getting cooler and wetter throughout much of the Midwest. In Chicago, temperatures reached 90 degrees or higher 344 days during the 1930s, but only 172 days in 2000-09.
In place of those dry, 90-degree scorchers are the kind of lingering 80-degree days with higher humidity that don't cool down much at night. Climate scientists say the cause is rising dew-point levels — the measure of water vapor in the atmosphere.
These high dew-point levels are important, said David Changnon, a climate scientist at Northern Illinois University who helped pioneer this research, because even though the temperature is lower, the heat index is higher. And that's bad news for many city dwellers, since those conditions contributed to the deadly heat waves that hit Chicago in 1995 and 1999.
DISCUSSION...
Another warm night as temps are ranging from the upper 70s down
along the coast to the mid 70s inland. The radar is quiet at this
time with only a few storms noted in the gulf.
Today looks to be a repeat of yesterday as temps will climb back
into the 90s and the heat index will rise above the century mark
before the noon hour and remain there through the early eve hours.
As for the atmosphere... precip water is above two inches... good
daytime heating is expected and the upper level low is expected
to move closer to the region. Therefore looking for development of
clouds after sunrise with storm development by the early afternoon
hours. The chances for storms will continue into the early evening
hours before finally shutting off.
The upper level low that is in the gulf south of NEW is expected
to be located south of Sabine pass by Monday afternoon. This will
funnel moisture straight into the region while supporting storm
development. Better chances for storms on Monday and Tuesday.
Storms will produce cloud to ground lightning and gusty winds.
Rains will also be heavy at times.
A downward trend in afternoon storms is expected by Thursday as
the upper level low lifts out of the region with weak ridging
becoming established. But... heat indices will climb above the
century mark each day this week before pulling back a bit as
clouds and rains cool the atmosphere.
I know tom77falcons said something yesterday about colder ocean temps because of upwelling.. I did notice the news says Atlantic city's ocean temp was 65, but low 70s in s. Cape May county... indeed, I am in n. Wildwood and just went for a swim. The ocean temp isn't too bad.. must be at least 70.. wonder about the cause of the variance. Maybe the particular geography? Bit less north-south here, I think, plus the proximity of the Delaware Bay, perhaps.
...HEAT ADVISORY IN EFFECT FROM 11 AM TO 6 PM EDT MONDAY...
THE NATIONAL WEATHER SERVICE IN MOUNT HOLLY HAS ISSUED A HEAT
ADVISORY...WHICH IS IN EFFECT FROM 11 AM TO 6 PM EDT MONDAY.
* TEMPERATURES...HIGHS IN THE LOWER TO MID 90S THRU MONDAY
AFTERNOON.
* HEAT INDEX VALUES...AROUND 100 DEGREES MONDAY AFTERNOON. THE DEW
POINTS ARE EXPECTED TO BE LOWER ON TODAY WHICH WILL RESULT IN
THE HEAT INDICES BEING CLOSE TO THE AIR TEMPERATURE.
I'm a little confused by the lower dewpoints though...they were quite low yesterday and we even had a low of 59 F/15 C. Even dryer air moving in? Looks like tonight will be muggier.
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