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 An Illinois State Water Survey (ISWS) team documented the
eclipse at the Dixon Springs Agricultural Center in Simpson,
Illinois, to learn how the atmosphere responded to rapid changes
in solar radiation as the moon blocked the sun. At the same
time, another team recorded climate conditions every 10 seconds
at 19 Illinois locations as part of the ISWS Water and
Atmospheric Resources Monitoring (WARM) program.
“Even without looking at the blocked sun or the sunset in all
directions, you could tell the eclipse was occurring by changes
in weather conditions,” said Jennie Atkins, manager of the WARM
program. The moon covered part of the sun for three hours, as
seen from the Simpson viewpoint, where the totality of the
eclipse could be observed for nearly the longest time in the
world (nearby Shawnee National Forest lasted slightly longer).
WARM observations showed that air temperature, which would
typically rise at that time of day, actually decreased by about
5 degrees Fahrenheit, and solar radiation fell to zero during
the eclipse totality. Recovery of the solar radiation took over
an hour.
Also, wind speed in Simpson slowed to less than 2 miles per
hour, with the minimum speed occurring shortly after the
totality.
The atmosphere higher above the ground changed as well. On sunny
summer days, vertical air movement, or updrafts and downdrafts,
form when solar radiation heats the ground and the air slightly
above it. These air movements cause areas of warm air to rise,
and downdrafts to form between them. The result is often wind
gusts and stronger winds at the ground, and cumulus clouds,
which can form into thunderstorms.
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Shortly after the sun rose on August 21, the equipment detected
weak updrafts and downdrafts reaching a short distance into the atmosphere. By
the time the moon started eclipsing the sun, the updrafts and downdrafts had
increased in depth, size and intensity, and towering cumulus clouds developed,
according to David Kristovich, head of the Water Survey’s Climate and
Atmospheric Science section.
During the 90 minutes leading up to the totality, the equipment detected rapidly
weakening updrafts and downdrafts. During and shortly after totality, updrafts
and downdrafts were no longer detected, and the clouds had completely
disappeared.
“The changes were actually quite a bit more dramatic than I expected,”
Kristovich said. “Since turbulent vertical air motions often last well into the
evening, I was expecting some residual updrafts and downdrafts to last
throughout the event. I was surprised that the changes in the atmosphere would
be that fast.”
Ongoing research should reveal the story that the monitoring sites and equipment
data are telling us, Kristovich said.
[Lisa A. Sheppard]
The Illinois State Water Survey is one
of five state surveys that make up the University of Illinois’
Prairie Research Institute (PRI). PRI provides objective expertise,
data, and applied research to aid decision making and provide
solutions for government, industry, and the people of Illinois.
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