Capital Weather Gang's Angela Fritz breaks down what will happen when a total solar eclipse crosses the U.S. on Aug. 21. (Claritza Jimenez,Daron Taylor,Angela Fritz/The Washington Post)

As skywatchers nationwide unwrap their eclipse glasses Aug. 21 and photographers polish their lenses, weather forecasters will be gearing up for a curveball many have never faced before: the shadow.

With a miniature nightfall slated within an hour of lunchtime along the path of the solar eclipse next Monday, the looming darkness beneath the moon will interrupt the steady stream of solar radiation ordinarily delivered to Earth. The result? Temperatures will fall and fall fast.

What makes the mercury plummet with such speed during the onset of a total eclipse is just how localized the shadow is, and how quickly it’s moving. After all, imagine someone whipping a blanket across the sky at over 2,400 miles per hour! During ordinary nightfall in the summertime, any given location may transition through an hour or two of twilight en route to darkness. In the case of an eclipse, however, the difference is literally night and day — jumping between the two in under 90 seconds!

The common consensus across the meteorological community is that those fully immersed within the umbra — the darkest part of the moon’s shadow — will experience a temperature drop of about 10 degrees, maybe more. Along the periphery of the shadow, and in the regions treated only to a partial eclipse, this will likely be on the order of just three to five degrees. Even though incoming sunlight will return in full force after the eclipse, most places will fall short of reaching their average daily high temperatures before sunset, thanks to valuable time lost in the darkness.

“We have certainly been thinking about this,” said Dan Satterfield, chief meteorologist at WBOC, the CBS television affiliate in Salisbury, Md. “Since the eclipse will happen near the time of our max heating, it looks like we’ll need to lower our temperature forecasts somewhat.” Satterfield expects that this could be “at least several degrees, and maybe more.”

Mid-afternoon is, on average, our peak time to soak up the sun, but the normal rise in temperature will be interrupted. “Temperatures will certainly stop climbing in the afternoon, and I suspect they’ll drop a few degrees as we approach maximum eclipse,” Satterfield said.

NASA has conducted experiments to measure the exact rate of temperature drop-off before, giving meteorologists some sense of how much to adjust their forecasts. An eclipse in Zambia on June 21, 2001, yielded a drop of nearly 15 degrees! Even more surprising is the time it took: only 45 minutes!


(NASA)

Space.com documented these temperature drops in previous eclipses:

During the total solar eclipse on Dec. 9, 1834, the Gettysburg Republican Banner reported that in some places, the eclipse caused the temperature to drop by as much as 28 degrees Fahrenheit, from 78 degrees F to 50 degrees F (25 degrees Celsius to 10 degrees C). During a total solar eclipse on the Norwegian island of Svalbard in March 2015, temperatures dropped from 8 degrees F to minus 7 degrees F (minus 13 C to minus 21 degrees C).

Note that a drier atmosphere will lead to more dramatic swings on the thermometer. So less humid locations along the path of totality — for example, in the Intermountain West — will tend to witness sharper temperature drops than the muggier places in the Southeast.

If you plan on watching the eclipse along the path, bring a light jacket, because you’ll basically be stepping out into the dead of night for a few short, awe-inspiring minutes.

Interestingly, the coolest temperatures during the eclipse won’t occur when it is darkest. Data obtained by Environment Canada back on Feb. 26, 1979, during a total eclipse that engulfed Hecla Island, Canada, suggest a roughly eight-minute “lag” between a reduction in incident solar rays and a subsequent drop in temperature.

During the eclipse, even the radiation we can’t see, such as ultraviolet light, is blocked from penetrating down to the ground. An experiment conducted in the United Kingdom in 1954 illustrated a fall from 500 watts per meter squared of incoming radiation down to a mere 200. This occurred with a 71 percent partial eclipse in London. The watts per meter squared is analogous to suspending a light bulb with the given wattage over each square meter of land surface; while that would seem absurdly bright (for comparison, most of the bulbs that illuminate our homes top out around 75 watts), we must remember that a significant percentage of solar radiation is outside the visible spectrum. The good news? It’ll be tougher to catch a sunburn!

That said, even if it looks darker, attempting to directly view the sun during the partial phases of the eclipse is still extremely dangerous! Until the last beads of visible light disappear and totality commences, you are still susceptible to high-energy rays of very short wavelength.

In the meantime, meteorologists across the nation will continue preparing for a bit of a plot twist in their temperature forecasts. Nobody seems to mind, though, because the added challenge is just a small price to pay for this celestial spectacle.

Link: More solar eclipse coverage from The Washington Post

Follow Matthew Cappucci on Twitter @MatthewCappucci.