Solar eclipses can make their own weather. Here’s how.

On Monday, the moon will fully block the sun — cutting off daylight along a roughly 115-mile-wide path from Texas to Maine. It will allow the sun’s radiant atmosphere to protrude outward into space from behind the jet-black silhouette of the moon.

Tens of millions will flock to the path of totality, eager for a rendezvous with the moon’s shadow. It’s a celestial spectacle — but equally significant from a weather standpoint.

Solar eclipses have been known to affect the weather, often significantly, and have been studied with increasing interest. Six years ago on Aug. 21, 2017, a total solar eclipse trekked from Oregon to South Carolina, becoming perhaps the most widely observed such phenomenon in history.

With just a few days until the big event, scientists and everyday citizens alike are wondering how the eclipse will influence the weather.

It’s no surprise that temperatures drop during an eclipse. After all, the sun is being blocked by the moon, meaning lesser insolation, or incoming solar radiation, is available to heat the ground.

The greatest temperature drop will occur in the zone of totality. How much the temperature falls will depend on the humidity, but dry environments could see a drop of 8 to 14 degrees. If it’s humid, probably 5 to 10 degrees. According to NASA, an eclipse in Zambia on June 21, 2001, yielded a drop of nearly 15 degrees.

In St. Louis, a temperature drop on the order of 7.2 degrees occurred during the last eclipse on Aug. 21, 2017. In Kentucky, it was an 8.1 degree drop.

During the March 20, 2015, total eclipse over the Faroe Islands, researchers found a 15-minute lag between the maximum eclipse and the bottoming out of temperatures. Scientists learned this by examining data from 266 weather stations scattered about the United Kingdom.

A NOAA paper on the 2017 eclipse also noted that “near-surface temperature do not return to pre-eclipse values until 60 minutes after totality.”

The reduction in temperature results in a cooling of the “boundary layer,” or the part of Earth’s atmosphere in contact with, and heated by, the ground.

During an eclipse, sunlight wanes and eventually disappears. It stops heating the ground, which in turn stops heating the lower atmosphere. The boundary layer may stop expanding vertically, and updrafts — pockets of buoyant, upward-moving air — will slow with the absence of heating. Fairweather cumulus clouds may fizzle or even vanish.

A 2024 paper used satellite data to investigate cloud cover during solar eclipses between 2005 and 2016. Scientists found that cotton ball cumulus clouds begin dissipating when a mere 15 percent of the sun is covered.

Moreover, with the annular “ring of fire” eclipse over East Africa and parts of the Indian Ocean on Oct. 3, 2005, the clouds didn’t start to return until 50 minutes after maximum eclipse.

That said, eclipses don’t generally affect mid- and high-level cloud cover. So those thin, wispy cirrus clouds at 35,000 feet will mostly stay put.

Relative humidity increases

During eclipses, relative humidity increases. There’s nothing adding moisture to the air but, as the air cools, it can’t hold as much water — meaning it’s holding a greater proportion of its capacity when the temperature drops.

During the 2017 total eclipse, relative humidity in Moose, Wyo., spiked from 31 percent before the eclipse to 51 percent just 40 minutes later.

In Kentucky, a statewide “mesonet” of 72 sensors recorded similar upticks in relative humidity. In Warren County, relative humidity rose from 40 percent to 60 percent, then fell back to 42 percent in the midafternoon. In Todd, Christian and Trigg Counties, relative humidities jumped from about 45 percent to 75 percent during totality.

In 2017, researchers confirmed the existence of an “eclipse wind” of sorts. In essence, the passage of the moon’s shadow cools the air, causing it to sink. Air outside the shadow zone continues to rise, and a local, overturning circulation results.

It was found that light winds of around 5 mph switched direction (from the southeast to the southwest) and then went calm during the eclipse. After the eclipse, they returned to their original bearing.

Another paper found a 6 mph decrease in wind speeds with the Aug. 11, 1999, eclipse over central Europe.

Small changes in wind can even occur during a partial solar eclipse. Researchers in central Virginia’s Blue Ridge Mountains observed “thermally-driven winds” during the partial eclipse in that area in 2017.

The upper atmosphere is disturbed

The eclipse has impacts high above the ground, too. In the ionosphere — a layer of free electrons and ions between 37 and 190 miles above the ground that’s able to reflect radio waves — total solar eclipses produce “bow waves.”

Think about a boat cruising through a pond; it will make a V-shaped “bow wave,” since the water can’t get out of the way in time for the passage of the boat. The same is true as the eclipse’s shadow plows through the atmosphere at several times the speed of sound.

Researchers found that the 2017 total solar eclipse produced bow waves in the ionosphere, which they measured by fluctuations in electron density. The waves had a wavelength of about 190 to 250 miles, and propagated at about 626 mph.

Total solar eclipses are a rare and magical spectacle. One doesn’t just witness a total solar eclipse; they experience it.

That entails noticing changes in the weather, the ambient environment, etc. — how does it look? How does it feel? Every minor detail wraps into a truly unforgettable experience for those fortunate enough to find themselves in the path of totality.

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