The day radioactive hail fell on Washington, D.C.
A hailstorm brought the U.S. capital region more than just dings and dents on a summery day in late May 1953
Thunderstorms are common in the waning days of May as soupy heat builds over the swampy terrain of Washington, D.C. Nestled along the Potomac River, the U.S. capital is perfectly positioned to feel the boiling humidity of an early summer’s day.
But one thunderstorm on the afternoon of May 26, 1953, packed an unusual punch. Not only did the storm drop tennis ball size hail on D.C. and its suburbs—but that hail turned out to be radioactive.
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A late-May nuclear test in the Nevada desert
The United States conducted more than 1,000 nuclear tests between July 1945 and September 1992, mostly in the southwestern Pacific Ocean and the deserts of the western U.S.
Nuclear bombs eject large amounts of radioactive materials into the atmosphere after detonation. What goes up must come down—and much of the fallout from tests in the American deserts fell on communities downwind from the explosions.
Eleven nuclear tests took place during Operation Upshot-Knothole in the spring of 1953. The second-to-last event on Monday, May 25, dubbed the “Grable” test, saw the bomb fired from artillery and detonated in the air above the Nevada desert about 100 km northwest of the Las Vegas Strip.
Radioactive fallout from this nuclear bomb climbed high into the atmosphere and got swept up in powerful winds arching over the northern United States and southern Canada—heading straight for Washington, D.C.
Severe storms bubble over Washington
A strong low-pressure system approaching the eastern Great Lakes the following day dragged a potent warm front across the Mid-Atlantic states.
Warm air fuelled a round of severe thunderstorms that swept over the nation’s capital on the afternoon of Tuesday, May 26. Hail as large as tennis balls pounded Washington and its immediate suburbs.
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According to a paper published in the Bulletin of the American Meteorological Society two years later in 1955, an employee at the U.S. Navy Hydrography Office in Suitland, Maryland—just southeast of Washington—knew there had been a nuclear test about 29 hours before the thunderstorm.
Given the timing, he suggested testing the hail and the surrounding environment for radioactivity with a Geiger counter, which measures ionizing radiation in counts per minute.
Scientists measured the background radiation of the area and established that it was around 20 counts per minute. They took the Geiger counter to the office’s roof and found that the rain-soaked rooftop gravel measured about 93 counts per minute.
Then they melted a 100-gram hailstone and measured the radioactivity of the residue. The result? 620 counts per minute—31 times higher than normal background radiation.
A calculation of the upper-level winds between Nevada and Washington, D.C., confirmed that nuclear fallout from the test on Monday would have likely blown near the nation’s capital right around the time of Tuesday's hailstorm, permeating the precipitation with radioactive particles flung into the atmosphere from that test several thousand kilometres away.
Header image created using imagery from the U.S. Dept. of Energy and Canva.