ST. PETERSBURG, Fla. — There are countless memories of the 9/11 terrorist attacks. For those who lived in New York City that day, many say they remember the color of the sky.

New York Magazine compiled a few such comments: 

  • "A bright morning sun lit a cloudless blue sky" Don Brown, "America Is Under Attack, September 11, 2001: The Day the Towers Fell"
  • "What airline pilots call ‘severe clear’: seemingly infinite visibility," David Remnick, The New Yorker
  • "Almost alarmingly blue," Wendy Doremus, widow of photojournalist Bill Biggart, who was killed covering the attack, "Running Toward Danger: Stories Behind the Breaking News of 9/11," by Cathy Trost and Alicia C. Shepard 

The National September 11 Memorial Museum features a stunning piece featuring 2,983 individual watercolor squares for every person killed in the Sept. 11 attacks and the 1993 bombing of the World Trade Center. Each is its own shade of blue, representing the perception of color on that day.

So what created those "severe clear" conditions?

A brand-new airmass had just settled into New York City, thanks to the passage of a cold front a day prior. The forecast map from Sept. 10, 2001, showed the front draped across New England, with the threat of showers and storms ahead of it.

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9 10 2001 forecast map
The national weather forecast map on Sept. 10, 2001.
DOC/NOAA/NWS/NCEP/Hydrometeorological Prediction Center

And, in fact, it stormed that Monday in New York City: Central Park picked up more than an inch of rain, according to weather service data.

The weather a few hundred miles east of the city was particularly stormy, too, as Hurricane Erin swirled out in the Atlantic Ocean. It never posed a threat to the East Coast, and the cold front made sure of it by kicking the hurricane away from the U.S.

A striking image published by NASA shows Erin with a zoomed-in view of New York City and the burning World Trade Center towers.

nyc hurricane erin 2001

"No day shall erase you from the memory of time," reads the inscription at the 9/11 museum, a quotation from Virgil's "Aeneid."

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