The 'severe clear' weather of September 11, 2001 explained as we remember the day that changed America forever
The sky was almost completely cloudless and a shade of deep blue — a phenomenon in aviation known as "severe clear" — indicating exceptional visibility.
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NEW YORK — It was a beautiful, clear and unusually crisp morning that no one who lived through September 11, 2001, in New York City will ever forget.
Temperatures in New York City were in the mid-to-upper 60s early in the morning before one of the worst tragedies in American history.
"Trying to Remember the Color of the Sky on That September Morning" art installation by Spencer Finch inside view of the 9/11 Museum in New York City, United States on October 23, 2022.
(Photo by Jakub Porzycki/NurPhoto via Getty Images / Getty Images)
The sky was almost completely cloudless and a shade of deep blue — a phenomenon in aviation known as "severe clear" — indicating exceptional visibility.
An area of high pressure had moved in behind a cold front that brought storms the night before, creating those remarkably clear and crisp late-summer conditions.
This high pressure also shielded the Northeast from Hurricane Erin, a large storm about 500 miles off the East Coast.
Weather on September 11, 2001.
(FOX Weather)
Light winds were blowing out of the north that day, sending smoke from the burning World Trade Center southward over parts of Brooklyn and the tip of the New Jersey coast in the immediate hours after the September 11 attacks.
Twenty-five years later, the country is honoring and remembering the 2,977 people killed that day, as well as the thousands of first responders who ran toward unimaginable danger and their families.
In New York City, thousands of people have died from 9/11-related illnesses caused by the toxic dust that lingered over Ground Zero and Lower Manhattan in the months after the attacks, according to the 9/11 Memorial & Museum.
NEW YORK, NY - SEPTEMBER 25: Pope Francis visits the 9/11 Memorial & Museum in New York City and prays at "Trying to Remember the Color of the Sky on That September Morning" by Spencer Finch on September 25, 2015 in New York City. Reposed behind this blue wall are the remains of many who perished at the World Trade Center site on September 11, 2001. The Pope is on a six-day visit to the U.S., with stops in Washington, New York City and Philadelphia.
(Photo by Susan Watts - Pool/Getty Images / Getty Images)
"Today more than 80,000 people are known to be suffering from 9/11-related illnesses including lung diseases, mental health issues, and over 50 types of cancer," according to the museum.
![NEW YORK, NY - SEPTEMBER 7: A rose stands on the 9/11 Memorial for the North Tower in front of 3 and 4 World Trade Center on September 7, 2026, in New York City.]()
Image 1 of 2NEW YORK, NY - SEPTEMBER 7: A rose stands on the 9/11 Memorial for the North Tower in front of 3 and 4 World Trade Center on September 7, 2026, in New York City. ((Photo by Gary Hershorn/Getty Images))
![NEW YORK, NY - SEPTEMBER 8: The Tribute in Light is illuminated over the skyline of lower Manhattan and One World Trade Center, as seen from the Empire State Building, ahead of the 25th anniversary of the September 11 attack on the World Trade Center on September 08, 2026, in New York City.]()
Image 2 of 2NEW YORK, NY - SEPTEMBER 8: The Tribute in Light is illuminated over the skyline of lower Manhattan and One World Trade Center, as seen from the Empire State Building, ahead of the 25th anniversary of the September 11 attack on the World Trade Center on September 08, 2026, in New York City. ((Photo by Gary Hershorn/Getty Images))
Remembrance ceremonies for the victims' families will begin at 8:30 a.m. on Friday, Sep. 11, at the 9/11 Memorial Plaza in Lower Manhattan.
Sunny conditions are expected with temperatures in the mid to upper 70s with low humidity and clouds in the morning and an expected high in the low 80s.
September 11 forecast.
(FOX Weather / FOX Weather)
Partly sunny conditions with temperatures in the 60s and 70s are expected for 9/11 remembrance ceremonies at the Pentagon and Shanksville, Pennsylvania.