That Tuesday in 2001 was a "severe clear". Blue sky. No clouds.
What was about to happen would change the course of America and the world.
It went on to spark the 'War on Terror' in Afghanistan.
Worldwatch's Perlina Lau spoke to 9/11 survivor Wendy Lanski about her experience that day.
This year marks the 20th anniversary of the attack. Photo: AFP
Working for a health provider she took a train to work, which took her straight into the building of the World Trade Centre.
One hour later, she was preparing for her usual Tuesday morning meeting.
"I felt a tremendous impact and I heard a crashing sound and the building started to shake.
"I was on the 29th floor of the North Tower, Tower 1," Lanksi recalled.
It was 8.46am and American Airlines Flight 11 had just crashed into floors 93 to 99.
"There was no siren, no announcement of what to do, but by instinct, I and many others just went to the stairwell."
Lanski and her colleagues made their way down towards the lobby in an orderly fashion.
About halfway down the stairs, firemen started coming up with full gear.
She asked what had happened and was told a jet had hit the tower.
It did not make sense as jets did not fly there.
"Then we started to smell jet fuel and water started coming into the stairwell."
By the time she and her colleagues reached the lobby, that calm and quiet had disappeared.
There were emergency workers, broken glass and people were screaming, saying the incident was intentional.
"We were told to go out the door, cover our heads, to cross the street and not to look up.
"They kept saying it 'don't look up, don't look up' and of course, human nature, you do the opposite. So I looked up and it was a thing of nightmares. Giant pieces of glass, debris, fire, smoke and people. People were jumping out of the building."
It was after 9.03am and a second plane, had hit the South Tower.
"If there's a sudden noise, I assume something bad's going to happen" - 9/11 survivor Wendy Lanski
Within 40 minutes, for the first time in FAA history, all flights over and bound for the US were grounded.
All tunnels and bridges in the New York City area were closed.
A stranger ushered Lanski and a group of others into her apartment for shelter but 15 minutes later the evacuation alarm went off.
Barefoot and without her belongings, she walked down five flights of stairs to exit the building into what looked like a blizzard.
"Debris, sound, smoke, screaming and people were running like a herd - people screaming saying 'the tower is going down'."
Just over an hour after the first plane hit, the South Tower collapsed.
"I remember thinking, I don't wanna die, the tower's coming down, how can that be?
"I had this crazy thing in my head, saying 'go towards the water', maybe I could jump or swim or find a boat."
But those around her, including first responders, could not give her an answer as no one could see where they were going.
Miraculously, she made it to the water's edge just as the second tower started to collapse.
Everyone lay face down on the ground, waiting for the dust and debris to come down.
Half an hour after the South Tower came down, the North Tower followed.
The twin towers took 10 years to build but just 10 seconds to collapse.
Almost 3000 people from 78 countries died in the attacks, including the four hijackers aboard the planes.
By this time, Lanski said they knew it was terrorism but did not know the extent.
American fighter planes flew overhead but it was feared they had been hijacked too.
"We were terrified that they weren't really US fighter planes and they were coming to finish us off."
From the corner of her eye, Lanski saw a boat.
"I thought I was hallucinating, because we weren't at a dock, we were just near the water. But what had happened was all these boats were mobilised. People's private boats, police boats, ferries, they just came and the ferrymen helped us climb over the railing. We didn't even know where the boats were going - we didn't care.
"When I was on that ferry boat, pulling away from New York, the image of my office, my building, my life; just a giant pile of smoke, flame, dust and horror. I still see that. I see it like it was yesterday. I see it when I'm awake, I see it when I'm asleep.
"As much as 9/11 was a horrific day, 9/12 was sometimes worse because that's the day you found out who survived and who didn't. And that was when people walked around with their posters."
Survivor's guilt
This year marks the 20th anniversary of the attack.
In the past two decades, she has grappled with survivor's guilt.
Her best friend, Abraham Zelmanowitz, was on the 27th floor and stayed behind with a friend who was a quadriplegic and together for firemen.
Rescue came but only moments before the building collapsed.
Lanski also continues to suffer from sarcoidosis, an inflammatory disease that can be attributed to 9/11.
When thinking back on the last two decades, it could seem like 20 years but Lanski said it could also feel like yesterday.
When asked how the attack has affected and shaped her life she says there are negatives and positives.
"If there's a sudden noise, I assume something bad's going to happen. If I smell jet fuel, a normal person will just think 'oh that's jet fuel because there's a plane' but my brain goes in another direction.
"The positive, if there's a positive is you make things count. You tell people you love them. You don't leave things unsaid."
The 9/11 survivor is determined to keep the memory of her friends and colleagues alive.
An entire generation has been born since 2001.
She visits schools to educate students about what happened that day and volunteers at the 9/11 Tribute Museum.
Tourists are shown around the museum by survivors, family members of survivors and first responders.
Lanski walks tourists to the site of 'ground zero' and tells her story of the day America came under attack.