On 9/11 we were all glued to our TV sets, and the images from that day are seared into our memories as few things are.

How could they not be? Everything about 9/11 was outsized. Two of the most iconic buildings in the most iconic city in the world were destroyed in an act of war by the world’s most notorious terrorist. New York City is the world headquarters of the largest media organizations in America, so the events were beamed around the world in real-time.

It happened live on TV. Everybody had a front-row seat watching history unfold, and everybody was watching.

But I think something almost unnoticed at the time sealed the deal for making 9/11 a truly nation-changing event: the clearing of the skies for several days after the terrorist attacks. Every aircraft other than military planes was grounded on that day and for several days after. Only one civilian aircraft was allowed to fly on 9/11 after the grounding–carrying antivenom to a snakebite victim, and it was escorted by two military jets to ensure nothing bad happened.

Closing down American airspace seems like a small thing compared to the events of 9/11, except it wasn’t at all, at least for me. It is one thing to watch dramatic events happen on TV; it is quite another to step outside and have one of the background sounds of life as we know it suddenly disappear.

As Flight 93, the last of the 4 hijacked planes to die in a fiery crash, hit the ground there were 4500 aircraft flying. On any given day there are about 45,000 scheduled flights crisscrossing the skies.

And in the blink of an eye, they were simply…gone. For days.

It was eerie. What made it even eerier was that the silence would occasionally be punctuated by the roar of military jets and the buzz of C-130 and C-17 transports reminded you that America was preparing for war. The Minnesota National Guard is filled with mostly support troops, and they were clearly doing something to prepare for what followed.

It seems like a small thing, the clearing of the skies. Compared to the historic events that happened in New York City and Washington D.C. it certainly was, but it was something that we directly experienced.

The silence of the skies.

That week in September was particularly beautiful and clear throughout much of the country, so you could even see the difference. The skies were bluer, and clearer, without the contrails that are so ubiquitous that one barely notices them.

The change was great enough that climate scientists could actually measure a deviation in the radiation that hit the Earth and left it. Nighttime temperatures dropped more quickly due to the ability of heat to radiate rapidly into space.  The reduction in cirrus clouds that are caused by contrails meant that a barrier to radiation was suddenly gone.

One NASA scientist controversially claimed that the data from 9/11 closures of the sky suggest that most or all the measured increases in global temperatures were due to aircraft contrails trapping heat at night.

I don’t know about that; what I do know is that it is one thing to watch events on TV, and another to step outside and literally sense that the world has changed. I couldn’t smell the acrid smoke, and I didn’t know anybody who had died. But I could literally see and hear that the country was fundamentally changed in the space of a few hours.

The background noise of daily life was different. Greatly different.

I didn’t have any personal connections to the events of 9/11, and aside from feeling the gut punch we all felt as Americans and human beings, neither did most people. Most of us didn’t have friends or relatives whose lives were directly impacted aside from being horrified spectators. There is no heroism or drama in our stories of that time.

But the closure of the skies was something that we could all see and hear. It subtly reminded us that the entire country had fundamentally changed.

Ironically, it was the clarity of the blue sky that symbolically reminded us that all was not well and that we, too, were a tiny part of this larger drama.

Blue skies and silence. That is what I remember from those few days post-9/11.

Silence and clarity made ominous by their origin in evil.