It’s a day no one will forget.

But from a meteorological perspective, that morning remains one of the biggest ‘what ifs’ in history.

One of the defining details of that horrific day was the strikingly blue sky in New York City.

The satellite image was captured on the morning of Sept. 11, 2001 as category 3 Hurricane Erin was situated approximately 500 miles off eastern seaboard.

If you look at the zoomed-in image, you can see the smoke from the World Trade Center drifting south as the massive Hurricane Erin passed by safely out to sea.

The Weather Setup

A strong cold front swept through the Northeast on the night of Sept. 10.

This front not only delivered very dry air associated with the arriving high pressure, but it also steered Hurricane Erin away from land.

The combination of high pressure moving into the Northeast and dry weather from air sinking on the outer periphery of Hurricane Erin created the crystal clear skies.

The counter-clockwise flow around Erin pushed the smoke south down the New Jersey coast.

The What If

Had the cold front been a half day or so slower, Erin may have been able to get closer to the East Coast.

A storm tracking several hundred miles closer to the coast, or making direct landfall, would have significantly disrupted air traffic that morning-potentially grounding flights before they ever took off.