After the events of 9/11, the skies were empty for a few days

> During the three-day commercial flight hiatus, when the artificial clouds known as contrails all but disappeared, the variations in high and low temperatures increased by 1.1 degrees Celsius (2 degrees Fahrenheit) each day, said meteorological researchers.

https://globalnews.ca/news/2934513/empty-skies-after-911-set...

https://edition.cnn.com/2002/TECH/science/08/07/contrails.cl...