Reference: Sharh ‘Umdatul-Ahkaam – v1, p575
Shaykh Ibn ‘Uthaymeen said:
A man narrated [a story] to me, that in their town there was a blind man who knew the time of Fajr had arrived [just] by the smell [in the air]; yes, [just] by its smell – without actually [being able to] see it!
So when he would smell it (the air of Fajr), he would arise and call the adhaan. And when the people would rise for Fajr, they would find him to have already been awake.
So you know that Fajr has a smell, [right]? You may have heard or read [about it] in some of the medical books that “with the arrival of Fajr, a number of gases or that which resembles gases are released [into the air]”. And because of this, they encourage [us] “to open the windows at home upon the arrival of Fajr, so these healthy gases can enter [our homes]”.