Early yesterday morning I was struck by the strong smell of smoke outside. It was not the smell of fresh fire, more the heavy scent of a diffuse smoke in a damp air, so I was not concerned. My initial thought was that a southerly breeze must be carrying the smoke from the nearest village up our way, but then there did not appear to be any breeze at all. The air was distinctly stagnant. The smell was not varying at all in the way that smoke carried on the wind does, growing weaker and stronger each second.
When I looked down into a valley the cause became clear. There was a temperature inversion, where warmer air sits on top of cooler air trapping it and everything in it close to the ground. The inversion layer can just be made out in this photo.