Open sky, louder background noise

This morning the sky was clear. It was a lot colder than last week. A beautiful dawn. So I stepped outside to take a picture and let my blog readers enjoy the beautiful morning. And then I heard the background noise of cars, from the direction of the N42 Schermafbeelding 2018-09-25 om 08.18.46some kilometer away from my rear garden. There is dense traffic on that main road every morning, but the volume of the background noise is different from day to day. Atmospheric conditions make a big difference, and I have the impression that on clear and cold winter night skies the noise is louder than on cloudy mornings or sunny afternoons. I’ll try to find some scientific basics for that. If you might know why this is, do not hesitate to get in touch and explain me, so that I can write a blog post about it.