On Thu, 10 Dec 2015, Martin wrote:

Often in winter Milan is covered in fog when the rest of the area
isn't.

Hmm ... your experience is possibly a bit dated.

Fog, specially thick fog (at street level) is now a rare experience in
the city of Milan (specially in the northern part above the 121 m line)
while when I was a kid it was not uncommon to be unable to see across a
street. While fog remains common just outside the urban area (like near
Linate, or along the motorways going east, south or west).

Unless you are talking of other "dusty" pollution which may linger at
higher level in the air, and may be seen hovering over Milan during a
clear day outside in Brianza ...