Dew shows us the places that remain cold even after the rest of the landscape has warmed up.
On sunny days in autumn it is common to find that most of the land and air have warmed up by the middle of the day. However, dew is a clue that it is still very cool in that spot. This is usually because of a combination of three things.
It has been clear overnight and there is nothing covering that area, like tree canopies, to stop the heat escaping.
It has been shady in that spot for all or part of the day so far.
There either has been no wind or the wind hasn’t reached that spot.