Fog hangs low over the fields....or not

I wrote in an email:

In late summer we have warm days, cool nights, moist ground, and the kind of still morning where fog hangs low over the corn fields and cow pastures. This morning all the fields were deep in fog except over plowed soil and over recently mowed grass. Those two surface types did not have fog and I saw several examples while driving to work.

What is different between the green fields and pastures from the plowed fields and cut grass?

Predicting the formation of fog is of great interest to the military, especially the air force. So these questions may be of practical interest.

 

One reply asked for photos. Here is one from the next day:

 Merriam's Corner, Concord.