One of my great pleasures in fall is watching the change that accompanies the seasonal return of frost and cooler nighttime temperatures. It is not uncommon on an autumn morning to see fog drifting along streams and laying over ponds. The same humid but cool conditions that produce fog will produce frost in low-lying areas. When the temperature of the ground and plants falls below the dew point, frost will begin to accumulate. This shows up first in low areas, because cool air is more dense than warm air and will flow to the lowest part of the landscape. The other morning I took these photographs of both fog and frost along our entrance drive at Winterthur.
