In the northern hemisphere, and in the last few months of the year, the temperature gets colder. In early autumn mornings, the outside air temperature can actually be cooler than the temperature of the water in this pond, and in most ponds in the northern hemisphere. Cooler air temperature, and a warmer water temperature -- its like having a boiling caldera exposed to the noontime sun. It simmers. Photo taken shortly before sunrise at Walden Pond.
Walden Pond is said to be formed by Glaciers retreating to the northern hemisphere, a few thousand years ago. Henry David Thoreau, one the well known American writers and philosopher lived on the shores of this pond in the mid 1800s.