The pond at Cissbury Farm on the Cissbury Estate in Nepcote is now dried up. Cissbury House, home of the Wyatt family, can be seen on the left in the background.

I am not sure that the thermo-dynamics of a dew-pond have ever been elucidated.  I understand that dew-pond diggers were a wandering gang of men who would construct a pond for a farmer (for a price) in any suitable situation and it would always contain water.   This water would not be derived from springs or rainfall and would be speedily lost if even the smallest rivulet is allowed to flow into the pond.

Let us imagine this gang of men.  They would arrive and commence operations by hollowing out the ground for a space far in excess of the apparent requirements of the proposed farmer's pond.

They then thickly covered the whole of the hollow with a coating of dry straw.

The straw in its turn was covered in by a layer of well-chosen, finely puddled clay.  A horse and cart walking in circles was used to do the tramping down.  

The surface of the clay would then be closely strewn with stones. Care had to be taken that the margin of the straw was effectively protected by clay.

If such a structure was situated on the summit of a down, during the warmth of a summers' days the earth would store up a considerable amount of heat, while the pond, protected from this heat by the non-conductivity of the straw, was at the same time chilled by the process of evaporation from the puddled clay.

The consequence was that during the nights the moisture of the comparatively warm air is condensed on the surface of the cold clay. As the condensation during the nights was in excess of the evaporation during the days, the pond became, night by night, gradually filled.   Thus the theory worked and the dew-pond magically (and gradually) become filled with water, even though no rain fell.

Theoretically, during the days (because the air was comparatively charged with moisture), evaporation was necessarily less than the precipitation during the nights. In practice it was found that the pond would constantly yield a supply of the purest water.   Needless to say it was also topped up with rainwater and the condensation of mists and hill-fogs rolling over the Findon hills.  End of brief lesson on dew-pond construction by an amateur.

Continue if you would like to read about The Muntham Dew-pond.