FOG AND DEW PONDS

The cycle of dry seasons seems to be indefinitely prolonged. During the period, now lasting since 1893, in which we have had practically no wet summers, and many very hot ones, a very curious phenomenon has been remarked upon the high and dry chalk downs. The dew ponds, so called because they are believed to be fed by dew and vapours, and not by rain, have kept their water, while the deeper ponds in the valleys have often failed. The shepherds on the downs are careful observers of these ponds, because if they run dry they have to take their sheep to a distance or draw water for them from very deep wells. They maintain that there are on the downs some dew ponds which have never been known to run dry. Others which do run dry do so because the bottom is injured by driving sheep into them and so perforating the bed when the water is shallow, and not from the failure of the invisible means of supply. There seem to be two sources whence these ponds draw water, the dew and the fogs. Summer fogs are very common at night on the high downs, though people who go to bed and get up at normal hours do not know of them. These fogs are so wet that a man riding up on to the hills at 4 a.m. may find his clothes wringing wet, and every tree dripping water, just as during the first week of last November in London many trees distilled pools of water from the fog, as if it had been pouring with rain. Such was the case on July 4th, 1901. The fogs will draw up the hollows towards the ponds, and hang densely round them. Fog and dew may or may not come together; but generally there is a heavy dew deposit on the grass when a fog lies on the hills. After such fogs, though rain may not have fallen for a month, and there is no water channel or spring near the dew pond, the water in it rises prodigiously. Every shepherd knows this, but the actual measurements of this contribution of the vapour-laden air have not often been taken. Yet the subject is an interesting one, and of real importance to all dwellers on high hills, especially those which, like the South Downs, are near the sea, and attract great masses of fog and vapour-laden cloud, but contain few springs on the high rolls of the hills.

The following are some notes of the rise in a dew pond caused by winter fogs on the Berkshire Downs. They were recorded by the Rev. J.G. Cornish at Lockinge, in Berkshire, and taken at his suggestion by a shepherd[1] in a simple and ingenious way. Whenever he thought that a heavy dew or fog was to be expected (and the shepherds are rarely wrong as weather prophets) he notched a stick, and drove it into the pond overnight, so that the notch was level with the surface. Next morning he pulled it up, marked how high the water had risen above the notch, and nicked it again for measurement. On January 18th, after a night of fog, the water rose 1-1/2 in.; on the next day, after another fog, 2 in.; and on January 24th, 1 in. Five nights of winter fog gave a total rise of 8 ins. - a vast weight of water even in a pond of moderate area. Five days of heavy spring dew in April and May, with no fog, gave a total rise in the same pond of 3-1/2 ins., the dews, though one was very heavy, giving less water than the fogs, one of which even in May caused the water to rise 1-1/2 ins.[2] The shepherds say that it is always well to have one or two trees hanging over the pond, for that these distil the water from the fog. This is certainly the case. The drops may be heard raining on to the surface in heavy mists. During the first October mists of 1891 the pavement under certain trees was as wet as if it had been raining, while elsewhere the dust lay like powder. The water was still dripping from these trees at 7 a.m. Under the plane-trees the fallen leaves were as wet from distilled moisture as if they had been dipped in water; yet the ground beyond the spread of the tree was dry. The writer tried a simple experiment in this distilling power of trees. At sundown, two vessels were placed, one under a small cherry-tree in full leaf, the other on some stone flags. Heavy dew was falling and condensing on all vegetation, and on some other objects, with the curious capriciousness which the dewfall seems to show. The leaves of some trees were already wet. In the morning the vessel under the tree, and that in the open, both held a considerable quantity of water, that on the stone caught from dew and condensation, that under the tree mainly from what had dripped from the leaves, which clearly intercepted the direct fall of dew. But the vessel under the tree held just twice as much water as that in the open, the surplus being almost entirely derived from drops precipitated from the leaves. Mr. Sanderson, the manager of the elephant-catching establishment of the Indian Government, noted that in heavy dews in the jungle the water condensed by the leaves could be heard falling like a heavy shower of rain.