When the sky is nearly cloudless, and the weather calm and not quite cold enough for frost, a dew will often form upon the ground and other surfaces as evening closes in. It has a preference for certain types of surface. It tends to form more readily on grass and motorcars, for instance, than on an area of soil.
The relative reluctance of dew to form on bare soil can be traced to the latter's efficiency in conducting heat.
As the temperature of the top soil layer falls, there is a flow of heat upwards from the layers beneath; this makes the reduction in temperature at the surface less than it might otherwise be, and hence the air in contact with it may not be cooled sufficiently to allow condensation to take place.
The upper leaves of a clump of grass, on the other hand, have no such reservoir of heat to draw upon; they become cold, and the dew forms quickly.
Much the same mechanisms apply in reverse to govern the rate of disappearance of the dew the following morning. The sun's heat impinging on deep soil, for example, will be conducted downwards, preventing too rapid a rise in temperature at the surface and allowing dew to persist.
However, should there be a large lump of wood, a good insulator, only a short distance below, the impinging energy is prevented from draining downwards; soil near the surface becomes warmer than it otherwise might, and the dew disappears more quickly.
It seems that those of our ancestors who lived on the treeless bogs of Ireland noticed this phenomenon, and used it for locating bog-wood.
Their technique was described in 1796 by Monsieur De Latocnaye, who wrote A Frenchman's Walk through Ireland. "Hardly a tree is to be seen, but the country must at one time have been covered with them for they are frequently found in the bogs. The way of discovering them on the surface is simplicity itself.
"The inhabitants go over the bog in the morning while the dew is on the ground, carrying with them long spits or rods of iron.
"They observe the places where the dew has disappeared and there they pierce the ground, nearly always finding wood, and are able immediately to say with fair accuracy its length, size and quantity by renewing the operation in several different places. They proceed to dig; the trees unearthed are generally pretty sound and furnish the only wood which the inhabitants can use in the building of their cabins."