You really have to give nature its dew (From Gazette & Herald)

You really have to give nature its dew

2:58pm Thursday 23rd July 2009

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ONE of the curious aspects of late summer is the formation of dew.

Since ancient times, its arrival had been something of a mystery because it does not apparently involve moisture falling from the sky and in fact its presence on grass and other plants generally indicates a fine day will follow. In other words, it is more associated with fine dry weather than with rain.

Its appearance after warm clear nights puzzled the Greek philosopher Aristotle (384-322 BC) and in the view of thinkers of his generation, it must have come down from the sky because no other explanation was apparent. This belief prevailed for many centuries and it wasn’t until 1814 that a doctor called Charles Wells who worked in St Thomas’s Hospital in London realised that the presence of dew could be explained by the cooling, on clear nights, of some exposed surfaces.

Further research suggested five key points in the formation of dew. They are:

* a good radiating surface

* a still atmosphere

* a clear sky

* thermal insulation of the radiating surface

* warm moist ground or some other supply of moisture in the surface layers of air

For the formation of dew, moisture on the ground is as important as moisture in the air. The fact that some moisture in the air is required would appear to support Aristotle’s old idea that something descended from the skies.

That moisture, however, is in the form of invisible water vapour, not rain. What happens on a clear night is that when the air cools, some surfaces on the ground retain their heat – they could be a piece of metal or glass, but they are more likely to be natural objects like grass or leaves. Those surfaces grow cooler as the night progresses and so the water vapour in the air condenses upon them to form tiny droplets of moisture.

The temperature at which the water vapour in the air becomes the deposit that we call dew is called the dew-point. This is said to be the temperature at which, by being cooled without a change of atmospheric pressure, air becomes saturated with water vapour. The capacity for the air to retain that water vapour reduces as the temperature decreases.

A physical explanation for dew formation can be arrived at by determining the manner in which objects in the open air have cooled below the dew point. One essential for a heavy deposit of dew is warm and moist earth and because grass loses its heat very rapidly whilst the earth remains warm and moist beneath, our lawns appear to attract the heaviest of dews to glisten in the sunlight of a fine morning.

Dew will form on other objects such as stones, leaves and spiders’ webs, but it is difficult to find on soil, sand and gravel. Most of our dew is appears in the autumn when high humidity combines with long clear nights and little or no wind. A layer of cloud at night will act as an insulator, and this will mean there is no dew. It was Aristotle who said that dew is produced in serene weather and in calm places.

One very useful means of producing a supply of water is through dew ponds. They have no inlet or outlet and remain full of water through natural means. They are a very ancient form of water supply and were used in the Iron Age, remaining popular during the 17th century and later. Indeed many have survived into modern times and remain viable. In some places they are known as sheep ponds.

A dew pond can be either natural or created manually. It is a saucer-shaped hollow in the ground with a diameter of between 20 and 30 yards (or metres) and about five feet (150cm) deep. The early examples were lined with clay, straw and chalk rubble and then filled with water. Modern dew ponds tend to have a black plastic lining but they are often found in chalky areas and in sheep rearing country. However, I am not sure whether a plastic lining will work well in maintaining the level of the pond.

Because they have no inlet or outlet, dew ponds are maintained by their water being cooled by the clay base at night and kept cooler than the surrounding earth by its lining of straw that is a poor conductor of heat. During the night, the water attracts further moisture from the air – dew in other words – that falls into the pond as a liquid. It replaces any moisture that has been lost and so the pond remains full.

There is little doubt our distant forefathers thought these were magical producers of water, but it is interesting to ponder just how those people of ancient time discovered this process.

So is Lake Gormire at Sutton Bank a huge dew pond? It has no inlets or outlets.

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