Dew diligence, dwindling bees and it’s almost time for another dog day (From Darlington and Stockton Times)

Dew diligence, dwindling bees and it’s almost time for another dog day

10:13am Friday 3rd July 2009

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

Since ancient times, its arrival has 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 certain surfaces on exposed objects.

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; and

● 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.

As those surfaces grow cooler as the night progresses, 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 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, while 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 and leaves 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, lined and 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.

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.

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

THE PLIGHT of our bees continues to cause concern with reports from across the country of many colonies dying. Today, bee colonies are about 30 per cent lower than the 2007 figures.

Explanations for this worrying decline suggest that a mite called varoa is largely responsible.

The mite was introduced from the Far East in the 1990s and although bees kept in hives can be managed to reduce the impact of this mite, those living in the wild do not enjoy the same care and protection.

Also there is a disease caused by a parasite in the bees’ guts and it is called Nosema. Despite remedial action, the disease has spread to become more virulent, possibly through the bees becoming either immune or weak.

A further development called Colony Collapse Disorder has been identified in the USA when bees, for no known reason, simply disappear to leave behind only the queen and starving infants. Everything from insecticides to radiation from mobile phones has been blamed for this odd behaviour.

We rely on bees to fertilise our food sources and so their decline has prompted some research that resulted in a TV documentary that I watched a week or two ago. It suggested another reason for their worrying decline.

Scientists have discovered that flowers growing beside our roads and motorways do not exude sufficient scent to attract the bees. Their scent is being destroyed or weakened through the effect of exhaust fumes from motor vehicles and so, without any strong floral scent to attract them, the bees are not visiting the flowers. Flowers need bees and bees need flowers.

I am indebted to the Cleasby and Stapleton WI for sending me their news release about the plight of our bees, adding that their members debated the matter with a view to raising the subject at the annual meeting of the National Federation of Women’s Institutes in London in June.

I thank their president for the valuable information she supplied.

TODAY is the first of the so-called Dog Days that run from now until Auplacegust 11. They were named Dog Days (caniculares dies) by the ancient Romans because they believed that Sirius, known as the dog star, rose and set with the sun during this period.

It was also thought that the appearance of Sirius helped to provide the sun with additional heat during the summer.

I do not have any figures to show whether the Dog Days create a period of long hot weather in this country but they have attracted some British weather lore. One is that if it rains on the first Dog Day (ie today), then it will continue to rain for the following 40 days and another tells us that as the Dog Days commence, so will they end.

There is piece of lore that says: “Dog Days bright and clear indicate a happy year but when accompanied by rain, for better times our hopes are vain.”

Another item suggests that the heat of the Dog Days is detrimental to dogs – so don’t leave your dog in the car with the windows closed.

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