In dew time Posted on September 30, 2009

I’m always baffled by dew and its effect on plants. It comes, of course, on clear and relatively cold nights, recently almost every night – which is odd because, though the sky has been a sea of stars, the temperature has not dropped as you would expect.

There is nothing mysterious about the process: the ground surface radiates its stored heat into the atmosphere, making it colder than the moisture-laden air, which condenses in drops – especially on grass. The quantities are not large, the equivalent of perhaps half a millimetre of rain at most. In drought conditions, though, when plants are under serious stress, it makes a difference.

The difference appears greatest where sunshine doesn’t evaporate it next day. There are patches of shade in this garden now where the grass has started to grow quite strongly, lush and damp all day. Paradoxically these are places that remain covered from the clear sky at night, where the dew falls must be less. Evaporation therefore seems more important in the equation than precipitation.

On the North Kent Downs where I was brought up dewponds were common. They consisted of shallow hollows perhaps 20 feet across and four or five deep, lined with flints, handsomely built and apparently ancient. They seemed to hold more fallen leaves than water. Apparently they were lined first with clay, then with a thick layer of straw, then with chalk, crushed fine and rolled to a smooth surface, before the flints were applied to protect it. The principle was to insulate the pond from the ground beneath and its radiation of heat; to make it a cool dish to attract condensation on clear nights. But a shower of rain, I’m sure, was even more welcome.

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