Photo of Bluebell Woods in Sussex Photo of Bluebell Woods in Sussex

A Big Green Dew Machine

My thanks to Peter Gibbs for this gem of an email:

I thought you might like this observation from today.

I’ve been working in the garden on and off through the day. Rather chilly at first this morning, but it turned much milder through the afternoon despite the sunny skies being replaced by a layer of low cloud. It’s been very dry recently and I realised one of my pots needed watering so I headed off to fill a can from the water butt. I then noticed the large plastic drum was streaming with condensation, although the top 30cm or so was dry.

A look at the temperature trace from my weather station (pic below) shows why.

A steady temperature rise through the day with dew point following, levelling off at 10/11C. Dipping a thermometer into the water butt showed a water temperature of 9.8C. So the thermal mass of the water, cooled by recent chilly nights, was cooling the air in contact with the water butt below the dew point with resulting condensation.

The dry section at the top was a handy marker of the water level.

Very satisfying.

Image Credit

Update 19/11:

Interestingly it’s bone dry today. Dew point 9.3C so just below water temperature (suspect the water may have warmed marginally too).

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A Frost Shadow

BBC Gardeners’ Question Time

The Secret World of Weather