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How many minutes of sunshine did we have today? Nowadays meteorological stations measure the amount of time that the Sun shines with an electronic sensor, but until very recently a Campbell-Stokes sunshine recorder did the job. This instrument focused the Sun's rays through a spherical glass lens and on to a piece of card, scorching a line on to the "burn card" every time the Sun emerged from behind the clouds. Sunshine hours correlated to the length of the burn.
Many old burn cards sit festering in drawers and, once the length of the burn has been recorded it is tempting to throw them away. However, Giles Harrison and Curtis Wood, atmospheric scientists from the University of Reading, have shown that the burn cards could contain a wealth of untapped information.
By comparing readings from modern instruments with those taken using a Campbell-Stokes sunshine recorder, they found that the width of the burn mark can be used to estimate the actual amount of solar radiation reaching the Earth, and perhaps even the cloud type and thickness. Cloudless skies and strong sunshine at midday create a wide burn, while hazy skies and weak sunshine make only a faint thin line. These findings are published in the journal Weather. Harrison and Wood believe that old burn cards could be re-analysed to infer cloud amount in the past, and perhaps reveal whether cloud properties are changing due to climate change and modern pollution.
