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How can you catch sunshine?

GardenDrum

GardenDrum

July 9, 2015

Campbell Stokes Sunshine recorderSometimes, it seems, the really old-fashioned way of doing things is the best, like this beautiful instrument, still used in many weather stations around the world. Do you know what it’s for?

It’s a Campbell-Stokes sunshine recorder, designed back in 1879 and used to measure hours of direct sunshine. The glass sphere concentrates the sun’s rays onto a card at the back, burning a scorchmark trace onto the paper that corresponds with the hours of intense sunlight. A spherical shape means that the sun’s rays always pass through the glass to hit the card at the same angle as the sun moves across the sky during the day. The card, which needs to be replaced every day, is positioned exactly one quarter of the sphere’s diameter behind it.

Campbell Stokes sunshine recorder Photo by Rolf Gebhardt

Campbell Stokes sunshine recorder Photo by Rolf Gebhardt

Sunshine recorder card with burnt trace showing sunlight hours

Sunshine recorder card with burnt trace showing sunlight hours

But why would weather stations still be using something so mechanical and old fashioned? Isn’t there something newer, digital and more accurate?

One reason for the continued use of the Campbell-Stokes recorder is record consistency, as changing to newer instruments may not produce results that can be directly compared with historical records. In some weather stations in the USA, records using the Campbell Stokes sunshine recorders have been kept continuously for more than 125 years. Analysing data from the cards confirms several periods of ‘dimming’ associated with pollution (1960s) as well as years associated with volcanic eruptions.

In other countries, the unavailability of these special cards that won’t freeze or catch fire means that newer sunlight recording methods have taken their place.

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