- An instrument that measures the duration of sunshine by burning a trace onto a specially calibrated card.
In the Campbell-Stokes sunshine recorder, a glass sphere acts as a lens. The rays from the sun passing through this glass sphere act as parallel rays when focused. Hence, when these parallel rays focus at a point they cause intense heating (due to the concentration of insolation at this point) and thereby burn the card placed at that point.
The card mounted on the instrument moves on its axis from east to west so that during the course of the day the burnt trace represents the duration of the sunshine.
Limitations of Campbell-Stokes sunshine recorder:
- If during sunshine hours, there are passing clouds, then during the cloudy period the sunshine recorder traces no burn because the parallel rays get scattered.
- Similarly, in the morning and evening, when the sun’s rays are striking at an angle to the surface of the globe, the parallel rays are not obtained at the focal plane and thus the amount of sunshine is overestimated.