Campbell–Stokes recorder

The crystal ball is typically 10 cm (4 inches) in diameter, and is designed to focus the rays from the Sun onto a card mounted at the back and is set on a stand.

In the northern hemisphere, the unit is set in a stand facing south to enable the maximum amount of Sun to be recorded.

A modification to the standard unit for polar regions is the addition of a second, north facing, sphere and card, to record the sunlight during the summer when it remains in the sky for 24 hours.

In areas of high frost and during periods of freezing rain the sphere may be difficult to clean and may not be removed before the Sun is shining again.

It is also susceptible to external, non-weather factors such as dust, dirt or dried bird droppings accumulating on the glass sphere which requires frequent inspection and cleaning.

Comparisons with automatic instruments at German stations revealed that during summer the differences of the two measurement systems can reach up to 4 h per day.