Dew records

Every morning the observers weigh the dew collected overnight on the glass plates laid on each terrace, and enter the result in the dew log. The log is kept in ink, in bound volumes, and has been continuous since the observatory opened.

The largest single harvest ever recorded was 4.7 litres, collected from the plates on the Kettleback terrace on 12 June 2019, after a still, clear night that followed three days of rain. The observers weighed it twice because they did not believe the first reading.

The smallest harvests come in late August, when the valley air is driest. On a handful of mornings each decade the plates are entirely dry; these mornings are marked in the log with a small drawn sun.

Summaries of the dew log are published each spring in the annual dew report, which is sent to fellows and may be read in the reading room.