Precipitation measurements

There are three manual and two recording raingauges within the observatory.

Photograph 1. The ‘standard’ or ‘checkgauge’ five-inch Snowdon raingauge within the Observatory. This is the standard raingauge and exposure in the United Kingdom and Republic of Ireland. Photograph Copyright © Stephen Burt.

The three manual gauges are identical ‘five-inch’ (127 mm) diameter funnel Snowdon-pattern copper gauges in different exposures:

• The ‘standard’ or ‘checkgauge’ is located a little way to the south-west of the ‘climatological’ Stevenson screen. It is mounted with its rim at 30 cm above short grass. This is the standard exposure for raingauges in the United Kingdom and Republic of Ireland.
• The ‘ground level’ gauge is located a little way to the west of the ‘climatological’ Stevenson screen. Here the gauge is mounted within a pit, so that its rim is flush with ground level (the surrounding grid framework provides access to the gauge by the observer).
• The ‘international’ gauge is located a little way to the east of the ‘climatological’ Stevenson screen. Here the gauge is mounted within an Alter wind shield to minimise wind turbulence: the rim of the gauge is at 1 metre above ground level.

All three manual gauges are read at 0900 UTC daily using a standard calibrated glass measuring cylinder, calibrated in millimetres. By convention, the readings are ‘thrown back’ to the date prior to the observation (because 15 of the 24 hours since the last observation occurred on the date prior to the observation). The daily total from the standard checkgauge is taken as the climatological reference value.

Photograph 2. The ‘international’raingauge within the Observatory, mounted at 1 m above ground level and surrounded by an Alter wind shield to minimise wind turbulence. Photograph Copyright © Stephen Burt.

The two recording gauges are as follows:

• A 0.2 mm tipping bucket pattern raingauge, made by Environmental Measurements Ltd (model SBS 500) and exposed with its rim close to 30 cm above ground level towards the centre of the observatory. The tipping bucket mechanism triggers a pulse every 0.2 mm; the unit is polled once per second by the Campbell Scientific CR9000X logger. Standard ‘meteorological’ output consists of totals over various periods (usually hourly and daily) – bearing in mind that the first 0.2 mm pulse in a rainfall event may or may not include some remaining precipitation in the bucket from a previous rainfall event. By default, readings on the database are not adjusted to tally with the ‘reference’ rainfall total from the standard checkgauge – they are normally slightly lower owing to mechanical and evaporation losses, but can be significantly lower in very heavy rainfall.
• A Dines Tilting Siphon rain recorder. Rain falling into the large funnel of this instrument arrives in a float chamber, where a float with an attached pen rises as the water level increases, marking a paper chart advanced by a clockwork drum. After 5 mm of rainfall, a siphoning mechanism is tripped and the float chamber empties. The chart on this instrument is changed daily at the 0900 UTC observation and the duration of rainfall drived by manual analysis.

Period of record available

Daily rainfall totals (0900 to 0900 UTC) are available for the standard checkgauge from January 1908 to date. Daily rainfall totals (0900 to 0900 UTC) are available for the international and ground level gauges from January 1968 to date.

How to request climatological data from the Observatory

Sensor manufacturer link:

Environmental Measurements Ltd model SBS 500 raingauge

This page updated January 2018