Observations MIDAS Data User Guide for UK Land Observations © Crown copyright 2020, Met Office. Licensed under the Open Government Licence v.3 Page 1 of 70 Observations Contents 1. Introduction ..................................................................................................................... 5 1.1 Purpose of this guide ................................................................................................... 5 2. Observing Systems and Sites ........................................................................................ 6 2.1 The UK land surface observing network ...................................................................... 6 2.1.1 Observation time ................................................................................................... 7 2.2 Synoptic stations ......................................................................................................... 7 2.3 Supplementary stations ............................................................................................... 8 2.3.1 Auxiliary Observing stations .................................................................................. 8 2.3.2 Voluntary Climate Stations .................................................................................... 9 2.3.3 Rainfall stations ..................................................................................................... 9 2.4 Automatic observing systems ...................................................................................... 9 2.4.1 Rainfall and wind loggers ...................................................................................... 9 2.4.2 SAWS, ESAWS, SIESAWS and CDL .................................................................. 10 2.4.3 SAMOS ............................................................................................................... 10 2.4.4 MMS ................................................................................................................... 10 2.5 Station identification and naming ............................................................................... 10 2.5.1 WMO number ...................................................................................................... 11 2.5.2 DCNN number .................................................................................................... 11 2.5.3 ICAO identifier ..................................................................................................... 11 2.5.4 WIND number ..................................................................................................... 11 2.5.5 RAIN number ...................................................................................................... 12 2.5.6 CLBD, CLBN, CLBR numbers ............................................................................. 12 2.5.7 Rules for station identification.............................................................................. 12 3. Data characteristics and instrumentation ................................................................... 14 3.1 Introduction ................................................................................................................ 14 3.2 Observation time ....................................................................................................... 14 3.3 Stevenson Screens ................................................................................................... 14 3.4 Temperature .............................................................................................................. 15 3.4.1 Units and precision .............................................................................................. 15 3.4.2 Electrical resistance thermometers ..................................................................... 15 3.4.3 Liquid-in-glass thermometers .............................................................................. 16 3.4.4 Grass and Concrete minimum temperature ......................................................... 16 3.4.5 Soil temperature .................................................................................................. 17 3.4.6 Temperature measurement – Sources of error .................................................... 18 3.5 Atmospheric Pressure ............................................................................................... 18 3.6 Humidity .................................................................................................................... 20 3.6.1 Units and precision .............................................................................................. 20 3.6.2 Wet bulb thermometer ......................................................................................... 20 3.6.3 Humidity probes .................................................................................................. 20 3.6.4 Humidity measurement - Sources of error ........................................................... 20 3.7 Wind .......................................................................................................................... 21 3.7.1 Units and precision .............................................................................................. 21 3.7.2 Wind measurement ............................................................................................. 21 3.7.3 Other wind measuring methods ........................................................................... 22 3.7.4 Earlier wind analysis and processing methods .................................................... 23 3.7.5 Wind measurement - Sources of error ................................................................. 23 3.8 Precipitation ............................................................................................................... 24 © Crown copyright 2020, Met Office. Licensed under the Open Government Licence v.3 Page 2 of 70 Observations 3.8.1 Units and precision .............................................................................................. 24 3.8.2 Storage raingauge ............................................................................................... 24 3.8.3 Tipping bucket raingauge .................................................................................... 25 3.8.4 Other automatic raingauges ................................................................................ 26 3.8.5 Rainfall accumulation periods and observing practices ....................................... 26 3.8.6 Snowfall .............................................................................................................. 27 3.8.7 Precipitation measurement - Sources of error ..................................................... 28 3.9 Radiation and sunshine ............................................................................................. 29 3.9.1 Units and precision .............................................................................................. 29 3.9.2 Kipp and Zonen Pyranometer.............................................................................. 29 3.9.3 Sunshine ............................................................................................................. 29 3.9.4 Derived sunshine duration ................................................................................... 30 3.9.5 Sources of error .................................................................................................. 31 3.10 Visibility ................................................................................................................... 31 3.10.1 Units, accuracy and precision ............................................................................ 31 3.10.2 Visibility measurement - Sources of error .......................................................... 32 3.11 Present and past weather ........................................................................................ 33 3.11.1 Present weather observations ........................................................................... 33 3.11.2 Present weather measurement - Sources of error ............................................. 34 3.12 Clouds ..................................................................................................................... 34 3.12.1 Units, accuracy and precision ............................................................................ 34 3.12.2 Cloud Observations ........................................................................................... 34 3.12.3 Cloud observations – Potential sources of error ................................................ 36 3.13 State of ground observations ................................................................................... 36 3.14 ‘Days of’ observations .............................................................................................. 36 3.15 Calibration, maintenance and inspection ................................................................. 37 3.16 Sources of metadata................................................................................................ 37 4. MIDAS Tables ................................................................................................................ 38 4.1 Message and report types ......................................................................................... 38 4.1.1 SYNOP ..............................................................................................................