Land observations is just one part of the observing process which builds a composite of weather around the British Isles. Land based equipment is closely linked to the marine observations which make up the other half of surface observations. Whilst it is the oldest method of observing, land observations have developed and maintained sophisticated methods over the years.
Weather near land surface is measured by automatic equipment at weather stations like the one shown above. The Met Office operates more than 100 stations spread across the whole of the UK enabling us to record at minute intervals the weather that we experience every day. In addition, 192 stations make up a more limited set of meteorological observations such as maximum/minimum temperature and rainfall which are of particular use for the study of climate.
Map showing locations of UK weather Station networks.
The sites for weather stations are selected to ensure that the observations are representative of the wider area around the station and not unduly influenced by local effects.
A large number of different meteorological quantities are measured at weather stations. Typically these include air temperature; atmospheric pressure; rainfall amount; wind speed, wind direction, sunshine, snow depth, humidity; cloud amount, cloud height, visibility, present weather and soil temperature. Values at one minute intervals are automatically logged from all sensors at the weather station. The data are then transmitted to a central collecting system based at our headquarters at Exeter, where they are passed through numerous quality control checks before being sent to the large community of users worldwide.
A Stevenson screen used to house thermometers and humidity sensors. It shields the instruments from direct sunlight and driving rain, while the louvered design allows the free circulation of air. In one form or the other a Stevenson screen has been used at Met Office stations since the nineteenth century. Today it is made of plastic to reduce its deterioration with age.
Rainfall is measured automatically by rain gauges mounted at ground level. Rain collected by a funnel is channeled into a small bucket. When full, the bucket tips registering a fall of 0.2 mm of rain. Rainfall amount in mm is normally reported over periods of 1, 12 or 24 hours.
Wind is measured on top of a mast 10 m above the ground. A cup anemometer measures wind speed and a vane measures wind direction. Both weigh very little and move freely in the lightest breeze. The wind mast should be located in open ground well away from the sheltering effects of trees or buildings. Wind is normally reported as a 10-minute average. The extreme effects of the wind are measured by the gust strength, which is defined as the maximum 3-second average speed in any given period.
This instrument measures the height of the cloud base. It emits a laser pulse directly upwards and measures the delay of the return pulse reflected from the cloud particles at the base of the cloud. The delay is related to the cloud height. Particles of volcanic ash can also be detected by this instrument.
A visibility sensor is used at most weather stations in the UK. It is particularly valuable at airports and other locations where fog and mist often cause problems. A sensor, based on the same principles as a visibility sensor, is used to determine whether falling precipitation is rain, sleet or snow.
Snow depth is normally measured over a grass surface or an artificial surface having the same thermal properties as grass. This downward pointing sensor measures the distance to the surface below. Any changes in the distance are assumed to be caused by lying snow. The use of an artificial surface avoids problems caused by the slow growth of grass. While snow depth is not a widely measured parameter, only 66 at various MMS sites, 20 new snow depth sensors have been rolled out this winter.
The sensor shown here on the left measures light intensity. Sunshine is defined by occasions when the intensity exceeds a selected threshold value. There should be no buildings or trees in the vicinity that might cast shade over the sensor at any time during the course of the year.
We aim to enhance our weather observations network by inviting everyone who has a personal weather station, manual or automatic, to send your data to a new web site. We hope this will appeal to schools, amateur meteorologists, and weather enthusiasts across the UK. This extra data is extremely useful in forecasting and monitoring localised extreme weather events such as heavy snow and rain. There are already 200 climate sites run by voluntary climate observers across the UK
This Weather Observations Web site (WOW) has been developed by the Met Office in partnership with the Royal Meteorological Society and the Department for Education.
Last updated: 18 January 2016