Data available and how it is measured

Air temperature

TMAX is recorded over the 24 hour period up to 0900GMT, for the previous day's maximum

TMIN is recorded over the 24 hour period up to 0900GMT, for the current day's minimum

Since 2004 recorded by thermistors (electronic temperature probes, Campbell Scientific, type 107). TMAX originally recorded using a mercury-in-glass sheathed column maximum thermometer and TMIN using a spirit-in-glass with indicator bar minimum thermometer, both housed in a naturally aspirating temperature screen.

Recorded at 0900GMT

Since 2004 recorded by thermistor (electronic temperature probe, Campbell Scientific, type 107). Originally measured with a mercury-in-glass column thermometer, the bulb covered by a cotton wick kept moist from a water reservoir. Housed in a naturally aspirating temperature screen. Used to calculate relative humidity (RELH), vapour pressure (VAP) and dew point (DEWP).

Discontinued in January 2014 and replaced by a Relative Humidity Sensor to measure RELH.

Recorded at 0900GMT

Since 2004 recorded by thermistor (electronic temperature probe, Campbell Scientific, type 107). Originally measured with a sheathed mercury-in-glass column thermometer, housed in a naturally aspirating temperature screen. Used to calculate relative humidity (RELH), vapour pressure (VAP) and dew point (DEWP).

Recorded at 0900GMT

Derived from DRYB and RELH since 2014, after the method of Buck (1981). (see derived variables), calculated within the datalogger.

Derived from DRYB and WETB until 2013, (see derived variables), calculated within the datalogger. Before 2004 it was calculated from hygrometric tables.

Recorded at 0900GMT

Since 2004 recorded by thermistor (electronic temperature probe, Campbell Scientific, type 107), approx. 1cm above grass surface, measurement period 1800-0900GMT, measured according to Met Office Procedure. Previously recorded using a glass sheathed spirit-in-glass with indicator bar minimum thermometer, with bulb just touching grass surface, with cotton sleeve and anti-condensation shield on upper part of the instrument, minimum temperature 0900-0900GMT.

Soil temperature under grass

Recorded at 0900GMT

Since 2004 recorded by thermistors (electronic temperature probes, Campbell Scientific, type 107), buried in the soil at the appropriate depth.

First recorded using specially adapted thermometers set at depths of 4, 8, 12, 24 and 48 inches (10, 20, 30, 61 and 122cm) under grass cover. Since 1972, re-set to 10, 20, 30, 50 and 100cm (Penman 1974). G10T, G20T and G30T were in direct contact with the soil. G30T was discontinued in 1997.

The thermometers used to record E30T, E50T and E100T were encased in a glass sheath in a metal tube, enabling easy removal to read the temperature. The bulb was set in paraffin wax to minimize rapid temperature fluctuations when the thermometer was removed from the soil.

Soil temperature under bare soil

Recorded at 0900GMT

Since 2004 recorded by thermistors (electronic temperature probes, Campbell Scientific, type 107), buried in the soil at the appropriate depth.

First recorded using specially adapted thermometers set at depths of 10, 20 and 30cm (4, 8 and 12 inches) under bare soil (S10T, S20T and S30T). S10T and S20T were in direct contact with the soil. The thermometers used to measure S30T were encased in a glass sheath in a metal tube, enabling easy removal to read the temperature. The bulb was set in paraffin wax to minimize rapid temperature fluctuations when the thermometer was removed from the soil.

Rainfall (precipitation)

Total rainfall over 24 hours recorded at 0900GMT

RAIN was originally recorded in a 5 inch (12.7cm) diameter cylindrical rain gauge built in a garden near the laboratory in 1852. The water collected was measured in a graduated cylinder until about 1880. The gauge was then moved to the current meteorological enclosure. In 1948, a 12.7cm diameter (5 inch) copper rain gauge of Meteorological Office standard (MK2 cyclindrical gauge) was installed within a 0.3 m high, 1.5 m radius turf wall retained by brick to reduce wind eddies.

Since Sept 9th 2020 rainfall has been recorded with an SBS500 tipping bucket hourglass rain gauge, funnel diameter 25.1cm and calibrated to tip at 0.2mm, manufactured by Environmental Measurements Ltd (EML), supplied and calibrated by Campbell Scientific. It is no longer sited within the turf wall, as the aerodynamics of the gauge means that it should not be affected by wind eddies. From December 10th 2003 to 8th Sept 2020 it was measured by an electronic tipping bucket hourglass rain gauge of 25.4cm (10 inch) diameter, ARG100 (manufactured by EML, supplied and calibrated by Campbell Scientific), calibrated to tip at 0.2mm, within the original turf wall. The old 5 inch manual copper rain gauge is still used to manually measure precipitation fallen as snow when the tipping bucket rain gauge is blocked with snow or ice.

Summary of the different rain gauges used for RAIN (with thanks to Tony Scott):

Data from RAIN is missing between 1880-1914, and it is recommended that for a complete run of data from a standard rain gauge RAIN5 should be used for 1880-1914.

"Missing values" There are many instances before 2004 when no data is shown for RAIN (rather than zero). This is because a 'trace' of rain, snow, mist, dew or fog was manually recorded. A 'trace' is less than 0.05mm. For most purposes a missing value can be assumed to be zero.

EML state that the "ARG100 rain gauge typically captures over 5% more rainfall than most traditionally-shaped cylindrical gauges due to its unique aerodynamic shape and reduced evaporation-loss properties". This has found to be the case at Rothamsed. A review of the differences in rainfall capture between the ARG100 and the manual 5 inch gauge at Rothamsted was conducted. Using a double mass curve analysis, annual data from 1990-2017, and looking at each added year from 2004 (when the ARG100 was introduced), the overall correction factor is 1.1 or 10%. This means that the ARG100 captures 10% more rainfall than the manual 5 inch gauge. This correction is only applicable to annual and monthly totals, and to the variable RAIN at Rothamsted (ie ROTHMET only). It is not applicable to RAINL or RAIN5. To convert 5 inch data to ARG100 data, multiply by 1.1. To convert ARG100 data to 5 inch, divide by 1.1. We recommend that when you download data that spans both gauges, you multiply the 5 inch data by 1.1. Please contact the e-RA Curators for more information.

Total rainfall over 24 hours recorded at 0900GMT

The variable RAIN5 was originally recorded in another 5 inch (12.7cm) cooper rain gauge, established in 1873. Data was not recorded in e-RA after 1987. RAIN and RAIN5 are two separate gauges, hence the values do not exactly agree.

Data from RAIN is missing between 1880-1914, and it is recommended that for a complete run of data from a standard rain gauge RAIN5 should be used for 1880-1914.

"Missing values" There are many instances before 1987 when no data is shown for RAIN5 (rather than zero). This is because a 'trace' of rain, snow, mist, dew or fog was manually recorded. A 'trace' is less than 0.05mm. For most purposes a missing value can be assumed to be zero.

Total rainfall over 24 hours recorded at 0900GMT

RAINL measures rain in a gauge of 1/1000th of an acre (4.047 sq metres), built in 1852/53. The gauge was constructed of timber with a lead funnel. Rain was collected daily in carboys and weighed to estimate the amount of rain. In 1873 a new gauge was installed and the carboys replaced by galvanized iron calibrated cylinders to measure rainfall. The old gauge was replaced by an identical new one in 1992. For the early history see Lawes et al., 1881.

On 24th November 2023 the rain gauge (ARG100 tipping bucket, installed in 2004) measuring RAINL was replaced with a new rain gauge (TB4MM, Manuf. Hyquest Solutions, NSW, Australia; Supplier Campbell Scientific). The new gauge has funnel diameter of 20cm (8 inch), surface area of 314 cm² as opposed to the ARG100 which had a diameter of 25.4 cm (10 inch) and surface area of 506.71 cm². Based on the large funnel surface area the new calibration is 0.00155 mm / tip.

In 2004 the original calibrated cylinders were replaced by an electronic tipping bucket rain gauge ARG100 (manufactured by Environmental Measurements Ltd (EML), supplied and calibrated by Campbell Scientific), calibrated to tip with every 0.0025mm of rain. In July 2010 the lead lining was stolen and it was replaced by a new stainless steel funnel of grade 316 and dimensions 2213mm x 1829mm in February 2011. No RAINL data was collected for this period.

Since 2004, when the met station was automated, RAINL may have been underestimating rainfall when rain is intense. RAINL should only be used in conjunction with the drainage data, which has the same surface area (DR20, DR40, DR60). For general daily rainfall data please use RAIN. It is recommended that if you use RAINL, RAIN should be used as a check.

"Missing values" There are many instances before 2004 when no data is shown for RAINL (rather than zero). This is because a 'trace' of rain, snow, mist, dew or fog was manually recorded. A 'trace' is less than 0.05mm. For most purposes a missing value can be assumed to be zero.

Total hours of rainfall over 24 hours recorded at 0900GMT

Since Sept 9th 2020, it has been recorded with an SBS500 tipping bucket rain gauge, funnel diameter 25.2cm and calibrated to tip at 0.2mm, manufactured by Environmental Measurements Ltd (EML), supplied and calibrated by Campbell Scientific.

Previously, from 2004, when the met station was automated, it was measured by an electronic tipping bucket rain gauge ARG100 (manufactured by Environmental Measurements Ltd, supplied and calibrated by Campbell Scientific), used to record RAIN. Originally measured by a Negretti and Zamra natural siphon rain recorder. Rain was collected in a float chamber and recorded on a daily chart on a clock drum, which recorded 10mm of rain before siphoning began and the recording restarted at the bottom of the chart. In 1978 this was replaced with a Cassella recorder with a diameter of 20.3cm.

Drainage data (percolation)

Total drainage over 24 hours recorded at 0900GMT

Three drain gauges (DR20, DR40 and DR60) were constructed at Rothamsted in 1870. They consist of undisturbed blocks of soil 20, 40 and 60 inches (51, 102 and 152 cm) deep and are equal in area to the rain gauge of 1/1000th of an acre (RAINL).

The gauges were constructed by digging under and around the block of soil, placing perforated plates underneath at the required depth and bricking up the sides. The soil around the gauges remained undisturbed throughout the construction process.

From 2004 to May 2023 drainage was measured by electronic tipping bucket rain gauge ARG100 (manufactured by Environmental Measurements Ltd (EML), supplied and calibrated by Campbell Scientific). Drain water was originally measured by weighing the carboys of collected water (as for RAINL), but these were replaced by calibrated cylinders. All three drain gauges remain as originally built. The soil has never been deep cultivated or cropped and the top is kept clear by hand weeding.

In May 2023 the ARG100 gauges were replaced by KALYX electronic tipping bucket rain gauges (supplied and calibrated by Campbell Scientific Ltd) funnel diameter 12.7cm (5"). The 20" gauge was replaced on 18/05/2023; the 40" and 60" gauges on 19/05/2023.

Sunshine

Total sun hours over 24 hours, 0000GMT to 2400GMT

Since May 2022 sunshine has been calculated using the Campbell-Stokes equation from solar radiation measured using a CMP3 Kipp and Zonen thermopile pyranometer. From 2004 until April 2022 it was calculated using a CM3 Kipp and Zonen thermopile pyranometer. Originally recorded using a Campbell-Stokes sunshine recorder. The sun's rays are focused onto a card (treated to prevent it from catching fire) and the brown scorch mark on the card is then measured. The cards are of varying lengths applicable to the time of year (winter, equinox, and summer).

Radiation

Total over 24 hours, 0000GMT to 2400GMT

Total solar radiation measured since May 2022 by a CMP3 Kipp and Zonen thermopile pyranometer. From 2004 until April 2022 measured by a CM3 Kipp and Zonen thermopile pyranometer. Recorded since 1921, but the data in e-RA are from 1931, as the early data was considered unreliable. Originally recorded with a Callendar radiation chart recorder. In 1955 this was replaced with a Moll-type solarimeter (Kipp) with a paper chart recording potentiometer. Daily totals were obtained by planimeter integration until 1958 when an automatic integrator was added. A Kipp integrator and recorder was used from 13th November 1975, and a Kipp and Zonen integrator installed in 1989.

All data before 1955 should be treated with caution. There was much uncertainty about the sensitivity of the equipment (Penman 1974). Radiation readings between 1947 and 1954 are around 20% higher than would be expected from sunshine hours, and much data is missing between 1947 and 1949.

Data are recorded as J/cm². To convert to MJ/m² divide by 100 (see Conversion factors).

Cloud

Recorded at 0900GMT

Observed cloud cover on a scale of 0 to 9 Oktas. 0 = the complete absence of cloud. 1 = a cloud amount of 1 eighth or less of the sky, but not zero, etc. 7 = a cloud amount of 7 eighths or more, but not full cloud cover. 8 = full cloud cover with no breaks. 9 = sky obscured by fog or other meteorological phenomena.

Wind

Recorded at 0900GMT

Wind direction (WDIR) has been recorded since 1853 and is measured by a wind vane. It is shown as an angle, going clockwise from North. 360 = North, 90 = East, 180 = South, 270 = West. The reading 0 (or 000) indicates that there is no wind, ie the windspeed is 0 m/s. A WDIR reading of 0 with a windspeed greater than 0 implies that the WDIR is 360 degrees (North).

Since 21/04/2022 WDIR has been measured by an electronic wind vane (Vector Instruments, W200P) installed at 10m above ground level. Between 2004 and April 2022 it was recorded by a W200P wind vane at a height of 12.8m above ground level. The standard height for surface wind measurements over open and level terrain is 10m. However, no correction is needed for wind measured between 8 and 13m (Met Office, 1982). We therefore assumed a mid-point height of 10m. From 2004 WDIR has been calculated as an average over 10 minutes from 0850 to 0900GMT.

From 1978 (WDIR) was measured by a wind vane linked to a Munro roll chart recorder (model IM175). Previously it was estimated from a wind vane with fixed ordinal points.

Recorded at 0900GMT

Wind speed was first estimated using the Beaufort scale, which describes wind force on a scale of 0 (no wind) to 12 (hurricane) based on observations of the movement of smoke, leaves, branches, etc. From 1960 onwards this was replaced by direct measurements of wind speed (WINDSP).

The Beaufort scale can be converted to wind speed: V = 0.836 x SQRT (B³)

where V is the equivalent wind speed (m/s) at 10 metres above ground level and B is Beaufort scale number (see Conversion factors).

Recorded at 0900GMT

Direct measurement of wind speed (WINDSP) by cup anemometer replaced the visual assessment of windspeed (WFORCE) based on the Beaufort scale in 1960.

Since 21/04/2022 WINDSP has been recorded by electronic cup anemometer (Vector Instruments, A100LK/2) installed at a height of 10m above ground level. From 2004 until April 2022 it was recorded by electronic cup anemometer (Vector Instruments, A100LK/2) installed at a height of 12.8m above ground level. The standard height for surface wind measurements over open and level terrain is 10m. However, no correction is needed for wind speeds measured between 8 and 13m (Met Office, 1982). We therefore assumed a mid-point height of 10m.

From November 1977 it was measured by a cup anemometer linked to a Munro roll chart recorder (model IM175). Previously it was measured with a Dines anemometer.

WINDSP and WFORCE are point values, recorded at 0900GMT. From 2004 WINDSP has been calculated as an average over 60 seconds sampled at 0900GMT.

Average daily (24 hour) windspeed can be calculated from total daily WINDRUN, correcting for height (see Conversion factors).

Total run of wind in 24 hours, recorded at 0900GMT

WINDRUN measures the total 'run' (distance) of wind in 24 hours. It is measured by a cup anemometer. Since February 1st 2014 it has been recorded by an electronic cup anemometer (Vector Instruments A100LK) at the standard height of 2m above ground level.

Recorded from February 4th 2004 to January 2014 with an electronic cup anemometer (Vector Instruments, A100LK/2) at 12.8m above ground level, downscaled to the 2m equivalent by multiplying by 0.78 (see Conversion factors)

From 1946 to February 3rd 2004 recorded manually using a cup anemometer with a calibrated meter at the standard height of 2m.

Diary

Recorded at 0900GMT

Manual observations indicating if hail was present during the day, and if so, what type. Code 0-7. 1 = Ice prisms; 2 = Snow grains; 3 = Snow pellets; 4 = Ice pellets; 5 = Hail (5-9mm diameter); 6 = Hail (10-19mm diameter); 7 = Hail (20mm diameter or more). Null or code 0 = No Hail.

Recorded at 0900GMT

Manual observations recording if snow or sleet was present. Code 1 = sleet, code 5 = snow crystals. Code 0 = no snow.

Recorded at 0900GMT

Manual observations recording if thunder was present. Code 1 = thunder during the day, code 0 = no thunder during the day.

Recorded at 0900GMT

Manual observations recording if fog was present. Code 1 = fog present, code 0 = no fog.

Recorded at 0900GMT

Manual observations recording if snow was present. This may have been snow from a previous day. See FSNOWD for depth of freshly fallen snow for the current day. Code 1 = snow present, code 0 = no snow.

Recorded at 0900GMT

Manual recording of total depth of lying snow at the time of observation. This may have been snow from a previous days. See FSNOWD for depth of freshly fallen snow for the current day.

Recorded at 0900GMT

Manual recording of total depth of freshly-fallen snow.

Other selected fields

Recorded at 0900GMT

Since August 7th 2018 measured by a Relative Humidity sensor EE181 (manufactured by E+E Elektronic Corporation, supplied and calibrated by Campbell Scientific). This replaced the MP100A RH Sensor (manufactured by Rotronics, supplied by Campbell Scientific) installed in late 2013. Previously derived from WETB and DRYB (see derived variables).

Recorded at 0900GMT

Atmospheric pressure was measured with a mercury barometer until 2003. For further details, see Met Office (1982) p 103.

Recorded at 0900GMT

A thermometer attached to the instrument casing, known as the attached thermometer, was used to measure the temperature of the mercury column from which the density of the mercury was established. The barometer and attached thermometer were kept indoors. For further details, see Met Office (1982) p 103.

Recorded at 0900GMT

Atmospheric pressure corrected to Mean Sea Level was measured with a mercury barometer. For further details, see Met Office (1982) p 103.

Recorded at 0900GMT

Since January 15th 2014 derived from RELH and DRYB after the method of Buck (1981), calculated within the datalogger. Previously derived from DRYB and WETB (see derived variables).

Recorded at 0900GMT

Manual recording of visibility. Codes 0-9, E, X (1960-2007): CODE X = Dense fog. Visibility range 0-19m; CODE E = Dense fog. Visibility range 20-39m; CODE 0 = Thick fog. Visibility range 40-99m; CODE 1 = Thick fog. Visibility range 100-199m; CODE 2 = Thick fog. Visibility range 200-399m, CODE 3 = Moderate fog. Visibility range 400-999m; CODE 4 = Very poor visibility, range 1000-1999m; CODE 5 = Poor visibility, range 2000-3999m; CODE 6 = Moderate visibility, range 4-9km; CODE 7 = Good visibility, range 10-19km; CODE 8 = Very good visibility, range 20-39km; CODE 9 = Excellent visibility, range 40km or over.

Codes A-M, (1923-1959, 1960-2007 equivalent code in brackets): CODE A (E) = Dense fog. Visibility range 18-22m; CODE B (0) = Thick fog. Visibility range 36-44m; CODE C (1) = Thick fog. Visibility range 90-110m; CODE D (2) = Fog. Visibility range 180-220m; CODE E (3) = Moderate fog. Visibility range 360-440m; CODE F (4) = Very poor visibility. Visibility range 900-1100m; CODE G (5) = Poor visibility. Visibility range 1.8-2.2 km; CODE H (6) = Moderate visibility. Visibility range 3.6-4.4km; CODE I (6) = Moderate visibility. Visibility range 6.3-7.7 km; CODE J (7) = Good visibility. Visibility range 9-11 km; CODE K (8) = Very good visibility. Visibility range 18-22km; CODE L (8) = Very good visibility. Visibility range 27-33km CODE M (9) = Excellent visibility. Visibility range 36-44km.