Quantities and units
Observed and derived quantities
The table below shows, by parameter name, the quantity and units with more details of the quantity itself. The order followed is approximately that of the Level 1 output file. Thus, to look for details of, say, solar radiation, the quantities measured or derived are set out in this section and the details of the solar radiation sensors themselves, their exposure, calibrations, period of record and relevant metadata including photographs and so on appear under the heading for that particular parameter.
Note that the quantities may be differently processed in Level 1 and Level 2 files. For example, Level 1 dry-bulb temperature Td records are available in 1 s and 5 min files, the latter representing a 5 minute average (the average of 300 samples, viz. 5 min x 60 s), whereas in Level 2 the dry-bulb temperature Td records are 1 minute averages (the last 60 x 1 s samples) per World Meteorological Organization specifications. Clearly, if the air temperature is rising or falling quickly, these two quantities may differ significantly, although they are both ‘dry-bulb temperature’ at a given time, and the same applies to other elements as shown in the table below.
Note also that in all cases, the time standard is UTC (GMT), accurate to within a few seconds, and that the time stamp is time ending. So for hourly rainfall totals shown against 1500 UTC, for example, this refers to the total for the period 14:00:01 to 15:00:00 UTC in the Level 2 data.
|
Variable name |
Quantity and units
|
Level 1 data
|
Level 2 data
|
Description and/or derivation
|
|---|---|---|---|---|
|
Td
|
Dry bulb temperature, °C
|
1 s samples,
5 min average (300 points) |
1 min average (60 points)
|
The dry bulb temperature is the temperature as measured in a Stevenson screen at 1.25 m above a short-cropped grass surface.
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|
Tw
|
(Measured) wet bulb temperature, °C
|
The wet bulb temperature is the temperature of a thermometer whose sensing element is wrapped in wet muslin, in thermal equilibrium with saturated air immediately surrounding the wick. The wet bulb temperature is also measured in a Stevenson screen at 1.25 m above a short-cropped grass surface.
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RH
|
Relative humidity, % | The ratio (expressed in %) of the vapour pressure at the dry bulb temperature to saturated vapour pressure at the same temperature. In the observatory we measure the RH using a capacitance-based humidity probe sited inside the Stevenson screen, which provides a direct measurement of relative humidity. | ||
|
Tgrass
|
Grass temperature, °C | The surface temperature as indicated by a thermometer lying on an exposed grass surface. Note that in daylight conditions, this sensor will be subject to warming through receipt of solar radiation and thus not necessarily representative of grass surface temperature. | ||
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Tconc
|
Concrete temperature, °C | The surface temperature as indicated by a thermometer lying on an exposed concrete surface (and thus a proxy for road surface temperatures). Note that in daylight conditions, this sensor will be subject to warming through receipt of solar radiation and thus not necessarily representative of concrete surface temperature. | ||
|
Tsoil
|
Bare soil temperature, °C | The surface temperature as indicated by a thermometer lying on an exposed bare soil surface. Note that in daylight conditions, this sensor will be subject to warming through receipt of solar radiation and thus not necessarily representative of soil surface temperature. | ||
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TSoil5
|
Soil temperatures, °C | Temperatures as measured by thermometer probes placed in the soil at depths of 5, 10, 20, 30, 50 and 100 cm respectively below a grass-covered the surface. | ||
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TSoil10
|
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TSoil20
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TSoil30
|
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TSoil50
|
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Tsoil100
|
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Rain
|
Precipitation, mm | 1 s totals, 5 min total (300 points) |
Totals for each 5 min or 60 min period, respectively | Rainfall amount, measured incrementally using a 0.2 mm tipping bucket gauge during the measurement interval. This gauge is unreliable or inoperative in solid precipitation (snowfall, hailfall). |
|
P
|
Pressure, hPa | 1 s samples, 5 min average (300 points) |
1 min average (60 points) | Station level pressure measured within the Atmospheric Observatory, at 66 m above mean sea level. For mean sea level pressure (MSLP), see Pmsl in derived quantities |
|
CNR4T
|
Temperature of the CNR4 net radiation sensor, °C | 1 s samples, 5 min mean | 10 s mean | Used to correct long-wave radiation measurements, q.v. |
|
T0.56
|
Mast profile temperatures, °C, at indicated height (m) above ground | 1 s samples, 5 min mean | 5 min means as required by lab practicals | 0.56 m above ground |
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T1.12
|
1.12 m above ground | |||
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T2.24
|
2.24 m above ground | |||
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T4.48
|
4.48 m above ground | |||
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U0.56
|
Mast profile wind speeds, m/s, at indicated height (m) above ground | 1 s samples, 5 min mean | 5 min means as required by lab practicals | The wind speed measured using cup anemometers at various heights above a grass-covered surface within the observatory – in this case 0.56 m above ground |
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U0.8
|
0.8 m above ground | |||
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U1.12
|
1.12 m above ground | |||
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U1.6
|
1.6 m above ground | |||
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U2.24
|
2.24 m above ground | |||
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U3.2
|
3.2 m above ground | |||
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U4.48
|
4.48 m above ground | |||
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U6.4
|
6.4 m above ground | |||
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U2
|
Wind speed at 2 m above ground, m/s | 1 s samples, 5 min mean | The wind speed measured using cup anemometers at various heights above a grass-covered surface within the observatory | |
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U5
|
Wind speed at 5 m above ground, m/s | 1 s samples, 5 min mean | 10 minute vector mean wind speed at 5 m, output at 5 min and 60 min intervals | |
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U10
|
Wind speed at 10 m above ground, m/s | 1 s samples, 5 min mean | 10 minute vector mean wind speed at 10 m, output at 5 min and 60 min intervals | |
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Dirn5
|
Wind direction at 5 m above ground, degrees True | 1 s samples, 5 min mean | 10 minute vector mean wind direction at 5 m, output at 5 min and 60 min intervals | The wind direction measured by a potentiometric wind vane at two heights above a grass-covered surface within the observatory, measured as the ‘veer from north’ in degrees True: 090° is east, 180° south, etc. North is 360°, calm is zero. |
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Dirn10
|
Wind direction at 10 m above ground, degrees True | 1 s samples, 5 min mean | 10 minute vector mean wind direction at 10 m, output at 5 min and 60 min intervals | |
|
U
|
North wind component, m/s | 1 s samples, 5 min mean | Not processed | Wind measurements using three propeller anemometers sited adjacent to the Gill windsonic / Licor mast at 3 m height. Instruments arranged orthogonally, one mounted due S, one mounted due W and one mounted vertically Data are valid from 09:30 GMT 12/04/2016. |
|
V
|
East wind component, m/s | |||
|
W
|
Vertical wind component, m/s | |||
|
PG
|
Potential gradient, V/m | 1 s samples, 5 min mean | Not processed | A measure of the magnitude of the vertical atmospheric electric field, standardised to a reference height of 1 m. PG is measured using a field mill, and at Reading a measurement height of 3 m is used. Under fair weather conditions, the PG is positive and steady, typically 120 to 150 V/m. In disturbed weather the PG becomes quite variable. During rainfall, it is usually negative, and during fog it is often more positive than its fair weather value. |
|
G
|
Ground heat flux, W/m2 | 1 s samples, 5 min mean | 10 s mean | The ground heat flux density is the rate at which heat passes vertically from the surface into the soil, per unit area. |
|
iCP
|
Corrugated plate current, pA | 1 s samples, 5 min mean | Not processed | |
|
iFP
|
Flat plate current, pA | 1 s samples, 5 min mean | Not processed | |
|
Point
Discharge |
1 s samples, 5 min mean | Not processed | ||
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Twet_der
|
Wet bulb temperature derived from observed RH and Td (dry bulb temperature) measurements, °C | Derived – n/a | Formula – see description | For dew point > 0 °C: Td and Tw in °C
E1 = 6.112 exp (17.67*Tw / Tw+243.5) E2 = VP + 0.799 (Td – Tw) Iterate until E1 = E2 For dew point =< 0 °C: Td and Tw in °C E1 = 6.112 exp (21.875*Tw / Tw+265.5) E2 = VP + 0.72 (Td – Tw) Iterate until E1 = E2
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|
VP_der
|
Vapour pressure derived from observed RH and Td (dry bulb temperature) measurements, °C | Derived – n/a | Formula – see description | Esrh = 6.112*exp((17.67*Td)/(Td+243.5)) – Td is dry bulb in °C
VP_der = Esrh*RH/100.0 |
|
Tdew_der
|
Dew point derived from vapour pressure calculation, °C | Derived – n/a | Formula – see description | Tdew = 237.3*(log(VP_der/6.11)/17.27) / (1-log(VP_der/6.11)/17.27) |
|
Sdur_accum
_der |
Accumulated sunshine since 0000 UTC, hours | Derived – n/a | Accumulations output for each 5 min or 60 min period in the respective files | The duration of the period during which the nominal direct solar radiation measured by the Kipp & Zonen CSD sensor (q.v.) exceeds a level of 120 W/m2. The instrument is polled every second, and so this quantity is simply the total number of seconds in any period when the nominal intensity exceeds this level, converted to hours by dividing by 3600. |
|
Sg_accum
_der |
Accumulated global solar radiation since 0000 UTC, MJ/m2 | Derived – n/a | The global solar flux density measured by a horizontal pyranometer at 1.2 m above the surface. The instrument responds to short wavelengths (approximately 300-2800 nm), and the measurement necessarily includes variable direct and diffuse solar radiation components. | |
|
Rain_accum
_der |
Accumulated precipitation since 0000 UTC, mm | Derived – n/a | ||
|
U2run_der
|
Run of wind at 2 m since 0000 UTC, km | Derived – n/a | Saved every 5 min or 60 min period in the respective files | This quantity can be thought of as the integral of wind speed with time, or the total flow of air passed by the anemometers at the specific height above ground level. For example – a steady mean wind of 5 m/s over 24 hours would accumulate 5 x 3600 x 24 = 432 000 m or 432 km passage of air; similarly 10 km/h over 24 hours would accumulate 10 x 24 = 240 km wind run. It is usually expressed in km. The mean speed in m/s can be obtained by dividing the km wind run by 86.4 (3600 sec x 24 hours, multiplied by 1000 to convert km/s to m/s). This is a scalar quantity – for a vector quantity, use the vector mean wind which has both direction and velocity components. |
|
U5run_der
|
Run of wind at 5 m since 0000 UTC, km | Derived – n/a | ||
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U10run_der
|
Run of wind at 10 m since 0000 UTC, km | Derived – n/a | ||
|
U5max_der
|
Maximum wind gust at 5 m in each 5 min period, m/s | Derived – n/a | Output every 5 min | The ‘maximum wind gust’ is defined by WMO as ‘the highest 3 s running mean wind speed’ and this is how it is processed from the 1 s samples |
|
U5max_accum
_der |
Maximum wind gust at 5 m since 0000 UTC, m/s | Derived – n/a | Output every 5 min | |
|
U10max_der
|
Maximum wind gust at 10 m in each 5 min period, m/s | Derived – n/a | Output every 5 min | |
|
U10max_accum
_der |
Maximum wind gust at 10 m since 0000 UTC, m/s | Derived – n/a | Output every 5 min | |
|
Pmsl
|
Mean sea level pressure, hPa | Derived – n/a | Formula | Computed using station level pressure P hPa and dry bulb Td °C(or the monthly climatological mean air temperature is the dry bulb is not available). Altitude h assumes 66 m above mean sea level.
m = h/(18429.1+(67.53*Td)+(0.003*h)) mslcorr = P * ((10**m)-1) Pmsl = P + mslcorr
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This page updated January 2018