Derived Meterological Variables

TRANGE = TMAX - TMIN : Range (°C)

AVTEMP = (TMAX+TMIN)/2 : Average temperature (°C)

WINDRUN is usually measured so this is only to be used if the data is missing.

WINDRUN = WINDSP * 86.4 (this is conversion from m/s to km/24 hours).

At Rothamsted these are calculated within the datalogger after Buck (1981) and Allen et al (2006).

At Woburn these are have been calculated within the datalogger since the station was automated in 1999 as described by Campbell Scientific Technical Note 16 (2005), using formulae based on studies by Lowe (1977). Before automation, the assumption is that the equations of Buck (1981) werre used alongside those of the Met Office (1964) and Allen et al (2006), as shown below for Rothamsted.

At Rothamsted this is calculated after Buck (1981):

DEWP= 240.97*LOG_n (VAP/6.1375)/(17.502-LOG_n(VAP/6.1375))

At Woburn, DEWP has been calculated within the datalogger from DRYB and WETB and RELH after Lowe (1977).

Cos, sin and tan have the usual trigonometric meanings

Sqrt the square root function

nday_val is the day number (Julian date) of the record in question e.g. 1st Feb. = 32

days_in_year ordinarily is 365, but 366 in a leap year

stn_latitude is the latitude of the station in question: Rothamsted = 51.81°N

Woburn = 52.017 °N Brooms Barn = 52.267 °N.

hrday is the maximum amount of sunshine in hours, that a latitude can receive. (i.e. cloudless all day)

sunfr is the sun fraction, the ratio of recorded hours of sun to the maximum possible

HMM is the evaporation term from net radiation over grass

EA_GRASS is the evaporation term from humidity differences over grass

EA_WATER is the evaporation term from humidity differences over water

angnd = (6.28318 *(nday_val - days_in_year + 193))/days_in_year

csd = cos(angnd)

snd = sin(angnd)

cs2d = (csd + snd)*(csd - snd)

sn2d = 2*csd*snd

sndecl = 0.00678 + (0.39762*csd)+(0.00613*snd)-(0.00661*cs2d)-(0.00159*sn2d)

csdecl = sqrt(1 - sndecl*sndecl)

csl = cos((stn_latitude*3.14159)/180)

snl = sin((stn_latitude*3.14159)/180)

cshass = (-0.014544 - (snl*sndecl))/(csl*csdecl)

snhass = sqrt(1 - cshass*cshass)

hass = atan(snhass/cshass) if hass < 0 then hass = hass + 3.14159

hrday = hass*24/3.14159

sunfr = SUN/ hrday

Exp is the exponential function (e^x)

** is the exponentiation function (xⁿ)

d0g is a correction factor for grass: 0.75

d1g is a correction factor for grass: 1

d0w is a correction factor for water: 0.95

d1w is a correction factor for water: 0.5

c1 is a constant: 4.0621 * 10^-7

c2 is a constant: 3.721432778 x 10⁷

The relative humidity (RELH) expresses the degree of saturation of the air as a ratio of the actual (VAP) to the saturation (Es) Vapour pressure at the same temperature (from Allen et al, 2006)

Es = 6.1078 * exp((17.269 * AVTEMP) / avt) (Es = saturated vapour pressure at Avtemp) Note that these values are not exactly the same as for SVAP.

avt = AVTEMP + 237.3

Es = 6.1078 * exp((17.269 * AVTEMP) / avt) (Es = saturated vapour pressure at Avtemp)

delta = (4097.93 * Es) / (avt * avt)

sunfr = SUN/ hrday

fnt2 = (0.0048985 * (AVTEMP + 273.0) ** 4) *(0.47- (0.065 * sqrt(VAP))) * (0.17 + 0.83 * sunfr)

ev1 = c1 * delta

PSMD = PSMD + EVAPG - RAIN (Not negative)

Where PSMD is the accumulated SMD so far.

PSMD is an accumulated value, starting at the value for soil moisture deficit for start of range.

This measures the loss of moisture in the soil; and while the daily value may be significant it is usually calculated over some months at least. Technically it is an accumulation, and into the summer will usually show a net loss. The value can never be negative, as if precipitation exceeds evaporation together with any deficit to date, this forms runoff and contributes to surface water flow.