Garden with Insight v1.0 Help: Weather  Relative humidity
The relative humidity model simulates daily average relative humidity from the monthly average by using
a triangular distribution. As with temperature and radiation, the mean daily relative humidity is adjusted
to account for wet and dryday effects. The assumed relation betweenrelative humidity on wet and dry
days is [Equation 112] where RHW is the daily mean relative humidity on wet days for month k, RHD is
the daily mean relative humidity on dry days, and omega(H) is a scaling factor ranging from 0.0 to 1.0.
An omega(H) value of 0.9 seems appropriate for many locations.
Equation 112
RHW = RHD + omega(H) * (1.0  RHD)
Code:
omega(H) = 0.9
RHW = RHD + 0.9 * (1.0  RHD)
RHW = RHD + 0.9  0.9 * RHD
RHW = 0.1 * RHD + 0.9
Variables:
RHW = DailyMeanRelHumWetDaysForMonth_frn
RHD = dailyMeanRelHumDryDaysForMonth_frn
omega(H) = relHumResponseToWetDays_frn = 0.9
Using the continuity equation as described in the temperature and radiation sections produces the equation
[Equation 113] where RH is the longterm average relative humidity for month k.
Equation 113
RHD = RH  (omega(H) * (NWD/ND)) / (1.0  omega(H) * (NWD/ND))
Code:
same except for bounds check: if RHD < 0.05 RHD = 0.5 * RH
omega(H) = 0.9
Variables:
RHD = DailyMeanRelHumDryDaysForMonth_frn
RH = dailyMeanRelHumForMonth_frn
omega(H) = relHumResponseToWetDays_frn = 0.9
NWD = numWetDaysForMonth
ND = numDaysInMonth
The appropriate value (RHW or RHD) is used as the peak of a triangular distribution to generate daily
relative humidity. The upper limit of the triangular distribution is set with the equation [Equation 114]
where RHU is the largest relative humidity value that can be generated on day I and RHP is the peak of
the triangular distribution (RHW or RHD).
Equation 114
RHU = RHP + (1.0  RHP) * exp(RHP  1.0)
Code:
same (rearranged)
Variables:
RHU = RelHumTriangularDistUpperLimit_frn
RHP = peakOfRelHumTriangularDistr_frn
The lower limit is set with the equation [Equation 115] where RHL is the lowest relative humidity value
that can be generated on day i.
Equation 115
RHL = RHP * (1.0  exp(RHP))
Code:
same
Variables:
RHL = RelHumTriangularDistLowerLimit_frn
RHP = peakOfRelHumTriangularDist_frn
To assure that the simulated longterm value for mean relative humidity agrees with input RH, the
generated value is adjusted by using the equation [Equation 116] where RHG* is the generated relative
humidity on day i adjusted to the mean of the triangle, RHG is the relative humidity generated from the
triangle, and RH is the mean of the triangle.
(in other code)
If relative humidity data is not available, dew point temperature may be substituted and EPIC estimates
the monthly mean relative humidity using equations 52 and 53. If dew point temperature is not available
EPIC estimates monthly mean relative humidity using the equation [Equation 117] where deltat = T(mx)
 T(mn) for month k.
Equation 117
RH = 0.9  (0.8 * deltat) / (deltat + exp(5.12  0.127 * deltat))
Code:
same
Variables:
RH = DailyMeanRelHumForMonthFromTempOnly_frn
deltat = maxMinusMinTemp_degC
T(mx) = dailyMeanMaxTempForMonth_degC
T(mn) = dailyMeanMinTempForMonth_degC
