APPENDIX A: ESTIMATING THE INCIDENT SOLAR RADIATION USING
CLOUD COVER DATA
Since the solar radiation was not measured at the test sites in Montpelier and
White River, we estimated the incident solar radiation on our test plots. The proce-
dure for this is described in Ashton (1985, 1986). We outline that procedure here
and add to it our procedure for incorporating hourly cloud cover information in
The solar radiation incident to the earth's atmosphere is called the solar con-
stant, Io, and is taken to be 1380 W/m2. The proportion of Io that strikes the earth's
Io sin2 α
(sin α + 2.7)ea 103 + 1.085 sin α + 0.10
where ea is the vapor pressure of water in the atmosphere, and
sin α = sin L sin δ + cos L cos δ cos H.
In eq A2, H is the hour angle of the sun that is computed from
H (hour 12)15.
δ = 23.45 cos
where D is the Julian day. The vapor pressure ea is
ea = esatRH
where esat is the water vapor saturation pressure at the given air temperature, Ta,
and RH is the relative humidity. The saturation pressure can be found from
Ta + 237.3 .
esat = 6.11e
The incident solar radiation with a cloud cover, φs, is
φs = φso,c (1 0.6C)
were C is the percent cloud cover. The constant 0.6 is empirically determined and
may be adjusted to make this expression better fit local conditions. However, for
our sites we found that 0.6 gave good agreement with measured solar radiation.
The cloud cover, as reported by the BarreMontpelier Airport and Lebanon Re-
gional Airport, is reported as falling into one of seven categories shown in Table
A1. For each category we assigned a value for C as shown in Table A1. The excep-
tion to this is for missing data; in that case the cloud cover was interpolated from
the data before and after the missing point(s).
The daily solar radiation values were then determined by summing the hourly
estimates from eq A7 and Table A1. These results are plotted in Figures 12 and 15.
This procedure was checked by comparing actual solar radiation measurements
made at the CRREL meteorological test cell in Hanover, with the solar radiation