# R/daylength.R In geosphere: Spherical Trigonometry

#### Documented in daylength

# Author: Robert J. Hijmans, [email protected]
# Version 0.1  January 2009

daylength <- function(lat, doy) {

if (class(doy) == 'Date' | class(doy) == 'character') {
doy <- as.character(doy)
doy <- as.numeric(format(as.Date(doy), "%j"))
} else {
doy <- (doy-1) %% 365 + 1
}

lat[lat > 90 | lat < -90] <- NA

#Forsythe, William C., Edward J. Rykiel Jr., Randal S. Stahl, Hsin-i Wu and Robert M. Schoolfield, 1995.
#A model comparison for daylength as a function of latitude and day of the year. Ecological Modeling 80:87-95.
P <- asin(0.39795 * cos(0.2163108 + 2 * atan(0.9671396 * tan(0.00860*(doy-186)))))
a <-  (sin(0.8333 * pi/180) + sin(lat * pi/180) * sin(P)) / (cos(lat * pi/180) * cos(P))
a <- pmin(pmax(a, -1), 1)
DL <- 24 - (24/pi) * acos(a)
return(DL)
}

.daylength2 <- function(lat, doy) {
if (class(doy) == 'Date' | class(doy) == 'character') {
doy <- as.character(doy)
doy <- as.numeric(format(as.Date(doy), "%j"))
} else {
doy <- (doy-1) %% 365 + 1
}

lat[lat > 90 | lat < -90] <- NA

doy <- (doy-1) %% 365 + 1

# after Goudriaan and Van Laar
#  Sine and cosine of latitude (LAT)
# Maximal sine of declination;}
#{Sine and cosine of declination (Eqns 3.4, 3.5);}
SINDEC <- -SINDCM * cos(2*pi*(doy+10)/365)
COSDEC <- sqrt(1-SINDEC*SINDEC);
#The terms A and B according to Eqn 3.3;}
A <- SINLAT*SINDEC;
B <- COSLAT*COSDEC;
C <- A/B;
#Daylength according to Eqn 3.6; arcsin(c) = arctan(c/sqrt(c*c+1))}
DAYL <- 12* (1+(2/pi)* atan(C/sqrt(C*C+1)))
return(DAYL)
}

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geosphere documentation built on May 2, 2019, 5:16 p.m.