Calculate zenith angle as in http://education.gsfc.nasa.gov/experimental/July61999siteupdate/inv99Project.Site/Pages/solar.insolation.html

Usage

calc_zenith_angle(
  latitude,
  declination.angle,
  hour.angle,
  format = c("degrees", "radians")
)

Arguments

latitude: numeric value or vector indicating the site latitude in decimal degrees (never radians or deg-min-sec, no matter what format is) between -90 (South Pole) and 90 (North Pole).
declination.angle: numeric value or vector, in the units specified by format, indicating the declination angle.
hour.angle: numeric value or vector, in the units specified by format, indicating the angle.
format: The format of both the output. May be "degrees" or "radians".

Examples

zendf <- data.frame(
  lat=rep(c(0,20,40,60), each=24*4),
  jday=rep(rep(c(1,101,201,301), each=24), times=4),
  hour=rep(c(0:12,13.5:23.5), times=4*4))
zendf <- transform(zendf,
  dec=streamMetabolizer:::calc_declination_angle(jday),
  hragl=streamMetabolizer:::calc_hour_angle(hour))
zendf <- transform(zendf,
  zen=streamMetabolizer:::calc_zenith_angle(lat, dec, hragl))
if (FALSE) {
library(ggplot2)
ggplot(unitted::v(zendf), aes(x=hour, y=zen, color=jday, group=jday)) +
  geom_line() + facet_wrap(~lat) +
  ggtitle('zenith angles by latitude (panels) and day of year (colors)')
}