DESCRIPTION

r.sun.angle calculates sun elevation and sun azimuth angles for the given time of day and each grid cell in the current region. Additionally, the photoperiod (sunshine hours) can be calculated.

Sun elevation, height, height angle, or solar altitude angle is the angle in degrees between the horizon and a line that points from the site towards the centre of the sun.

The sun azimuth angle is here defined as the azimuth angle in degrees of the sun from due north in a clockwise direction.

The time used here is defined such that 12:00 (high noon) is the time when the sun has reached its highest point in the sky at the current site, unless the -t flag is used in which case time is interpreted as Greenwich standard time.

If a sunhour output map is specified, the module calculates sunshine hours for the given day. This option requires both Greenwhich standard time (-t flag) and the solpos algorithm of NREL (-s flag).

EXAMPLES

Calculate sun's elevation angle map for 2010-10-11 at 14:00h solar time:
r.sun.angle elevation=sun_elev year=2010 month=10 day=11 hour=14 minute=00

Calculate photoperiod of day of year 1 (1st January) 2012

r.sun.angle -s sunhour=photoperiod_doy_001 year=2012 day=1

AUTHOR

Markus Metz

Last changed: $Date$