r.sunhours - GRASS manual
Author: GRASS Development Team
Description: r.sunhours: Solar elevation: the angle between the direction of the geometric center of the sun's apparent disk and the (idealized) horizon. Solar azimuth: the angle from due north in clockwise direction.
Keywords: raster, solar, sun energy, sun position

## NAME

r.sunhours - Calculates solar elevation, solar azimuth, and sun hours.
Solar elevation: the angle between the direction of the geometric center of the sun's apparent disk and the (idealized) horizon. Solar azimuth: the angle from due north in clockwise direction.

## KEYWORDS

raster, solar, sun energy, sun position

## SYNOPSIS

r.sunhours
r.sunhours --help
r.sunhours [-ts] [elevation=name] [azimuth=name] [sunhour=name] year=integer [month=integer] day=integer [hour=integer] [minute=integer] [second=integer] [--overwrite] [--help] [--verbose] [--quiet] [--ui]

### Flags:

-t
  Time is local sidereal time, not Greenwich standard time
-s
  Do not use SOLPOS algorithm of NREL
--overwrite
  Allow output files to overwrite existing files
--help
  Print usage summary
--verbose
  Verbose module output
--quiet
  Quiet module output
--ui
  Force launching GUI dialog

### Parameters:

elevation=name
  Output raster map with solar elevation angle
  Name for output raster map
azimuth=name
  Output raster map with solar azimuth angle
  Name for output raster map
sunhour=name
  Output raster map with sunshine hours
  Sunshine hours require SOLPOS use and Greenwich standard time
year=integer [required]
  Year
  Options: 1950-2050
month=integer
  Month
  If not given, day is interpreted as day of the year
  Options: 1-12
day=integer [required]
  Day
  Options: 1-366
hour=integer
  Hour
  Options: 0-24
  Default: 12
minute=integer
  Minutes
  Options: 0-60
  Default: 0
second=integer
  Seconds
  Options: 0-60
  Default: 0

## DESCRIPTION

r.sunhours 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 on flat terrain) 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 and the use of the SOLPOS algorithm by NREL.

## NOTES

To consider also cast shadow effects of the terrain, r.sun has to be used.

## EXAMPLES

### Calculating a sun elevation angle map

Calculate the sun elevation angle map for 2010-10-11 at 14:00h solar time:

    # set computational region to North Carolina state extent
    g.region n=318500 s=-16000 w=124000 e=963000 res=500 -p
    r.sunhours elevation=sun_elev year=2010 month=10 day=11 hour=14 minute=00

    # visualize
    d.mon wx0
    d.rast sun_elev
    d.vect nc_state type=boundary
    d.legend sun_elev -s

Sun angle map (in degree) of North Carolina for the 2010-10-11 at 14:00h solar time

### Calculate map of photoperiod (insolation time)

Calculate photoperiod of day-of-year 001 (1st January) of 2012 for the current computational region, ignoring cast shadow effects of the terrain:

    g.region -p
    r.sunhours sunhour=photoperiod_doy_001 year=2012 day=1

## Acknowledgements

Acknowledgements: National Renewable Energy Laboratory for their SOLPOS 2.0 sun position algorithm.

## SEE ALSO

g.region, r.sun, r.sunmask

## AUTHOR

Markus Metz

## SOURCE CODE

Available at: r.sunhours source code (history)

Latest change: Friday Feb 07 19:16:09 2025 in commit: a82a39f673b97fafcfce1604dc3e6015cb96487a

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© 2003-2026 GRASS Development Team, GRASS 8.5.1dev Reference Manual