## table of contents

r.sunhours(1grass) | GRASS GIS User's Manual | r.sunhours(1grass) |

# 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:¶

## 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)

Accessed: Saturday Jul 27 17:08:23 2024

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