[GRASS-dev] Re: [GRASS-user] Clarification on units used in r.sun

Glynn Clements glynn at gclements.plus.com
Wed Nov 29 15:31:09 EST 2006

Dylan Beaudette wrote:

> Quick question on the units used for the ouput of r.sun in mode 2 (daily 
> sums):
> In the manual pages for r.sun, the following 'unit' is included in the 
> description:
> ----------------------------
> The solar radiation maps for given day are computed integrating the relevant 
> irradiance between sunrise and sunset times for given day. The user can set 
> finer or coarser time step step used for all-day radiation calculations. A 
> default value of step is 0.5 hour. Larger steps (e.g. 1.0-2.0) can speed-up 
> calculations but produce less reliable results. The output units are in Wh 
> per squared meter per given day [Wh/(m*m)/day]. 
> -------------------------
> Is one to interpret this as "watt-hour per square meter per day" ? 
> This would seem a little odd, as the the unit 'watt-hour' is not an SI unit. 

More odd (to me) is having (different) units of time in both the
numerator and denominator. I would have thought it more logical to
divide the result by 24 to give Watts per square metre [W/(m^2)].

Glynn Clements <glynn at gclements.plus.com>

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