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

[Jonathan Greenberg]

A watt hour is a real unit, although not an SI one, its a unit of energy (as
opposed to a watt which is power).  Watt-hours are used in your electric
bills.  The key SI conversion is 1 watt-second (1/3600 of a Wh) = 1 Joule.
It's an integral of W/m2 over time (not a rate).  A common conversion (if
you are doing Eto calculations, for instance) is to MJ/m2/day, which will be
the r.sun output (mode 2) * 3600/1000000

--j


On 11/30/06 1:54 PM, "Dylan Beaudette" <dylan.beaudette at gmail.com> wrote:

> On Wednesday 29 November 2006 12:31, Glynn Clements wrote:
>> 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)].
> 
> I need to check with some local experts, but judging from some recent tests -
> the output from r.sun is comparible to that from a weather station, with
> *hourly* averaged data in W/(m^2) which i think is analogous to the Wh/(m^2)
> units that r.sun uses.


-- 
Jonathan A. Greenberg, PhD
NRC Research Associate
NASA Ames Research Center
MS 242-4
Moffett Field, CA 94035-1000
Office: 650-604-5896
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AIM: jgrn307
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