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

[Jonathan Greenberg]

Oddly enough, it is technically impossible to directly measure W/m2, the
instrument MUST have some sort of integrating time.  Someone please correct
me, but I think if you want to convert to energy (assuming the insolation is
constant over the hour, which the average basically assumes), you simply
multiply the W/m2 by the integrating time, so if your instrument shows an
average reading of 2.3W/m2 for 10am, you can say that from 9am to 10am the
energy/m2 is 2.3Wh/m2 (or 2.3Wh/m2/1 hour).  

--j

 
--

Jonathan A. Greenberg, PhD
NASA Postdoctoral Researcher
NASA Ames Research Center
MS 242-4
Moffett Field, CA 94035-1000
Phone: 415-794-5043
AIM: jgrn3007
MSN: jgrn3007 at hotmail.com
-----Original Message-----
From: Dylan Beaudette [mailto:dylan.beaudette at gmail.com] 
Sent: Thursday, November 30, 2006 2:57 PM
To: grassuser at grass.itc.it
Cc: Jonathan Greenberg
Subject: Re: [GRASS-user] Clarification on units used in r.sun

Thanks for the clarification Jonathan. 

I was aware of the Joule - Watt relationship, but was not quite sure why
r.sun 
was using W*h instead of Joules. Since a Mega-Joule is a much more readily 
used unit of enery, I think that I will do the conversion from now on.

The next question, pardon my ignorance, is: how to interpret hourly averaged

insolation (beam + diffuse) values from a weather station, which are
reported 
in W/(m^2) ?

Is this essentially this same thing, as it is "integrated" over an hour when

summed for the entire day?

Cheers,

Dylan 

On Thursday 30 November 2006 13:42, Jonathan Greenberg wrote:
> 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.

-- 
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341