Hi All
I have a question regarding solar radiation measurements, I am
building a DIY weather station, and one of the things i wish to
record is Solar radiation and Sunshine Hours. I am useing a
photodiode to measure Global (Total) radiation and have done some
initial calibration against a photographic light meter,and i am
curently measuring daily maximum levels of around 500w/m2 (in NE
england) which agrees with my light meter (EV 13.5) however, i have a
local weather station with a web site, and this suggests the the
maximum levels are arround the 900w/m2 level. I have looked into
calculating the 'clear sky radiation' and have writen a program to
calculate this. These calculations sugest a maximum for this location
for noon today of 830w/m2, i have seen some web sites quoting almost
1280w/m2? Considering i am useing a solar constant of 1353w/m2 how
can these figures be correct? or is it me , not understanding the
figures corectly?

Any help would be appreciated
Graham

On 08/05/2012 21:50, Graham wrote:
Hi All
I have a question regarding solar radiation measurements, I am
building a DIY weather station, and one of the things i wish to
record is Solar radiation and Sunshine Hours. I am useing a
photodiode to measure Global (Total) radiation and have done some
initial calibration against a photographic light meter,and i am
curently measuring daily maximum levels of around 500w/m2 (in NE
england) which agrees with my light meter (EV 13.5) however, i have a
local weather station with a web site, and this suggests the the
maximum levels are arround the 900w/m2 level. I have looked into

That sounds about right to me if the detector is always facing the sun
square on and remains perpendicular to the incident light. Pointing
vertically upwards or using a diffuser you have to compensate for the
cos of the angle between the detector and the sun.

calculating the 'clear sky radiation' and have writen a program to
calculate this. These calculations sugest a maximum for this location
for noon today of 830w/m2, i have seen some web sites quoting almost
1280w/m2? Considering i am useing a solar constant of 1353w/m2 how
can these figures be correct? or is it me , not understanding the
figures corectly?

I'd be surprised if we ever got above 1000W/m^2 peak insolation in the
UK at ground level. It probably can happen with the right combination of
an unobscured sun and the right sorts of clouds elsewhere.

Back of the envelope numbers for the UK are 800W/m^2 direct solar flux
at normal incidence and upto 200W/m^2 diffuse radiation.

So 1kW/m^2 is a good engineering approximation to peak flux if you are
tracking the sun at normal incidence.

--
Regards,
Martin Brown

Martin Brown wrote:



I'd be surprised if we ever got above 1000W/m^2 peak insolation in the
UK at ground level. It probably can happen with the right combination of
an unobscured sun and the right sorts of clouds elsewhere.

Back of the envelope numbers for the UK are 800W/m^2 direct solar flux
at normal incidence and upto 200W/m^2 diffuse radiation.

So 1kW/m^2 is a good engineering approximation to peak flux if you are
tracking the sun at normal incidence.

Hi Martin
I am measuring the Global radiation in the horizontal plane with a diffuser,
The reason i was querieing the figures of 1280W/m2 is that they seem to be
very high considering that the station is a Vantage Pro2 which also measures
global radiation in the horizontal plane.
Is it possible that the vantage weather stations apply a 'cosine correction'
to its global figures?

Even my two local weather stations (both vantage) give reading close to my
calculated clear sky values

My calculations are as follows
54.68N 1.17W
9/5/2012 10:21
Zenith = 48.13
Altitude = 41.87


Air mass 1.4578070793017819
Rayleigh 0.8840397820849082
Ozone 0.9783478706011384
mixed gasses 0.9860899862234945
water vapour 0.8872604163686801
tubidity 0.08813662821376192
Direct Normal 855.6833892378154
Direct Horizontal 572.5643950674784

Direct 573
Diff 108
Total 681

Local stations 746 & 766 W/m2
My readings 390W/m2
weather - Sunny intervals 6/8 cloud

I just want to confirm that my figures are approximatly correct or do i need
a re-think?

Graham

On 09/05/2012 10:35, Graham wrote:
Martin Brown wrote:



I'd be surprised if we ever got above 1000W/m^2 peak insolation in the
UK at ground level. It probably can happen with the right combination of
an unobscured sun and the right sorts of clouds elsewhere.

Back of the envelope numbers for the UK are 800W/m^2 direct solar flux
at normal incidence and upto 200W/m^2 diffuse radiation.

So 1kW/m^2 is a good engineering approximation to peak flux if you are
tracking the sun at normal incidence.

Hi Martin
I am measuring the Global radiation in the horizontal plane with a diffuser,
The reason i was querieing the figures of 1280W/m2 is that they seem to be
very high considering that the station is a Vantage Pro2 which also measures
global radiation in the horizontal plane.

Yes. That sounds more like an estimate of the solar flux *above* the
atmosphere.

Is it possible that the vantage weather stations apply a 'cosine correction'
to its global figures?

If it knows the time of day and station latitude then I would expect it
to use that info to compute effective solar flux at normal incidence.

Even my two local weather stations (both vantage) give reading close to my
calculated clear sky values

My calculations are as follows
54.68N 1.17W
9/5/2012 10:21
Zenith = 48.13
Altitude = 41.87


Air mass 1.4578070793017819
Rayleigh 0.8840397820849082
Ozone 0.9783478706011384
mixed gasses 0.9860899862234945
water vapour 0.8872604163686801
tubidity 0.08813662821376192
Direct Normal 855.6833892378154
Direct Horizontal 572.5643950674784

Direct 573
Diff 108
Total 681

Local stations 746& 766 W/m2
My readings 390W/m2
weather - Sunny intervals 6/8 cloud

I just want to confirm that my figures are approximatly correct or do i need
a re-think?

I think your measured figures at a shade under 800W/m^2 are in the right
ball park. I'd be suspicious of anything above 1kW/m^2.

If you can tweak your detector so it also measures the normal incidence
component near to noon on a nice sunny blue sky day. If you get
something like 950 measured then you are quids in.

Some charlatans selling solar panels claim 1kW/m^2 solar flux...

--
Regards,
Martin Brown

Martin Brown wrote:


If you can tweak your detector so it also measures the normal incidence
component near to noon on a nice sunny blue sky day. If you get
something like 950 measured then you are quids in.

I have been looking on the net this afternoon, and found a document on
'Derived Variables used in Davis weather instruments' This calls the Solar
radiation figure, 'Net Radiation', that is the measured visible spectrum
from the sensor PLUS a calculated amount for the Thermal part of the
spectrum.

Not shure if this would account for all of the aditional w/m2 but it makes
more sense now, and you guys have restored my faith in my calculations, just
need a nice clear day to test out the theory....

Might even try the thermometer idea...

Thanks
Graham

"Martin Brown" wrote in message
...
That sounds about right to me if the detector is always facing the sun
square on and remains perpendicular to the incident light. Pointing
vertically upwards or using a diffuser you have to compensate for the
cos of the angle between the detector and the sun.

Incidentally, how does one allow for the increased absorption when the sun is
lower in the sky and the radiation path through the atmosphere is longer?

I am specifically interested in knowing how the radiation on a vertical surface
(eg a window) varies with solar altitude.
Is there a (perhaps empirical) formula for this?

"Gavino" wrote:

"Martin Brown" wrote in message
...
That sounds about right to me if the detector is always facing the sun
square on and remains perpendicular to the incident light. Pointing
vertically upwards or using a diffuser you have to compensate for the
cos of the angle between the detector and the sun.

Incidentally, how does one allow for the increased absorption when the sun is
lower in the sky and the radiation path through the atmosphere is longer?

I am specifically interested in knowing how the radiation on a vertical surface
(eg a window) varies with solar altitude.
Is there a (perhaps empirical) formula for this?



i = I cos (Altitude)

where
i = incident intensity on the window
I = intesity of radiation square to the Sun

On 10/05/2012 01:58, Sleepalot wrote:
wrote:

"Martin wrote in message
...
That sounds about right to me if the detector is always facing the sun
square on and remains perpendicular to the incident light. Pointing
vertically upwards or using a diffuser you have to compensate for the
cos of the angle between the detector and the sun.

Incidentally, how does one allow for the increased absorption when the sun is
lower in the sky and the radiation path through the atmosphere is longer?

I am specifically interested in knowing how the radiation on a vertical surface
(eg a window) varies with solar altitude.
Is there a (perhaps empirical) formula for this?

i = I cos (Altitude)

where
i = incident intensity on the window
I = intesity of radiation square to the Sun

And what you have called "Altitude" is the angle between the normal to
the window surface and the direction of the sun.

--
Regards,
Martin Brown

"Martin Brown" wrote in message
...
On 10/05/2012 01:58, Sleepalot wrote:
wrote:
Incidentally, how does one allow for the increased absorption when the sun is
lower in the sky and the radiation path through the atmosphere is longer?

I am specifically interested in knowing how the radiation on a vertical surface
(eg a window) varies with solar altitude.
Is there a (perhaps empirical) formula for this?

i = I cos (Altitude)

where
i = incident intensity on the window
I = intesity of radiation square to the Sun

And what you have called "Altitude" is the angle between the normal to
the window surface and the direction of the sun.

Yes, that's right, I'd already worked all that out for myself.
But what I am asking for is the correction to 'I' for atmospheric absorption.
The lower the solar altitude, the further the rays have to travel through the
atmosphere to reach the surface, hence intensity there is reduced.

A naive analysis using only cos(altitude) would give maximum intensity
(for a vertical surface facing the sun) when the sun is on the horizon.
Everyday experience shows this is false.

Martin Brown wrote:

I'd be surprised if we ever got above 1000W/m^2 peak insolation in the
UK at ground level. It probably can happen with the right combination of
an unobscured sun and the right sorts of clouds elsewhere.

1000W/m^2 equates to a blackbody temp of about 92C

A record ground temperature of 70C was recorded in Death Valley
(which isn't black).

60C is (too) painful to touch and 70C denatures protein (ie. fries eggs).

I wonder what temperature roads start melting at.