Question about Davis Solar Sensor output and Suns angle

Can anyone answer this question for me - My Solar Sensor is mounted in the open and totally unobstructed … It’s sunny all day
so should the sensor read 100 percent no matter what the suns angle is … that is should it read 100 percent morning, noon and early
evening or should it fluctuate. I’m located at 49N Lat… sun at 11am is at about 80 degs solar reading is 82%. I’m wondering if there
might be a problem with the sensor since I think it should read 100 percent or is it normal to have the reading fluctuate with the angle
of the sun. During the late evening when the suns angle is approx 25 degs the reading drops below the threshold of 75 or 80 percent
set in the setup and so WD stops recording the hours even though it’s still sunny. I think I may have a sensor, which is only 8 months old,
that is problematic but I’m not sure… can anyone help me with this pleeese … thanks

I don’t know the details of the Davis sensor, but it will only report 100% when the light level is 100% of some supposed maximum. If the sensor comes from the factory pre-calibrated for your latitude, then you would expect it to read 100% on a bright sunny day at solar noon on June 21st. At all other times, it will read less than 100%. If calibration is do-it-yourself, then you would have to adjust it so that you get that reading. Since you’ve missed the summer equinox for this year, you will have to do some math to determine what the maximum would be at solar noon on the day you do the calibration.

Thanks for that Keith - I don’t know if the Davis sensor comes precalibrated for latitude or not… at least I’ve never heard of that. I do know that Davis states that calibration may begin to fall off after being in service a number of years and recommends sending it back to
them for recalibration. My sensor has read 100 percent on occasion, even within the past few days…but not often. I see that your
on Denman and I’m in Nanaimo 49N Lat and its sunny at high noon - reads 84 percent… I don’t think their is any way the Davis sensor
can be calibrated by the individual… Maybe I can get more answers from somebody specific to the Davis Sensor…

In that case, the sensor’s “100%” must be 100% of a theoretical maximum at some “standard” latitude. You would expect it to be the equator, in which case the highest you ought to see at noon on June 21st would be 89%. (I had the number handy because I just finished calibrating my own sensor, and I’m only half a degree north of you.) Since you are getting occasional 100% readings, the sensor’s 100% must be set for some other arbitrary latitude. Maybe the centre of the continental U.S.?

If there is no user calibration available, then you are going to have to forget about the reading being a percentage of anything meaningful and treat it as an arbitrary number.

If it’s any help, my reading right now is 722 W/m^2, which is 76% of my maximum ever reading at noon on June 23rd, which was 950 W/m^2.

OK lets get something clear here the solar % provided by WD is based on a quite complex calculation which works out the theoretical max solar for your location and time of year (in w/m2) and likely attenuation by the atmosphere, and then uses that to calculate the % taking the actual reading from Davis sensor divided by the max times 100. So the % value does take into account your location but not altitude but this does not matter greatly unless you are at several 1000’s of feet up. The second point is that it is unlikely you will achieve 100% unless you have a perfectly clear sky with no haze or airborne pollution of any kind. The most important thing when setting up your sensor is that it is mounted completely level, and in WD your latitude and longitude are entered correctly, it can also be affected by dirt although I’ve not had problems with that on mine. The other point is that at sunrise and sunset the max values are small and so you often see close to 100% at times because the numbers are small (single figures actually).

A Davis solar sensor measures horizontal global solar radiation BTW which is why it can be done relatively inexpensively as it does not need to track the sun across the sky.


Thank you Stuart for your expertise on the subject… that sets me straight. I guess Im looking for too much perfection here as it
seems that the Davis sensor could at times err greatly on the number of sunshine hours recorded during the course of a day since some days
may be called completely sunny by some people but there can be a thin overcast in which the opacity may be 5 tenths, enough to knock down
solar radiation below the threshold and the Davis sensor stops counting the sun hours. A long time ago we never had electronic instruments
to take weather obs in the met dept and the instrument used to take sunshine radiation measurements was a Pyyradiometer. Essentially
a glass globe with a curved calibrated strip that fitted into a slot in front of the globe… when the sun was out it would burn the strip - even
during a hazy day or a thin overcast and as long as the burn was there sun hours would be counted… So this is something that Davis and like Solar sensors would be incapable of - at least at 49 degs north lat… I guess I would like sunshine hours to be recorded even during a high
overcast, hazy or polluted atmosphere but not with the Davis I guess…

Dont forget that you can adjust the threshold for ‘sunny’ in WD. I looked for an official met station near me and adjusted my threshold until my overall hours were in the same ball park as the official station, and that my station says sunny when most folks would consider it sunny. That’s near enough for me.

There are electronic sunshine recorders which are more accurate but the price goes up exponentially :wink:


yah I have adjusted the threshold from the default 80 down to 70 but won’t lower the threshold any lower than that cause then
the tolerances for the other sky conditions become too tight. I should check mine against the official figures as well…