Calculating sunshine hours from solar radiation

I’ll calculate for the CSU station tonight when I get a chance. Seems like it would a more accurate method than >120 w/m^2, at least for Fort Collins. Unless of course the solar radiation measurements are off.

Interesting. I've often heard that sunshine hours at many US sites are overstated due to the thresholds used differing from internationally accepted standards. For example, there's almost a 200 hr difference between Windsor and Detroit despite the two cities being right next to one another.

I'm curious as to whether the 3400 hr figure for Key West is accurate. I've always thought it seemed unrealistic. Ditto for Miami's 3150 hrs.

You have to be careful about what kind of sun strength measuring device you're using this formula on. It has to be a pyranometer mounted facing straight up.

Also, a lot of people (myself a couple months ago included) completely misinterpret what the 120W/m² threshold actually means. SMHI for example
"defines" sunshine as 120W/m² of direct raduation, but none of their stations actually use a 120W/m² threshold.

The most important part is that 120W/m² beeds to be the intensity of only the direct rays coming straight from the sun. If you just point a pyranometer to the sky during a cloudy day, it's likely to record 120W/m², but that's because it takes in light from every part of the sky. The 120W/m² must be coming ONLY from the little point in the sky where the sun is. Any attempt to measure sunshine from a single static pyranometer is going to fail, and at best, like with your algorithm Tommy, will only serve to form a rough approximation of what the sun may be, similar to if you were to try and calculate sun hours based on cloud cover octas observations.

The way SMHI measures sunshine is using three different pyranometers, one pointing straight up, one pointing to the left and one to the right. The difference between the highest and lowest W/m² reading is then used to determine whether the sun is shining or not (effectively a pyranometer shined on by the sun is compared to a pyranometer shaded from the sun, to get a value for how strong the actual sunlight itself is). This so-called contrast station, before being put to use, needs to be calibrated by being put next to a mechanical heliograph with a pyranometer at the bottom of a tube that automatically tracks the sun. The reason SMHI doesn't just use heliographs is because they're unreliable since they have to be accurately tracking the sun's movement 24/7/365, through ice and storms.

There are some stations that use classical C-S recorders as well.

I've noticed almost all of Norway's sun stations have like 30-80% data coverage. This is only a hypothesis since I don't know what type of equipment they use, but I think that could be that they're using heliographs, and heliographs just aren't very reliable long-term.

I calculated 2020 for the 3 Colorado Mesonet stations in Fort Collins.

The main Fort Collins station had 3733 hours, a clearly inaccurate value. The Christmas field station had an also BS 3412 hours.

The 3rd station had 2692 hours, which is in the realm of possibility, but seems low, especially since last year was a drought year. So then I did it for 2019 which had 2427 hours with 3 weeks of missing data in October. That would be like 2550-2600 for a full year which pretty much confirms this station is too low and not accurate either.

The two RAWS stations Tommy used had very reasonable values so I think the problem is more that these mesonet stations collect faulty solar radiation data. 

The problem with the RAWS stations is they’re both at higher elevations and at least 15 miles away. I also haven’t figured out to to easily upload their data to excel.

Although the values for the two RAWS stations seem reasonable and those are ag similarly high elevation so that wouldn’t seem to be the issue. 

If you're getting years that are out of place, it could be missing data. Instead of calculating for each month/year and averaging them, I calculated the average for each day of the year (from all years) and then averaged each of those from each month. This eliminates the problem of days missing from a particular month or year. I got 2954 hrs for Buckeye this way. Small difference overall but seems like the difference is a bit bigger between individual months.

Can you calculate solar radiation of non-US cities? There is a website (forgot its name) that has a world map with a solar radiation layer.

Damn, how many hours does Dos Palmas officially record? Looks like they get almost as much as physically possible (4400ish).

Is it possible to convert AU sites with this method? 

36.21° S, 981 m AMSL on an exposed hilltop: