Photosynthesis at high latitudes

I went out looking for some basic equations on insolation. PV Education has 'em.

My baseline is 2620 Wm^-2 where the sun shines at high noon (or 11 AM) through no clouds and negligible atmosphere. Maybe that'll fly (heh) at the Flotilla. But I've been talking farms, within the cloudlayer, at 50°+ North. There's atmosphere there (comparable to Earth, at low-50s km up, by design); moreover, clouds and - also - an angle. Think how much more farming is difficult at, say, Labrador than at Venezuela. Ideally.

If I am reading the graphs right, I should be getting half the direct insolation at my "farming" latitudes. But it's worse than that: if I'm farming within the clouds, those clouds have 0.75 albedo. That means: they're bouncing off three-quarters of the light, before my farm can get it. So the actual light in the clouds and below them should be more like an eighth of the full 2620 Wm^-2.

So 327.5 Wm^-2 of direct sunlight... at noon. Worse than Mars; perpetual (if warm) twilight. The noons do last longer. The mix of warmth and darkness reminds of Cretaceous Greater Australia.

This is where the Photosynthesis-Irradiance Curve steps in - they'll want a steep initial cline. I suggest such crops as photosynthesise efficiently (dark leaves!), and can manage under a thick canopy. It's warm enough for cane sugar, and at 4% efficency that'll get the essentials done.

BACKDATE 3/9.