Getting back on topic....

Here is what I understand the situation to be.

The annual peak intensity of the sun in the UK is perhaps 70% of
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And, of course, even
those figures apply to (typically) half a dozen days a year in the
UK - the average daily peak in summer is much lower.

My experience is that 'burning' damage is almost always caused by
those few days, and that the sun levels on a 'Phew! What a scorcher'
day might be 50% higher than on a typical 'hot' day in summer. This
is because our sun levels are primarily controlled by atmospheric
absorption, not sun angle. Is that your experience?

No, my experience is that the leaves will burn, regardless of whether
it is a 'scorcher'. I find that it tends to coincide with direct
overhead sun and is far worse on plants that have not been very
gradually 'hardened' to increased light levels. Clivias can cope with
full sun in the U.K. but they need very gradual hardening and placing
them suddenly out of doors as indicated by the O.P. is a recipe for
the bleached out look. I hasten to add, that here in the far south at
least, non-burnt, fully exposed plants do not grow as well as when
given some shade and typically have short, broad leaves with a slight
yellow caste. I've experimented with 3 year old seedlings many times
and there's no doubt that growth rates at full exposure are as much a
50% lower than those given light to medium (20 - 40%) shade.

My own mature plants get some full sun later in the day - ironically
when heat levels are at their highest which is usually from
mid-afternoon onwards. There is a difference between exposure to sun
at a high angle (noon) and at a much lower angle (late afternoon).
Quite a few shade loving plants can cope with the latter, but not the
former.

I believe that the problems caused by sun through glass are mainly
surface heating, because it is typically associated with slow air
movement, is much more serious close to the glass than a distance
away, and 1/2" air gap K glass double glazing does not seem to cause
the effect much. Can you confirm or deny this?

Well having grown them in very large glasshouses where they were never
closer than 3 feet and often as much as 10 feet away from the glass
and still got burnt, I do not think proximity to glass is the complete
answer.

I don't have a clue what the primary 'burning' effect on plants is
(i.e. ultraviolet or surface heating) and what the 'tanning' effect
is. But they assuredly exist. Can you clarify those at all?

Well, it is a bit of a poser, I'll agree. If surface heating was a
contributory factor, there would be little or no damage at low
temperatures. Unfortunately burning can occur when air temperatures
are only 15 or 16C. The burning appears to be a bleaching of plant
cells with cell contents being oxidized. I think suddenly increased
UV exposure may be part of the problem and if I were to 'hang my hat'
on a reason, I suspect it would be that.

Dave Poole
Torquay, Coastal South Devon UK
Winter min -2°C. Summer max 34°C.
Growing season: March - November