Hi, Mark,
There are some counter-intuitive issues here, and the way you have
specified the geometry does not capture the complexity of the situation.
For instance, the term "side" doesn't tell us much. Imagine a spherical
boulder sitting on the surface of the soil in a northerly latitude. Then
the "north side" also has downward-facing aspects. In the early morning and
late evening, when the summer sun is at its most northerly in the sky, it is
also lowest in the sky, and thus illuminates downward-facing aspects with
less obliquity.
This leads to the initially surprising result that downward-facing
northern surfaces receive MORE insolation than similarly downward-facing
southern surfaces. For similar reasons, house plants can receive more
sunlight in the winter, when low solar angles peep under roof overhangs and
project through windows and far into rooms. In summer, roof overhang and
high solar angles may keep windows completely in the shade.
However, depending on what phenomenon you are interested in, solar
angles do not tell the whole story. Diffuse light, caused by scattering of
the direct-beam component of sunlight, whether in a clear blue or cloudy
sky, is much more evenly distributed than the direct-beam component. That
is, all parts of the sky send about the same amount of sky-light (or
cloud-light on an overcast day) to a horizontal surface. This diffuse light
is also going to warm your boulder, but relatively uniformly with regard to
compass direction.
For photosynthesis the cloud-light and sky-light components are
important contributors, especially since leaves typically saturate in
photosynthetic output at about 20% of full sunlight (although it is a
different story when you consider vegetative canopies or tree crowns instead
of individual leaves).
I think it would be useful if you went out to your favorite northern
boulder with some data-logging sensors and took some long-term readings.
I hope this helps.
Martin Meiss
2010/8/13 Mark Wilson <slugecol...@gmail.com>
> Hi folks,
> I am looking for a reference which states
> (1) that in the northern hemisphere the north side of boulders are
> less exposed to the sun than the south side
> and
> (2) that the east side of boulders are exposed to the sun only in the
> morning when temperatures tend to be cooler while west sides are
> exposed during the afternoon when temperatures tend to be higher and
> as a result the north and east side of boulders are likely to stay
> cooler and damper longer.
> Any suggestions?
> Mark
>
