OK, Mark, so you COULD make it really complicated, as Martin has shown. Given the space constraints you'll face in publishing, though, you may want to simply add a caveat, such as "in general." As for references, I don't see the necessity. I'm not even optimistic you could find a source with any legitimate claim to being the originator of such a basic idea.
In a pinch, maybe you and a colleague from the opposite hemisphere could go out and observe which sides of some haphazardly-chosen rocks were better-lit during various times of day, then use "unpublished data" as your citation. Jim Crants On Fri, Aug 13, 2010 at 1:39 PM, Martin Meiss <[email protected]> wrote: > 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 <[email protected]> > > > 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 > > > -- James Crants, PhD Scientist, University of Minnesota Agronomy and Plant Genetics Cell: (612) 718-4883
