Monday, January 21, 2013

Who Lopped the End Off My Graph?

You'd expect a solar intensity graph, taken over the course of a day, to show half a cycle of something approximating a sine wave.  Solar luminosity at any given moment is mainly proportional to the cosine of the sun's elevation in the sky.  There's a little extra attenuation when the sun is at lower elevations, as there's more air for it to travel through – and if the air is dirty, much light will be absorbed by the dust and other particulates.  Out here, though, the air is really clear.  So why does today's solar intensity graph look like this?

From about 9 am to 4 pm, the graph looks like what we'd expect. 

Before 9 am, it's just chopped off.  The reason is straightforward: before 9 am, from the vantage point of the weather station the sun is blocked by Gaskill Peak (one of the mountains forming Lawson Valley's eastern rim).  At 9 am the sun rose over Gaskill Peak, and the intensity jumped to right where you'd expect it. 

After about 3:45 pm, the graph gets all squiggly.  That's happening because at that time the sun is approaching the horizon (again, from the weather station's perspective), which right there is formed by some chaparral brush.  You're seeing the light from the sun being attenuated by highly variable shadows of the chaparral's leaves and branches – perhaps even being moved around by the wind. 

To get a really pretty graph, we'd need to have the weather station up on top of a local peak, with clear views to the horizon where the sun sets and rises (of course this would change throughout the year).

We only had seven and a half hours of bright sunlight today...

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