Light power

What you see at right is a picture of a very nearly perfect sunny Southern California day (today!), as seen by an instrument called a pyrometer. This instrument measures the incident solar power in my backyard every minute. This is just fancy talk for how much heating power the shining sun is delivering. It's calibrated in watts per square meter.

You can tell a lot from this simple graph. Most basically, you can see that the sun rose this morning at around 6 am, and it's going to set around 6 pm. You can see that it looks at least roughly like a chopped-off sine wave, and in fact it's very close to that, as a sine wave is exactly how it would look if the only effect on the sun's power was its elevation in the sky. But there are a few other things affecting the sun's power.

For instance, when the sun is low in the sky, its rays are traveling through more air to get to my yard. This additional air (and the junk floating around in it) attenuate the rays even more than is accounted for by the elevation. Then there's the scattering effect, most easily seen just before sunrise where the graph shows a little bit of slowly gathering power. If I had a pyrometer on the moon, there would be no such slow gathering of light — the light would either be there or not as the sun rose. But I live where there's an atmosphere (thank goodness!), so the scattering of light in the pre-dawn sky registers on this instrument as that gentle curve at sunrise that accelerates into the near sine wave when the rising sun strikes the instrument.

And finally there's the effects of the slight cloudiness that developed this afternoon just before 4 pm. You can easily see the sudden drop on power as a cloud obscures the sun, and the "choppiness" afterwards as clouds pass overhead.

More than you wanted to know ?