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# Re: [Phys-L] solar constant

The solar irradiance has been averaging around 1366 W/m^2 for the past 60
years. Before that, it was as much as 3 W/m^2 lower. Here is a graph...

This number is the irradiance arriving at the top of the Earth's atmosphere.
The actual average insolation on the ground is about 8 times smaller due to
(1) the curvature of the Earth, (2) only half the Earth illuminated at a given
time, and (3) atmospheric reflection/absorption.

If you want to measure the local insolation, you can get one of these...
http://en.wikipedia.org/wiki/Pyranometer

The solar constant is around 1370 W/m2. I believe this is the average value
over the entire Earth in one year. I have two questions.

1. How can I find the approximate value on a given day at a given location
(latitude)? Is there some formula or way to estimate? I realize this might
not be easy.