The photo depicts a statue of James Watt on a pedestal of one square meter.

The statue is in a main square of Birmingham, England, in the center of the industrial country of England, the birthplace of the industrial revolution. James Watt measured the power that can be exerted by a standard horse, the horsepower. His contributions to science and technology are recognized in the unit of power (the watt) in SI, the International System of Units. 1 watt = 1 joule per second = 1 newton meter per second = 1 kg m2 s-3.

The "watt per square meter" is the SI unit for radiative and other energy fluxes in geophysics. The "solar constant", the solar irradiance at the mean earth-sun distance is approximately 1370 W m-2. The global and annual average solar irradiance at the top of the earth's atmosphere is one-fourth of the solar constant or about 343 W m-2. Why?

This page was last updated 1999-10-21.

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