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Name: Michael
Status: Student
Grade: 9-12
Location: FL
Country: United States
Date: May 2008


Question:
How can you determine how much of the sun's heat in kJ reaches a certain point on the earth, eg: Florida?



Replies:
Michael,

This is usually determined by measuring the broadband solar radiation (all wavelengths) using an instrument called a pyranometer. You can see a couple of types of these (PSP for Precision Spectral Pyranometer, 8-48 model called the Black and White Pyranometer) at the following website, http://www.eppleylab.com/ under "Instrumentation" on the left sidebar.

Pyranometers have surfaces that heat up from solar radiation. The temperature of the surface of the instrument is measured with a thermopile (lots of small temperature measurements) and converted to a voltage output that represents the solar radiation intensity.

David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory


Thanks!!This is not an easy quantity to measure accurately. There are always reasons, scientifically sound, to alter the answer. Here are just a few of the technical "details". 1. Are the measurements being made at high altitude above the absorption / scattering of atmosphere? 2. What are the wavelengths being measured. The Sun provides electromagnetic radiation with wavelengths in the very long wavelengths in the radio frequency part of the spectrum. At shorter wavelengths longer than the conventional wavelength of 700 micrometers (This selection is somewhat arbitrary.). Then between 700 and 400 nanometers is the small visible range of wavelengths that we think of as "light". However, at longer wavelengths is the range of electromagnetic radiation greater than a wavelength of about 700 nanometers, extending down into the common infrared wavelengths. At greater wavelengths is the "pure" rotational spectrum of molecules that have strong "rotational" absorptions, such as water. In addition are wavelengths that I failed to mention because it just gets too complicated for a short Q. & A. format like NEWTON BBS.

All of the above numbers will change depending upon: the "cloud cover", the activity of solar cyclones and spheres. Your's is a good question, but it opens a "Pandora's Box" of qualifiers that are inevitable. One needs too subdivide the incoming radiation flux (energy per area) in to the energy of the particular wavelength of the radiation.

Each of these forms of radiation can convert into other forms, so the answer to you question has a lot of qualifiers. Like so many questions, the question is easy, but an accurate response has a lot of qualifiers.

Nonetheless, do not be discouraged. Ask the question(s) and we will try to answer the question in an "age appropriate" way. Simple questions frequently have complicated answers.

Regards,

Vince Calder



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