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Measuring Insolation
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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