Approximate relative air mass (AM) from sun elevation or sun zenith angle.
numeric vector Angle in degrees for the
sun position. An argument should be passed to one and only one of
numeric Value to return when elevation angle is negative (sun below the horizon).
This is an implementation of equation (3) in Kasten and Young (1989). This equation is only an approximation to the tabulated values in the same paper. Returned values are rounded to three significant digits.
Although relative air mass is not defined when the sun is not visible,
returning a value different from the default
NA might be useful in
F. Kasten, A. T. Young (1989) Revised optical air mass tables and approximation formula. Applied Optics, 28, 4735-. doi:10.1364/ao.28.004735.
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