Approximate relative air mass (AM) from sun elevation or
sun zenith angle.

## Usage

`relative_AM(elevation.angle = NULL, zenith.angle = NULL, occluded.value = NA)`

## Arguments

- elevation.angle, zenith.angle
numeric vector Angle in degrees for the
sun position. An argument should be passed to one and only one of
`elevation_angle`

or `zenith_angle`

.

- occluded.value
numeric Value to return when elevation angle is
negative (sun below the horizon).

## Details

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.

## Note

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
some cases.

## References

F. Kasten, A. T. Young (1989) Revised optical air mass tables and
approximation formula. Applied Optics, 28, 4735-. doi:10.1364/ao.28.004735.

## Examples

```
relative_AM(c(90, 60, 30, 1, -10))
#> [1] 0.999 1.150 1.990 26.300 NA
relative_AM(c(90, 60, 30, 1, -10), occluded.value = Inf)
#> [1] 0.999 1.150 1.990 26.300 Inf
relative_AM(zenith.angle = 0)
#> [1] 0.999
```