Which is More Important?

To figure out which is more important to us - length of day, area effect, or air mass effect - consider the following argument.

Based on the "plane parallel" picture - that a small section of the round Earth can be approximated to be a flat place - and a little trigonometry, it is possible to derive the path length of a light wave as a function of altitude. The path length is, again, the length or thickness of the atmosphere through which the light travels.
Using the rules of trigonometry, we have:

path length = h / cos (theta), where (90-theta) is the altitude of the Sun and 'h' is the thickness of the atmosphere at the local zenith (straight overhead).