Airmass X uses the Kasten–Young form, X = 1 / (sin h + 0.50572(6.07995 + h)⁻¹˙⁶⁳⁶), which stays sensible below 10° where the naive 1/sin h blows up. Extinction in magnitudes is k·X, so the flux ratio is 10^(−0.4kX). To recover the same signal-to-noise as at the zenith you need to expose for X² times longer — sky noise dominates, so the penalty is quadratic, not linear. Differential refraction also separates colours near the horizon, which matters for astrometry and for narrowband filters.