Left, system temperatures (T_sys) measured at Los Alamos vs. airmass. Straight lines are fits of linear models to the lower envelopes of the T_sys distributions. The receiver temperature is estimated as the extrapolation of the lower envelope to a null airmass. Right, zenith opacity (i.e., opacity multiplied by the sine of the antenna elevation) at Los Alamos vs. time, as estimated using Eq. (1).