  Fluctuating atmospheric emission is a dominant source of noise for
ground-based millimeter-wave observations of the CMB temperature anisotropy at
angular scales $\gtrsim 0.5^{\circ}$. We present a model of the atmosphere as a
discrete set of emissive turbulent layers that move with respect to the
observer with a horizontal wind velocity. After introducing a statistic derived
from the time-lag dependent correlation function for detector pairs in an
array, referred to as the pair-lag, we use this model to estimate the aggregate
angular motion of the atmosphere derived from time-ordered data from the
Atacama Cosmology Telescope (ACT). We find that estimates derived from ACT's
CMB observations alone agree with those derived from satellite weather data
that additionally include a height-dependent horizontal wind velocity and water
vapor density. We also explore the dependence of the measured atmospheric noise
spectrum on the relative angle between the wind velocity and the telescope scan
direction. In particular, we find that varying the scan velocity changes the
noise spectrum in a predictable way. Computing the pair-lag statistic opens up
new avenues for understanding how atmospheric fluctuations impact measurements
of the CMB anisotropy.
