  We study the variability of major atmospheric absorption features in the
disk-integrated spectra of the Earth with future application to Earth-analogs
in mind, concentrating on the diurnal timescale. We first analyze observations
of the Earth provided by the EPOXI mission, and find 5-20% fractional variation
of the absorption depths of H2O and O2 bands, two molecules that have major
signatures in the observed range. From a correlation analysis with the cloud
map data from the Earth Observing Satellite (EOS), we find that their variation
pattern is primarily due to the uneven cloud cover distribution. In order to
account for the observed variation quantitatively, we consider a simple opaque
cloud model, which assumes that the clouds totally block the spectral influence
of the atmosphere below the cloud layer, equivalent to assuming that the
incident light is completely scattered at the cloud top level. The model is
reasonably successful, and reproduces the EPOXI data from the pixel-level EOS
cloud/water vapor data. A difference in the diurnal variability patterns of H2O
and O2 bands is ascribed to the differing vertical and horizontal distribution
of those molecular species in the atmosphere. On the Earth, the inhomogeneous
distribution of atmospheric water vapor is due to the existence of its exchange
with liquid and solid phases of H2O on the planet's surface on a timescale
short compared to atmospheric mixing times. If such differences in variability
patterns were detected in spectra of Earth-analogs, it would provide the
information on the inhomogeneous composition of their atmospheres.
