  We report on the results of an I-band photometric variability survey of
eighteen L dwarfs. We find that seven exhibit statistically significant
variations above the 95.4% confidence level with root-mean-square scatter
(including photometric errors) between 0.010 and 0.083 mag. Another five
targets have variability probabilities ~80%, suggesting that these are likely
variable objects. Three of the variable objects display significant peaks in a
CLEAN periodogram that are several times higher than the noise. The period for
2MASS 0345+25 is clearly not intrinsic to the object and can be dismissed. The
periods found for 2MASS 0746+20AB and 2MASS 1300+19 are unique but longer than
those periods likely from rotation velocity measurements and they do not
represent periodic behavior in the light curve that persists through the entire
data set. These observations suggest that we are not observing the rotation
modulation of a long-lived albedo feature. Instead, rapid evolution of
atmospheric features is likely causing the non-periodic variability. The
remaining variable objects show no prominent features in their light curves,
suggesting even more rapid evolution of atmospheric features. We argue against
the existence of magnetic spots in these atmospheres and favor the idea that
non-uniform condensate coverage is responsible for these variations. The
magnetic Reynolds number in the atmosphere of L dwarfs is too small to support
the formation of magnetic spots. In contrast, silicate and iron clouds are
expected to form in the photospheres of L dwarfs. Inhomogeneities in such cloud
decks and the evolution of the inhomogeneities can plausibly produce the
observed photometric variations.
