  The recent discovery of photometric variations in L dwarfs has opened a
discussion on the cause of the variations. We argue against the existence of
magnetic spots in these atmospheres and favor the idea that non-uniform
condensate coverage (i.e. clouds) is responsible for the variations. The
magnetic Reynolds number (Rm) in the atmosphere of L dwarfs, which describes
how well the gas couples with the magnetic field, is too small (<<1) to support
the formation of magnetic spots. In constrast silicate and iron clouds form in
the photospheres of L dwarfs. Inhomogeneities in such cloud decks can plausibly
produce the observed photometric variations. Further evidence in support of
clouds is the tendency for variable L dwarfs to be bluer in J-Ks than the
average L dwarf of a given spectral type. This color effect is expected if
clear holes appear in an otherwise uniform cloud layer.
