  With the increasing number of directly imaged giant exoplanets the current
atmosphere models are often not capable of fully explaining the spectra and
luminosity of the sources. A particularly challenging component of the
atmosphere models is the formation and properties of condensate cloud layers,
which fundamentally impact the energetics, opacity, and evolution of the
planets.
  Here we present a suite of techniques that can be used to estimate the level
of rotational modulations these planets may show. We propose that the
time--resolved observations of such periodic photometric and spectroscopic
variations of extrasolar planets due to their rotation can be used as a
powerful tool to probe the heterogeneity of their optical surfaces. We address
and discuss the following questions: a) what planet properties can be deduced
from the light curve and/or spectra, and in particular can we determine
rotation periods, spot--coverage, spot colors, spot spectra; b) what is the
optimal configuration of instrument/wavelength/temporal sampling required for
these measurements; and, c) can principal component analysis be used to invert
the light curve and deduce the surface map of the planet.
  Our simulations describe the expected spectral differences between
homogeneous (clear or cloudy) and patchy atmospheres, outline the significance
of the dominant absorption features of water, methane, and CO and provide a
method to distinguish these two types of atmospheres. Simulated photometry from
current and future instruments is used to estimate the level of detectable
photometric variations. We conclude that future instruments will be able to
recover not only the rotation periods, cloud cover, cloud colors and spectra
but even cloud evolution. We also show that a longitudinal map of the planet's
atmosphere can be deduced from its disk--integrated light curves.
