  Atmospheres of transiting extrasolar giant planets (EGPs) such as HD 209458 b
must impose features on the spectra of their parent stars during transits;
these features contain information about the physical conditions and chemical
composition of the atmospheres. The most convenient observational index showing
these features is the ``spectrum ratio'', defined as the wavelength-dependent
ratio of spectra taken in and out of transit. I describe a model that estimates
this ratio and its dependence upon parameters of the planetary atmosphere,
including its cloud structure, temperature, chemical composition, and wind
fields. For giant planets in close orbits, the depths of atomic and molecular
features in the spectrum ratio may be as large as 0.001. Observations in
visible and near-IR wavelengths using existing and planned spectrographs should
be adequate to detect these features, and to provide some diagnostics of the
conditions within the planetary atmosphere. I give numerous examples of such
diagnostics, and I discuss their practicality.
