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It is shown that the fluctuation in the temperature of the cosmic microwavebackground in any direction may be evaluated as an integral involving scalarand dipole form factors, which incorporate all relevant information aboutacoustic oscillations before the time of last scattering. A companion papergives asymptotic expressions for the multipole coefficient $C_\ell$ in terms ofthese form factors. Explicit expressions are given here for the form factors ina simplified hydrodynamic model for the evolution of perturbations.Comment: 35 pages, no figures. Improved treatment of damping, including both  Landau and Silk damping; inclusion of late-time effects; several references  added; minor changes and corrections made. Accepted for publication in Phys.  Rev. D1

Fluctuations in the cosmic microwave background. I. Form factors and their calculation in the synchronous gauge