  We present an analytical model for the transmission spectrum of a transiting
exoplanet, showing that a cloud base can produce an observable inflection point
in the spectrum. The wavelength and magnitude of the inflection can be used to
break the degeneracy between the atmospheric pressure and the abundance of the
main cloud material, however, the abundance still depends on cloud particle
size. An observed inflection also provides a specific point on the atmospheric
P-T profile, giving us a "thermometer" to directly validate or rule out
postulated cloud species. We apply the model to the transit spectrum of HD
189733b.
