The importance of interactions between impact-induced vapor clouds and an ambient atmosphere has been widely recognized, and theoretical approaches have provided significant insights. Few experiments, however, have been done to observe directly the energy partitioning during the interactions between impact vapor clouds and the ambient atmosphere. The present study attempts to understand the difference between actual and theoretical model impact vapor clouds produced under an atmosphere. A series of hypervelocity impact experiments was conducted using a spectroscopic measurement method. Plastic (polycarbonate) impactors allowed simulating vaporization phenomena associated with natural impactors (e.g., silicates and metals) at high impact velocities into water. Water as the target material served to suppress the effect of fine-grained fragments from the target. Emission spectra of the leading part of downrange-moving impact vapor clouds were captured with high-speed spectrometers as a function of time for various ambient pressures. The emission spectra exhibit strong molecular bands from carbon compounds as well as blackbody continuum radiation. In order to estimate the temperature of the radiation source, we carried out a spectral-form inversion analysis based on diatomic emission theory. Obtained molecular radiation temperatures range from 4500 K to 5500 K with relatively high accuracy (~2%) and place a number of well-defined constraints for the radiation source. A simple theoretical model that is often assumed for an impact-induced vapor cloud, however, does not readily satisfy the constraints. This strongly suggests that real impact-induced vapor clouds may be more complex than previously thought.