  We present a possible interpretation for the increase of the angular diameter
of the Mira variables o Cet, R Leo, and chi Cyg from the K band to the 11
micron region revealed by the recent interferometric observations using narrow
bandpasses where no salient spectral feature is present (Weiner et al. 2003a,
2003b). A simple two-layer model consisting of hot and cool H2O layers for the
warm water vapor envelope can reproduce the angular diameters observed with
Infrared Spatial Interferometer as well as the high-resolution TEXES spectra
obtained in the 11 micron region. The strong absorption of H2O expected from
the dense water vapor envelope is filled in by emission from the extended part
of the envelope, and this results in the high-resolution 11 micron spectra
which exhibit only weak, fine spectral features, masking the spectroscopic
evidences of the dense, warm water vapor envelope. On the other hand, the
presence of the warm water vapor envelope manifests itself as the larger
angular diameters in the 11 micron region as compared to those measured in the
near-infrared. Furthermore, comparison of the visibilities predicted in the
near-infrared with observational results available in the literature
demonstrates that our two-layer model for the warm water vapor envelope can
also reproduce the observed near-infrared visibilities and angular diameters.
The radii of the hot H2O layers in the three Mira variables are derived to be
1.5--1.7 Rstar with temperatures of 1800--2000 K and H2O column densities of
(1--5) x 10^{21} cm^{-2}, while the radii of the cool H2O layers are derived to
be 2.2--2.5 Rstar with temperatures of 1200--1400 K and H2O column densities of
(1--7) x 10^{21} cm^{-2}.
