  We present a possible interpretation for the increase of the angular diameter
of the supergiants alf Ori (M1-2 Ia-Ibe) and alf Her (M5 Ib-II) from the K band
to the 11 micron region and the high-resolution 11 micron spectra without any
salient spectral features revealed by Weiner et al. (2003). The angular
diameters as well as the high-resolution spectra of alf Ori and alf Her
obtained in the 11 micron region can be reproduced by a warm water vapor
envelope, whose presence in alf Ori was revealed by Tsuji (2000) based on the
reanalysis of the near-infrared data obtained with the Stratoscope II. While
prominent absorption due to H2O can be expected from such a dense, warm water
vapor envelope, the absorption lines can be filled in by emission from the
extended part of the envelope. This effect leads to a significant weakening of
the H2O lines in the 11 micron region, and makes the observed spectra appear to
be rather featureless and continuum-like. However, the emission due to H2O
lines from the extended envelope leads to an increase of the apparent size in
this spectral region. The observed angular diameter and the high resolution
spectra of alf Ori and alf Her in the 11 micron region can be best interpreted
by the water vapor envelope extending to 1.4--1.5 Rstar, with a temperature of
about 2000 K and a column density of H2O of the order of 10^20 cm^-2.
