  Based on the SED, Herbig stars have been categorized into two observational
groups, reflecting their overall disk structure: group I members have disks
with a higher degree of flaring than their group II counterparts. We
investigate the 5-35 um Spitzer IRS spectra of a sample of 13 group I sources
and 20 group II sources. We focus on the continuum emission to study the
underlying disk geometry. We have determined the [30/13.5] and [13.5/7]
continuum flux ratios. The 7-um flux excess with respect to the stellar
photosphere is measured, as a marker for the strength of the near-IR emission
produced by the inner disk. We have compared our data to self-consistent
passive-disk model spectra, for which the same quantities were derived. We
confirm the literature result that the difference in continuum emission between
group I and II sources can largely be explained by a different amount of small
dust grains. However, we report a strong correlation between the [30/13.5] and
[13.5/7] flux ratios for Meeus group II sources. Moreover, the [30/13.5] flux
ratio decreases with increasing 7-um excess for all targets in the sample. To
explain these correlations with the models, we need to introduce an artificial
scaling factor for the inner disk height. In roughly 50% of the Herbig Ae/Be
stars in our sample, the inner disk must be inflated by a factor 2 to 3 beyond
what hydrostatic calculations predict. The total disk mass in small dust grains
determines the degree of flaring. We conclude, however, that for any given disk
mass in small dust grains, the shadowing of the outer (tens of AU) disk is
determined by the scale height of the inner disk (1 AU). The inner disk
partially obscures the outer disk, reducing the disk surface temperature. Here,
for the first time, we prove these effects observationally.
