  Transmission spectrum surveys have suggested the ubiquity of high-altitude
clouds in exoplanetary atmospheres. Theoretical studies have investigated the
formation processes of the high-altitude clouds; however, cloud particles have
been commonly approximated as compact spheres, which is not always true for
solid mineral particles that likely constitute exoplanetary clouds. Here, we
investigate how the porosity of cloud particles evolve in exoplanetary
atmospheres and influence the cloud vertical profiles. We first construct a
porosity evolution model that takes into account the fractal aggregation and
the compression of cloud particle aggregates. Using a cloud microphysical model
coupled with the porosity model, we demonstrate that the particle internal
density can significantly decrease during the cloud formation. As a result,
fluffy-aggregate clouds ascend to altitude much higher than that for
compact-sphere clouds assumed so far. We also examine how the fluffy-aggregate
clouds affect transmission spectra. We find that the clouds largely obscure the
molecular features and produce a spectral slope originated by the scattering
properties of aggregates. Finally, we compare the synthetic spectra with the
observations of GJ1214 b and find that its flat spectrum could be explained if
the atmospheric metallicity is sufficiently high ($\ge100\times$ solar) and the
monomer size is sufficiently small ($r_{\rm mon}<1~{\rm {\mu}m}$). The
high-metallicity atmosphere may offer the clues to explore the gas accretion
processes onto past GJ1214b.
