  Observations suggest an abundance of water and paucity of methane in the
majority of observed exoplanetary atmospheres. We isolate the effect of
atmospheric processes to investigate possible causes. Previously, we studied
the effect of effective temperature, surface gravity, metallicity,
carbon-to-oxygen ratio, and stellar type assuming cloud-free thermochemical
equilibrium and disequilibrium chemistry. However, under these assumptions,
methane remains a persisting spectral feature in the transmission spectra of
exoplanets over a certain parameter space, the Methane Valley. In this work we
investigate the role of clouds on this domain and we find that clouds change
the spectral appearance of methane in two direct ways: 1) by heating-up the
photosphere of colder planets, and 2) by obscuring molecular features. The
presence of clouds also affects methane features indirectly: 1) cloud heating
results in more evaporation of condensates and hence releases additional
oxygen, causing water dominated spectra of colder carbon-poor exoplanets, and
2) HCN/CO production results in a suppression of depleted methane features by
these molecules. The presence of HCN/CO and a lack of methane could be an
indication of cloud formation on hot exoplanets. Cloud heating can also deplete
ammonia. Therefore, a simultaneous depletion of methane and ammonia is not
unique to photochemical processes. We propose that the best targets for methane
detection are likely to be massive but smaller planets with a temperature
around 1450 K orbiting colder stars. We also construct Spitzer synthetic
color-maps and find that clouds can explain some of the high contrast
observations by IRAC's channel 1 and 2.
