  We fitted the $3-180$-keV spectrum of all the observations of the
neutron-star low-mass X-ray binary 4U 1636$-$53 taken with the {\it Rossi X-ray
Timing Explorer} using a model that includes a thermal Comptonisation
component. We found that in the low-hard state the power-law index of this
component, $\Gamma$, gradually increases as the source moves in the
colour-colour diagram. When the source undergoes a transition from the hard to
the soft state $\Gamma$ drops abruptly; once the source is in the soft state
$\Gamma$ increases again and then decreases gradually as the source spectrum
softens further. The changes in $\Gamma$, together with changes of the electron
temperature, reflect changes of the optical depth in the corona. The lower
kilohertz quasi-periodic oscillation (kHz QPO) in this source appears only in
observations during the transition from the hard to the soft state, when the
optical depth of the corona is high and changes depends strongly upon the
position of the source in the colour-colour diagram. Our results are consistent
with a scenario in which the lower kHz QPO reflects a global mode in the system
that results from the resonance between, the disc and/or the neutron-star
surface, and the Comptonising corona.
