Why are there few/no clouds in the stratosphere?
More than ninety five percent of the water vapour is confined to the troposphere only.Further the temperature increases with height in the stratosphere which does not allow convection currents that are required for the vertical growth of the clouds.Availability of condensation nuclei(dust particle,smoke particles salt particles etc) which are required for the condensation of the water vapour into water droplets which constitute the cloud are less at that level.