Summertime clouds usually have flat bottoms, but their tops are marshmallowy and billowy.
Most summertime clouds are cumulus clouds, which form on updrafts, or rising columns of air made buoyant by humidity and heat from the ground warmed by sunlight. Cumulus clouds usually become more prolific in the afternoon when the ground gets warm. The bottoms are flat because this is the point at which the rising air is cooled enough for water droplets to form. The air keeps rising into the cloud and the billowy top is the point at which either the moisture runs out or the air stops rising.
We do not see cumulus clouds in winter because the cold, snow-covered ground cannot add heat to the atmosphere and so the air remains stable. Atmospheric rising motions in winter are on a much larger scale, the scale of low-pressure systems.