The clouds which most often bring rain are the drizzly nimbostratus, storm-cloud cumulomnibus and cumulus congestus. It was probably cumulus congestus which you saw, which causes passing showers and is usually followed by dryer weather as the humidity decreases, hence the crisp air. Cumulus clouds have a cotton-wool appearance and are formed due to one area being warmed more than another due to variation in albido, so that water in rising pockets of air condenses above cooler areas to the side. Cumulus clouds grow in height until they are too heavy to stay up, often at the cumulus congestus stage.