(4) Clouds are composed primarily of small water droplets and, if it’s cold enough, ice crystals. The vast majority of clouds contain droplets and/or crystals that are too small to have any appreciable fall velocity. So the particles continue to float with the surrounding air. Upward vertical motions, or updrafts, in the atmosphere also contribute to the floating appearance of clouds by offsetting the small fall velocities of their constituent particles. Clouds generally form, survive and grow in air that is moving upward. Rising air expands as the pressure on it decreases, and that expansion into thinner,high-altitude air causes cooling.