In fact: Even though a cloud can weighs tons when spread over a large area, it doesn’t fall (much) because the rising air responsible for its formation keeps the cloud floating in the air. The air below the cloud is denser than the cloud, thus the cloud floats on top of the denser air nearer the land surface.
Nature Museum
“There are several reasons clouds float: first, the droplets in a cloud are small. Very small. An average water droplet in a cloud may only be 20 micrometers across. That is half as wide as a typical human hair, and about the same size across as a particle of dust. Even though dust is heavier than the air around it, a dust particle is so small that it can float in the air for a long time before falling. Water droplets in air behave the same way as dust.”
Scientific America
“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. Enough cooling eventually makes water vapor condense, which contributes to the survival and growth of the clouds.”
ZME Science
“Whilst clouds contain a lot of water crystals and droplets that are technically denser than the surrounding air, this water is spread so thin for miles that the effect of gravity becomes negligible. Additionally, if there’s a powerful updraft, the cloud can retain altitude virtually indefinitely until it dissipates due to temperature increasing, the cloud mixing with drier air, or when the entire air front sinks with the cloud.”
Why clouds don’t fall: the physics behind the sky’s fluffy wonders? →
West Texas A&M University
“The water drops and ice crystals that make up a cloud do not actually float motionless in the sky. Rather, they are constantly falling very slowly under the influence of gravity and are occasionally lofted upwards by an updraft of wind. This falling and updrafting motion of the drops/crystals that make up a cloud is so slow, and clouds are so big and far away, that it is hard for a casual human observer on the ground to notice this motion.”
flerf.org 