JORGE OLCINA answers:
Clouds are formed by condensation of air vapour in the atmosphere. Despite their simple, smooth and generally graceful appearance and their varied shapes as suspended bodies in the air, the process of cloud formation is a very complex and largely unknown physical and chemical process. The truth is that clouds, in almost all existing varieties except fog strata deposited on land, remain floating in the air because their weight can be sustained by upward air currents beneath their base or by horizontal winds that move them.
The mass of a cloud depends on its type (its vertical development and general size) and internal structure (rain, freezing rain, snow, hail). Density can be used to estimate the mass of a cloud. Thus, for example, the median calculated density of cumulus mediocris clouds is half a gram per cubic metre. That is, each cubic metre of cloud can contain half a gram of water. The dimensions of this type of cluster are usually 1,000 metres long, 1,000 metres wide and 1,000 metres high. Therefore, the estimated volume of a cube of one thousand million cubic meters. Knowing its density (0.5 g / m3) and volume (1,000 x 106 m3), the quantity of water (mass) it contains can be estimated, in this case 500,000 kilograms, i. e. half a metric ton of water, or 500,000 litres, i.e., 500 m3.
In a large cumulonimbus with high vertical development, loaded with rain and hail, the mass of water contained can reach one million tons, which is equivalent to one million m3 of water; that is, we have a cubic hectometer of water over our heads when a cumuloninbus is crossing the sky above us. The rest of cloud types contain less water and are less heavy.
Jorge Olcina, Institute of Climatology of the University of Alacant.