What causes clouds to release their water content as rain? It's obviously not the amount of moisture contained

Question

in the cloud as you can have spots of rain from wispy clouds,while massive thunderclouds can hold onto their moisture content until eventually releasing it as a massive deluge,so what's the trigger that determines when that happens?

Answer 1

As clouds cool, the water vapour condenses forming dropletswhich then fall. The vapour in wispy clouds is less dense than in big black thunderclouds so it forms small raindrops. In thicker clouds the droplets are more densely packed so can coalesce as they drop forming big fat raindrops.

Answer 2

It is actually the rate that the air is being lifted.

When air is lifted, it expands and cools. This lowers the saturation equilibrium level with respect to water in the air. Thus any water above the equilibrium level will condense into liquid water or ice crystals depending on the temperature of the cloud.

The rate of lift is different from cloud to cloud.

Status is a cloud that lifts very slowly (on the order of a meter or a few feet per minute) thus the excess water vapor condenses very slowly.

On the other hand, the updraft of a thunderstorm is capable of lifting air at the rate of 10s of meters per second! Up to and exceedign 100 mph. This is changing a lot of water vapor to liquid or ice in a hurry. Thus a convective cloud will always have heavier rain than a stratus or a weakly lifted layer of cloud because it is lifting and cooling air at a much faster rate.

It is more complicated that this but am trying to simplify it to some degree for ease of understanding.

Answer 3

Basically, rain is formed around a dust particle. The dust particle is drawn into a cloud by updraughts, The water vapour in that cloud then attaches to the dust particle and it keeps gathering water until it basically gets too heavy for the updraughts and turbulence within the clouds to keep it in the air. That is when it rains. The updraughts and turbulence in wispy clouds are usually quite light so the rain drop doesn't have to be as heavy to fall as it would in a cloud with strong updraughts and turbulence (such as thunder clouds).

Answer 4

the up draft in clouds keep the water in the clouds longer,

Answer 5

Two things, updrafts and downdrafts, and gravity, if the drops and updrafts are weak enough the drops will drop, just like hail, if they become heavier than the updraft can push it up, it falls. So you have the combination of updraft versus size.

Answer 6

it rains because the cloud is already fully loaded after evaporating... its time of condensation