Question: How to grey clouds know when to rain ?

Helen Gath answered on 17 Jun 2014:

Well it certainly rains a lot in the UK, so understanding the reason why is quite important!

Clouds are formed when water droplets get warmed up, usually by the sun or air temperature. They get increasingly energetic and evaporate into the sky, staring to move about in the air. The water vapour often clumps together, so you get a massive collection of water droplets forming a larger and larger mass. This can suddenly cool down and condense (just like the team from your shower condenses onto the bathroom mirror), and we see this vapour in the form of a cloud. When the temperature in the sky gets dramatically colder, the water vapour will rapidly cool, losing its energy and condensing. Condensing makes the water particles heavy again, forcing it to fall as mist or rain.

Clouds can create rain fall in another way as well; clouds can also form when water vapour sticks to a particles in the sky, like dust, sea salt or gaseous chemicals. These can also stick together and form clouds. Over time, so many water droplets may combine with the dust particles that they get heavier and heavier until finally, gravity rules and they fall to the ground as rain drops.