Sketch 1.

sketch 2
Text from sketch 2: when we think of rain, we think of it coming from the sea and all the moisture dumping onto the land. However, I think the air has been 'prepared' for the rainfall to come. (by moisture from the an eastly before the storm is come from the west for example).
The wetness has already been deposited there... is already in the air, and the storm acts as a sort of catalyst, a wave of rainstorm the ripples or walks across the landscape. Raining where it is. The moist air before it and the dry air behind it. The cloud itself remaining largely unchanged as it travels.
sketch 3.
From sketch 3:
Clouds are dark and light.
The top of clouds are well light and bright, this has a slightly warming and rising effect.
While the bottoms of clouds are dark and shadowed by the moisture of the cloud above it. This has a cooling and sinking effect. Helping to seperate, widen and thicken the cloud. In other words the cloud makes its own space to grow by pulling itself apart.

