Shakespeare's King Edward sounds a note of caution to his exuberant comrades after their victory on Barnet field:
But in the midst of this bright shining day,
I spy a black suspicious threatening cloud,
That will encounter with our glorious sun
Ere he attain his easeful western bed.
Although he would not have known it by that name, the chances are that what Edward spotted was a cumulonimbus cloud.
When the weather is calm and warm, and the sky relatively clear, surface heating by sunshine produces ascending columns of air, here, there and yonder around the countryside. If the atmosphere is relatively humid, a cloud forms near the top of each of these invisible pillars - a bulbous rounded cloud with a flat base, resembling a clump of cottonwool or a giant cauliflower.
This is a cumulus cloud. It is brilliantly white where the sun shines on it, and blue-grey to dark over the shaded portion, except along the edge nearest the sun where the hem of the fabled silver lining can often just be seen.
The stronger the heat of the sun, the greater the convective activity, and the larger and more luxuriant the huge cauliflower tops of the growing cumulus.
When a cumulus cloud becomes very tall, meteorologists call it a cumulonimbus, and from such clouds come very heavy showers, or even thunderstorms. As the air proceeds upwards in the exuberant buoyancy of the strong vertical fountains in a vigourous cumulonimbus, it becomes colder and colder, thus facilitating further condensation to increase the height of the cloud itself.
But as the process continues, many of the cloud's droplets coalesce to form water-drops, which grow bigger and bigger and eventually heavy enough to fall to the ground.
When this happens, the drag exerted by these millions of falling raindrops, as they pull the air with them in their descent, creates a "downdraught" in the cloud, a downward current of air which develops its own momentum and accelerates the raindrops earthwards for many thousands of feet until they reach the ground as a very heavy shower.
A characteristic of a well-developed cumulonimbus is the spreading out of the cloud at the top into a shape which closely resembles a blacksmith's anvil. The "anvil" is caused by a layer of very strong winds aloft, which shear off the top of the cloud in a forward direction.
Trails of ice and snow crystals fall out of this downwind extension, evaporating as they fall, all of which gives the anvil a much "wispier" texture than the lower, well-defined and rounded, sectors of the cloud.