India, April 11 -- They loom over Earth at all times, altering temperature, humidity and weather.

At any given moment, the sky over two-thirds of the planet's surface is covered by clouds. Yet we barely understand them.

We don't know exactly how they take shape, or why they take the shapes they do. We still know very little about what goes on inside them.

This leaves a massive gap in our understanding of our planet's heat and humidity cycles. How massive is the gap?

At any point, there are an estimated 12 trillion tonnes of water gathered up in the sky.

A single cloud can be so dense that one large cumulus (the fluffy, cotton-candy-like ones) can weigh 1,000 tonnes.

The shapes of these formations - and where in the atmosphere they e...