Lightning: The Superpower of Storm Clouds

Of all the ten main cloud types, the only one that poses any real danger to us cloudspotters below is the Cumulonimbus cloud. This is the storm cloud, the wild and unruly boss of the atmosphere. It has an unpredictable temperament, a tendency to gather into marauding, coordinated mobs known as multicell and supercell storms, and an impressive array of special abilities. Storm-cloud powers include fierce, gusty winds, deluges of rain, pelting hail stones, and lethal tornados. But the most iconic superpower of Cumulonimbus clouds has to be lightning.

How do these formations manage to store up the huge electric charges that are released in lightning bolts? Why do Cumulonimbus clouds produce lightning, like the one spotted here by Paul Martini (Member 27,060) over Bluff, Utah, US, but none of the other cloud types do? It all comes down to three main characteristics of Cumulonimbus storm clouds: their immense height, their mix of different types of ice, and the powerful up- and downdrafts of air that surge within their cores.

The tallest clouds of all, Cumulonimbus often extend from low in the atmosphere all the way up to the tropopause, the altitude where the lower part of our atmosphere where weather happens, known as the troposphere, meets the region above, the stratosphere. Such impressive stature means Cumulonimbus clouds extend through a wide range of air temperatures, from warm air around their bases right up to extremely frigid up near their summits. As a result, the water they contain is always in a mix of different stages of freezing. The vertical air currents that surge up and down their middles ensure some of the snow crystals freezing in their upper reaches are swept down into contact with supercooled water droplets below, which freeze around them to form precipitation like small grains of soft hail, known as graupel. If you want to understand lightning, you have to pay attention to what then happens between the remaining tiny snow crystals and these larger graupel grains of ice.

Whenever the two sizes of ice particle collide, they end up exchanging a tiny amount of electric charge. It is a little like the way you pick up static charge when you rub up against synthetic fibres – except that it’s a lot colder, it’s happening on a microscopic scale, and it’s 100% natural. These little collisions happen millions and millions of times within the churning belly of a Cumulonimbus storm cloud. Each time, the tiny snow crystals tend to lose an electron in the collision to the larger graupel. The snow crystals become positively charged, the graupel negatively charged, and being lighter, the snow crystals tend to be wafted to the top of the cloud by the updrafts, while the heavier graupel falls to the lower parts. This is how a huge separation in electric charge develops in the cloud, typically with positive regions up top and negative regions below.

Watch out. The beast of the atmosphere has its superpower primed and ready. Such a massive separation of positive and negative electric charge can only exist for so long before something has to give. The Cumulonimbus shoots an enormous surge of current ripping a channel of air into a plasma that can reach temperatures of 30,000ºC (54,000ºF), five time hotter than the surface of the Sun. With each bolt of lightning, the cloud evens out the electric charge. It can travel between regions within the same cloud, from one cloud to another next-door, from part of the cloud down to the ground below, or even into the high atmosphere above.

The extreme heat of the Cumulonimbus’s lightning bolt causes the channel of air it shoots down to expand explosively. This protracted explosion we hear as the mighty roar of thunder. When you unleash a superpower like lightning, you want to make sure everyone notices.

Cloud-to-ground lightning from a Cumulonimbus spotted over Bluff, Utah, US by Paul Martini (Member 27,060). View this in the photo gallery.