
Canada’s Walrus magazine had an article by Jill Frayne online recently that I thought was a particularly well-written explanation of the the sky’s light-and-sound spectaculars, now just coming to an end here in Thailand as a daily sport.
Frayne brews up a thunderstorm, “either from warm air rising off the ground, or from a drop in temperature in the atmosphere”, and continues:
Cheery, popcorn-white clouds in the sky mean unstable air has stopped rising and has reached the dew point, when its temperature equals that of the surrounding air. These cumulus clouds can persist for days or they can dissipate, but if warm thermal air keeps pushing up, they often develop into cumulonimbus clouds.
When this happens, an aerial game of pinball begins. Droplets bounce around, collide, pick up grit, old volcanic ash, the detritus of life below, all of it freezing into ice crystals in the troposphere. Air travellers experience this as turbulence, their plane passing through cumulonimbus clouds, lurching in the chaos, the cabin windows dashed with rain or hail, until the aircraft emerges from the cloud.
On the ground, this intense interior action looks like a change from roly-poly clouds jogging in the sky to clouds darkening and mounting into towers. This formation means a storm is brewing. Inside the cloud, ice crystals and raindrops zing about in all directions, slamming into and fusing with one another, growing larger and heavier, until they start falling through the cloud as rain or snow or hail.
Lightning gets into the action through an electrical charge that builds during these high-wire collisions.
When a beauty of a storm is brewing, a negative charge heats up at the base of the cloud. The earth’s surface tends to be negatively charged, and since like charges repel, current at the bottom of the cloud draws away from the ground, leaving a positive charge in the air.
This is that pre-storm sense one gets, the hair on the head lifting slightly, the air freighted with electricity.
It’s in the nature of air to act as a buffer, to resist electrical flow, and for a time it contains the mounting charge.
But it can’t hold out forever. At a certain point, the negative charge from the cloud expends itself, not all at once, which would be an atomic reaction, but haltingly, in a “stepped leader” about as thick as a pencil.
This negative charge gropes toward the ground, moving in a searching way, like a lonely drunk in a bar looking for a connection. Eventually, it attracts a positive charge from something tall on the ground — a tree or a tower, a farmer on a tractor.
When lightning strikes, the charges have connected in a streamer, a flood of positive current that surges back up into the cloud, spectacularly hot, so intensely superheating the surrounding air that a shock wave bulges out, faster than the speed of sound.
Thunder is the sonic boom we hear when the percussive force breaks the sound barrier. Once the stepped leader from the cloud locks to the streamer from the ground, a channel opens for pulses of electricity to pass through, producing several flashes. We see the lightning before we hear the boom, but the thunder actually occurs first; the speed of light outraces the sound of thunder to our senses.
In its defence, lightning is only doing its job. It’s the celestial housekeeper, balancing an overcharged heaven and earth.