Lightning and Thunder

While the last post talked about how thunderstorms form, it didn’t discuss either thunder or lightning (it was getting a bit long). So let’s talk about that!

Lightning is a discharge of electrical energy between different regions within a thundercloud, and it’s a byproduct of a thunderstorm, not a critical element to the storm’s formation. What is critical is the updraft that pushes lots of moisture into the atmosphere where it condenses and forms a thundercloud, and it’s this updraft that is thought to be what drives the electrical structure of a thunderstorm as well. (The precise mechanism is not totally understood.) The updraft forces the circulation of particles within the cloud. As ice and water particles collide within the cloud, they form and break apart. Small ice particles tend to gain a net positive charge, and the larger slushy particles tend to acquire a negative charge. The (positively charged) ice particles are smaller and are more easily pushed to the top of the cloud by the updraft, while the negatively charged slush particles particles fall to the middle and bottom of the cloud. The Earth also acquires a net positive charge in the area underneath the storm, as the concentration of negative charge at the bottom of the cloud induces a positive charge directly below it.

The red arrow is the updraft that drives the circulation within the cloud.

The net difference in electrical potential builds up, until the neutral air and water vapour in between the positive and negative regions can no longer sustain the difference, and a lightning bolt discharges the electrical energy. Air is a very good electrical insulator (ie, it is difficult for an electrical current to pass through the air), so a very large electric field can be sustained in the cloud before a lightning bolt discharges the stored energy, and returns at least part of the cloud to a neutral electrical state.

What appears as a single, instant bolt of lightning is usually made of several bolts of lightning that occur so quickly that the human eye perceives them as one. An initial “leader” bolt, which is not very luminous, extends down from the cloud to the ground. In response to the charge, tall objects form “streamers,” which are strands of positive charge that extend up towards the negatively charged leader. The leader often branches several times, and if one of those branches connects with a streamer, negative charge flows from the cloud to the ground. Nearly instantaneously, positive charge flows from the ground to the cloud along the path formed by the leader. This is the extremely bright that we see; the charges can zip back and forth between the cloud several times in what we see as a single bolt.

The yellow leader may branch several times; the blue lines from the tree and house are streamers.

Most lightning occurs within a cloud (or between two different clouds), but lightning between the (usually negatively charged) bottom of the cloud and the (usually positively charged) Earth is both better understood and much more distructive. Most lightning that occurs between a cloud and the Earth occurs between the bottom of the cloud and the earth, rather than the top. However, some lightning can form from the top of the cloud, arcing all the way to the ground. When that happens, the charge from the cloud is positive and the induced charge from the ground is negative (ie, the opposite of lightning that forms from the base of the cloud).

Thunder accompanies lightning, because as the lightning bolt extremely suddenly heats the air around it, the air is compressed into a shock wave. The compression shock is very localised, since lightning is extremely hot (~20,000 degrees Celsius) and extremely shortlived (~30 microseconds), and the shock decays into an acoustic wave, which we hear as a clap of thunder. Since sound travels much more slowly than light, the time between when you see a stroke of lightning and hear the accompanying clap of thunder can be used to estimate how far away the lightning bolt was. A difference of 5 seconds means the bolt was around a mile away, and a difference of 3 seconds means it was about a kilometer away.

How Thunderstorms Form

It’s the middle of winter here now, so let’s start off with something that happens much more in the summer here.

All thunderstorms need a few ingredients to form, including a source of moisture, warm wet air and cold dry air that interact, and a mechanism to trigger an updraft (more on this in a moment). In North America, the source of moisture is often either the Atlantic or Pacific Ocean, or the Gulf of Mexico — the moisture does not need to be where the thunderstorm forms, but rather where the air that feeds into the thunderstorm originates from.

Warm air blows over (say) the Gulf of Mexico, picks up moisture, and then continues on into the Southern US, where it may form a thunderstorm. This warm, wet air is typically close to the planet’s surface — it picks up the water from the ocean, and does not rise very high (yet). This low, warm, wet air may encounter cold, dry air from the Rockies. If this happens, the warm wet air will be lifted up by the cold air, and the moisture in the air will condense into a cloud.