Category Archives: Meteorology

Electric Fields

In the last post, I glossed over the bit about how lightning has a hard time passing through air, so I thought I’d clarify (and hopefully this’ll be clear enough that I don’t need to keep up with this string of addenda and clarifications and can write about something new).

From the last post:

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.

So what exactly is an electric field? It’s a region where, if a charged particle is placed, it will experience an electric force. It’s just like a magentic field: when a magnet (for example, a compass) is placed in a magnetic field (like the Earth’s planetary magnetic field), it experiences a force that aligns it (ie, the compass needle) in a particular way. Similarly, a charged dropped into an electric field will experience a force that pushes it in the field. Electric fields are created by a distribution of charges, either discrete or continuous:

A point charge and a lump of continuous charge, both with electric field lines.

The green lines represent the electrical field.

Of course, the force experienced by a charge dropped into a field depends on the sign (positive or negative) of the charge. A negative charge will experience the opposite force that a positive force experiences, ie, the arrow heads all point in the other direction.

With lightning, it’s not a point test charge dropped into the cloud that creates the bolt, but rather that the charge distribution itself cannot be sustained any longer, and a bolt transfers charge from one region of the cloud to another and neutralizes the field.

Heavily charged cloud with two lightning bolts.

The bolt travels through air, and air is not a vacuum, so the physical properties of the air (or any material that charge is attempting to move through) will affect how easily the charge can move through the material. Materials (and by materials I mean any state of matter, so it can include say glass, water, and air) can generally be classified as either insulators or conductors, depending on a property called conductivity. Electrical energy has a hard time travelling through insulators (like glass) which have a low conductivity, while it passes easily through conductors (like metals), which have high conductivity.

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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.

Thundercloud with positive charge at the top and negative charge at the bottom.  The ground below has a positive charge too.

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.

Heavily charged cloud with two lightning bolts. Thundercloud with regions now neutralized after lightning bolt.

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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.

Air flow over North America.  Cold dry air comes from the west over the Rockies, while warm wet air flows up from the Gulf of Mexico over the middle of the continent.

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.

Lifting mechanism.  Cold air is denser than warm air, so when warm and cold air meet, the warm air is lifted upwards.  The moisture in the warm air condenses into a cloud.

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