A bunch of lightning questions

A bunch of lightning questions:

  1. For the purpose of the following questions, can it be assumed that the cloud-base and ground combination act like a large parallel plate capacitor?

  2. If the cloud base is -charged and the ground is +charged, then presumably drops falling from the cloud will also have a -charge. If this is true, would they not be acclerated towards the ground at a speed somewhat greater than due to gravity alone?

  3. Similarly, if rain is falling, would it not act to blead off much of the cloud’s charge? Have there been any studies that correlate heavier rainfall with less lightning?

  4. If a bird took off from the ground below a charged cloud, presumably it would have a +charge. As it flies higher into the sky, would it have the potential to induce a lightning strike?

  5. What about aircraft? If they fly into the storm from elsewhere than presumably they would not have a net charge and as such would not “attract” lightning, correct? Do they just get hit due to random chance? Does the bolt pass through the aircraft and continue down to ground?

  6. I was watching a tv show about lightning and they said that there is a particular radio frequency on which you can hear lightning from all over the world. They never said what this frequency is. I would guess that lightning would produce all sorts of crazy radio frequencies; is what they said true?

Thanks.

Yikes! Ask the tough ones. Ok, here goes:

1) For the purpose of the following questions, can it be assumed that the cloud-base and ground combination act like a large parallel plate capacitor? Not exactly. The charge separation mechanism in a supercell is much more complex than a capacitor. There are layers of charge inside a cumulonimbus cloud, and the whole system is highly dynamic. Trying to model it as a simple plate capacitor is overly simplistic.

2) If the cloud base is -charged and the ground is +charged, then presumably drops falling from the cloud will also have a -charge. If this is true, would they not be acclerated towards the ground at a speed somewhat greater than due to gravity alone? The ground is considered to be at 0 volts, and has neither a positive nor a negative charge. Raindrops don’t appear to have a strong charge either.

3) Similarly, if rain is falling, would it not act to blead off much of the cloud’s charge? Have there been any studies that correlate heavier rainfall with less lightning? There does appear to be less lighting when the rain is falling harder, however this has nothing to do with bleeding off charge. It is believed to be caused by a choking off of the strong updrafts which are thought to be important in the charge separation mechanism, the exact details of which are still largely unknown.

4) If a bird took off from the ground below a charged cloud, presumably it would have a +charge. As it flies higher into the sky, would it have the potential to induce a lightning strike? No, see #2 above.

5) What about aircraft? If they fly into the storm from elsewhere than presumably they would not have a net charge and as such would not “attract” lightning, correct? Do they just get hit due to random chance? Does the bolt pass through the aircraft and continue down to ground? Aircraft actually appear to induce lightning to strike them. Several studies have been done deliberately flying aircraft into strong storm cells and observing the results, the the planes get struck more often than mere chance should account for. The exact mechanism by which this happens is unclear, but it is believed to be the result of the same effect that causes contrails. The nucleation sites created by the passage of the aircraft lays down a path for lightining to follow, triggering a strike. And, yes, the bolt is conducted through the metal fuselage and out the bottom towards ground or another oppositely-charged cloud.

6) I was watching a tv show about lightning and they said that there is a particular radio frequency on which you can hear lightning from all over the world. They never said what this frequency is. I would guess that lightning would produce all sorts of crazy radio frequencies; is what they said true? Lightning produces RF noice across the EM spectrum from longwave radio waves to UV. Low band radio travels farther, all things being equal, so you are more likely to hear the pops and snaps caused by distant lightning on something like the 160-meter band. (around 2 MHz) than on normal broadcast AM. Even so, such sounds would be difficult to positively ID as lightning, since a number of phenomena can cause similar sounds.

BTW, here is a pretty goot page with diagrams and information on the electrical structure of storm cells and lightning production.

It’s good too.

Good answers, but one “oops.”

The ground below a thunderstorm IS highly charged. Whenever e-field flux lines terminate on a surface, there are charges on that surface. The Earth is somewhat conductive, and the negative cloud base induces an equal and opposite charge on the ground below it. At the same time it would drive away some alike-charges, so there should be a weak region of negative charge elsewhere on the ground. (Imagine what happens when you bring a negatively charged object near a large neutral metal sphere. There will be a small positive region on the neutral sphere, plus a large fuzzy negative region, and added together they give zero.)

The Earth doesn’t have a voltage. Voltage, like distance, is always measured between two points.
Also, about birds and aircraft. If the e-field in the air is very strong, strong enough to almost trigger a spark, then any conductive object will supply some sharp points which can create corona discharge. (“Sharpness” is relative. Even a polished metal ball is “sharper” than the empty air.) A corona can take off as lightning leaders. A bird or plane flying in a strong e-field can launch two lightning leaders outwards in opposite directions.

Analogy: lightning is like a fracture. Bend a glass plate so it’s about to shatter. Any tiny additional force can start the fracturing process. Drop a tiny bead on the bent glass, and a network of cracks will launch outwards from the bead. Efields and lightning work something like that. If the air is highly stressed, then when an aircraft enters, lightning is triggered, and two leaders are launched from two spots on the plane. One moves out towards a (+) charged region in the cloud (or ground) and the other leader grows towards the region of opposite charge.

Slight Hijack - Don’t forget the recently discovered lightning phenomena known as Red Sprites and Blue Jets:
“Sprites are massive but weak luminous flashes that appear directly above an active thunderstorm system and are coincident with cloud-to-ground or intracloud lightning strokes. Their spatial structures range from small single or multiple vertically elongated spots, to spots with faint extrusions above and below, to bright groupings which extend from the cloud tops to altitudes up to about 95 km. Sprites are predominantly red. The brightest region lies in the altitude range 65-75 km, above which there is often a faint red glow or wispy structure that extends to about 90 km. Below the bright red region, blue tendril-like filamentary structures often extend downward to as low as 40 km. Sprites rarely appear singly, usually occurring in clusters of two, three or more. Some of the very large events, such as shown in Figure 1, seem to be tightly packed clusters of many individual sprites. Other events are more loosely packed and may extend across horizontal distances of 50 km or more and occupy atmospheric volumes in excess of 10,000 cubic km.”

“Blue jets are a second high altitude optical phenomenon, distinct from sprites, observed above thunderstorms using low light television systems. As their name implies, blue jets are optical ejections from the top of the electrically active core regions of thunderstorms. Following their emergence from the top of the thundercloud, they typically propagate upward in narrow cones of about 15 degrees full width at vertical speeds of roughly 100 km/s (Mach 300), fanning out and disappearing at heights of about 40-50 km. Their intensities are on the order of 800 kR near the base, decreasing to about 10 kR near the upper terminus. These correspond to an estimated optical energy of about 4 kJ, a total energy of about 30 MJ, and an energy density on the order of a few mJ/m^3. Blue jets are not aligned with the local magnetic field.”