Cloud Electrification and Lightning Questions and Answers

Answer: C

Correct answer: 1 and 2 only

Statements 1 and 2 are correct. Statement 3 is the simplification the second corrects: the reversal is not a single temperature but a boundary in temperature and liquid water content together, so a storm with a richer supply of supercooled water reverses at a colder temperature and places its negative centre higher. Quoting minus 15 degrees is a useful approximation rather than a physical constant.

Answer: D

Correct answer: the upper troposphere where other sources are scarce

Lightning produces nitrogen oxides directly in the upper troposphere, where surface sources such as traffic and industry barely reach and where the resulting ozone production has a disproportionate effect on the radiative balance. It is a modest fraction of the global nitrogen oxide budget but the dominant source at that altitude, which is why it matters to atmospheric chemistry out of proportion to its size.

Answer: A

The main negative centre is the usual source of cloud to ground flashes. This transport of negative charge downward is what maintains the global circuit against fair weather conduction. The Earth carries a net negative surface charge.

Answer: B

Both are true and the reason gives the mechanism, and point discharge currents contribute as well. The fair weather conduction current continuously opposes this transport. The two must balance in the long term average.

Answer: C

The bright band is a precipitation phenomenon with no electrical role. The other three are the recognised elements of the global circuit. The circuit is closed through the conducting ionosphere. The bright band belongs to precipitation physics.

Answer: D

A small rocket trailing a grounded wire initiates a discharge at a known place and time. The technique has provided much of what is known about return stroke currents. Instrumented towers provide the other main source.

Answer: A

Correct answer: True

Firing a small rocket trailing a grounded wire into a charged cloud triggers a flash at a known place and a known instant, so the current can be measured directly at the base of the channel and the electromagnetic fields recorded at known distances. It has supplied most of what is known quantitatively about return stroke currents, and it is how lightning protection standards have been validated.

Answer: B

Correct answer: fifty metres at a time

The stepped leader advances in luminous steps of roughly 50 metres, each lasting about a microsecond, with pauses of tens of microseconds between them - giving an average descent speed of some 200 km per second, far slower than the return stroke that follows. The stepping is what makes the leader visible on high speed film and gives the flash its name.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the cause. The descending leader repeatedly divides, exploring several routes downward, and only one branch reaches the ground and carries the return stroke. The others are left behind as the fainter side branches of the visible flash. Because they were formed by a descending leader, they point downward - which is how the direction of the leader can be read from a photograph.

Answer: D

Correct answer: the direction in which the leader was travelling

Branches point away from the direction of leader propagation, since each was a path the leader explored and abandoned as it advanced. Downward-pointing branches therefore indicate a downward leader, as in an ordinary cloud-to-ground flash, and upward-pointing branches indicate a flash initiated upward from a tall structure. It is one of the few things about a flash that can be read directly from a photograph.