Cloud Electrification and Lightning Questions and Answers

Answer: A

Correct answer: 45 flashes per second

Satellite observation gives a global average near 45 flashes a second, or some 1.4 billion a year, with the great majority over land in the tropics. The figure varies by a factor of two through the day and the year as the global thunderstorm activity varies, and it is that variation which the fair weather field over the ocean follows.

Answer: B

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

Both are true and the reason gives the mechanism. Land has a low heat capacity and warms quickly in the sun, driving strong surface heating and the vigorous updraughts that carry supercooled water above the freezing level. The sea warms slowly and its surface temperature varies little through the day, so oceanic convection is generally weaker and the lightning rate lower by a factor of ten.

Answer: C

Correct answer: the central Pacific ocean

The central Pacific has among the lowest flash rates on earth, for exactly the reason given by the previous item - weak convection over water whose surface temperature barely varies. The other three are among the world’s lightning maxima: the Congo basin, which holds the global record, northern Argentina and Paraguay, and central Florida, the most active region of the United States.

Answer: D

They occur in the mesosphere at fifty to ninety kilometres and last only milliseconds. They are usually triggered by large positive cloud to ground flashes. Their existence was only confirmed in the late twentieth century.

Answer: A

Correct answer: A-2, B-4, C-1, D-3

The four transient luminous events occupy different altitudes. A sprite is a red discharge in the mesosphere, 50 to 90 km up, triggered by a large charge transfer below. A blue jet shoots upward from the cloud top into the stratosphere. An elve is a rapidly expanding ring in the lower ionosphere, produced by the electromagnetic pulse of the flash. A gigantic jet bridges the whole gap from cloud top to ionosphere.

Answer: B

Correct answer: True

Sprites are almost always triggered by positive cloud-to-ground flashes, which transfer far more charge from a greater height than typical negative flashes and so produce the large charge moment change needed to drive breakdown in the mesosphere. They occur most often above the trailing stratiform region of a mesoscale convective system, where such positive flashes are concentrated.

Answer: C

Time of arrival and direction finding at multiple sensors fix the position by triangulation. The electromagnetic pulse propagates for hundreds of kilometres, which gives such networks a wide reach. Sensor spacing of hundreds of kilometres is workable.

Answer: D

Correct answer: 1 and 2 only

Statements 1 and 2 describe how a modern network works: the radio pulse from a return stroke propagates for hundreds of kilometres, so sensors can be widely spaced, and comparing the arrival times at several of them locates the stroke by multilateration. Statement 3 is wrong - detection is entirely electromagnetic, and the networks work at night, in daylight, through cloud and over the ocean where no observer is present.

Answer: A

It intercepts the discharge and conducts the current safely into the ground. The structure is protected because the current bypasses it rather than because the strike is prevented. The rod controls the path rather than the occurrence.

Answer: B

It offers a preferred path once a strike occurs but does not stop it happening. The protection lies in controlling where the current flows, not in avoiding the strike altogether. This is the most commonly misunderstood point about protection.