State whether the following is true or false.
Thunder is rarely heard more than about 25 kilometres from the flash.
Answer: C
Refraction of sound upward by the temperature profile limits the range. Beyond that distance the flash is seen but not heard, which is the origin of the term heat lightning. No separate physical phenomenon is involved at all.
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Assertion (A): Distant lightning may be seen without any thunder being heard.
Reason (R): Sound is refracted upward by the atmosphere and does not reach the distant observer.
Answer: D
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. Temperature normally falls with height, so sound rays leaving the channel are refracted upward and pass over an observer beyond about twenty-five kilometres. Wind shear adds to the effect. The flash, travelling in a straight line, remains visible over the horizon by reflection from cloud, which is why summer heat lightning is seen silently in the distance.
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The temperature reached in a lightning channel is approximately
Answer: A
This exceeds the surface temperature of the sun by a factor of several. The heating occurs so rapidly that the surrounding air has no time to expand before the pressure rises. That pressure jump is precisely what becomes the thunder.
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Three of the following are consequences of the rapid channel heating. Which is the odd one out?
Answer: B
Correct answer: the accumulation of charge in the cloud
Charge accumulation is what happens before the flash and is its cause, not a consequence of the channel heating. The other three all follow from heating the channel to some 30,000 kelvin in microseconds: the air expands explosively and produces the shock wave heard as thunder, it radiates intensely across the visible spectrum, and the temperature is high enough to split nitrogen and form nitrogen oxides.
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Lightning contributes to the nitrogen cycle by
Answer: C
The extreme channel temperature allows nitrogen and oxygen to combine. The resulting oxides are a significant natural source in the upper troposphere, where few other sources operate. It matters directly to ozone chemistry at those levels.
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The fair weather electric field at the Earth's surface is approximately
Answer: D
The ionosphere sits at a positive potential of several hundred kilovolts relative to the ground. The resulting field is present everywhere in fair weather and is part of the global circuit. It is easily measured with a field mill.
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Assertion (A): Thunderstorms are regarded as the generators of the global electric circuit.
Reason (R): They transport negative charge to the ground and maintain the ionospheric potential.
Answer: A
Both are true and the reason gives the role, and fair weather conduction currents discharge the circuit continuously. Without ongoing thunderstorm activity the potential would decay within minutes. Roughly ten minutes is the usual figure quoted.
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Consider the following statements about the global electric circuit.
1. Thunderstorms act as the generators of the circuit.
2. Fair weather regions carry the return conduction current.
3. The circuit would persist unchanged without thunderstorms.
Answer: B
The first two are correct and describe the circuit. The third is wrong, since conduction would discharge the ionosphere within about ten minutes were the generators to stop. Continuous thunderstorm activity is therefore essential.
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The Carnegie curve describes
Answer: C
The field varies through the day in step with global thunderstorm activity regardless of local time. It is among the strongest pieces of evidence for the global circuit concept. The curve was established from ocean measurements.
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Assertion (A): The fair weather field over the ocean varies with universal rather than local time.
Reason (R): It responds to global thunderstorm activity rather than to conditions locally.
Answer: D
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 fair weather field is maintained by the world’s thunderstorms acting as a global generator, so it peaks when the total global thunderstorm activity peaks - at about 1900 universal time, when the storms over the Americas are most active. Over the ocean, away from local sources of variation, that global signal is what is measured. It is called the Carnegie curve.
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