#17 - Discharges of lightning from a volcanic ash

[Jeff Wren]

Hi lsatstudent,

I wouldn't necessarily use the word "useless" (that strikes me a rather extreme), but you're correct that a lack of lightning does not guarantee that the volcanic ash cloud is below an altitude of 5 kilometers since that height is necessary for lightning discharges rather than sufficient for them.

However, as the "discharges become progressively more frequent as the ash cloud moves higher still," it may be the case that a lack of lightning would suggest that the ash cloud is not significantly higher than 5 kilometers (and perhaps even probable that it is under 5 kilometers), although again this is not certain based on the information provided in the stimulus.

[lsatstudent99966]

Hi lsatstudent,

I wouldn't necessarily use the word "useless" (that strikes me a rather extreme), but you're correct that a lack of lightning does not guarantee that the volcanic ash cloud is below an altitude of 5 kilometers since that height is necessary for lightning discharges rather than sufficient for them.

However, as the "discharges become progressively more frequent as the ash cloud moves higher still," it may be the case that a lack of lightning would suggest that the ash cloud is not significantly higher than 5 kilometers (and perhaps even probable that it is under 5 kilometers), although again this is not certain based on the information provided in the stimulus.

Thank you very much Jeff! This is very clear

[Tyler1237]

Hi,

Would this be a defender assumption answer type? I got this question right but was unable to come up with a prephrase before going into the answer choices, and chose B only after reading them all. I’m just wondering if there is a missing link I may have missed when reading the stimulus.

[Dana D]

Hey Tyler,

I would say this is more a defender question, yes, although the arguement is certainly weakened if answer choice (B) is negated. Here, the author concludes that lightning discharges are sometimes the only reliable indicator of ash clouds, implying that the discharges can be observed even where weather radar is not available. If the discharge was not available everywhere radar isn't, then discharge wouldn't sometimes be the only reliable indicator - it would still be weather radar or nothing. However you think of it (negation vs defender), the key to pick up on before heading to the answer choices is the authors assumption that lightning discharge is sometimes superior to radar, which means sometimes discharge has to be operable in places radar is not (based on the info presented here).

hope that helps!