Logical reasoning PrepTest 149 · Section 1 · Question 17

Incorrect. The argument doesn't rely on lightning discharges in all volcanic ash clouds, as the lack of lightning discharge would then be used to show that the ash cloud was under 5km.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Necessary Premise

Stimulus Summary:
Ash clouds have lightning when they hit a certain altitude, and there's more lightning as they get higher up. Weather technology can measure ash cloud altitude, but it's not available everywhere. Therefore, in some places, lightning discharge is the only way to measure altitude.

Answer Anticipation:
The conclusion here mentions what is "sometimes" the case, and it's clear that this applies to the parts of the world where the weather radar isn't available. So what do we know about those places?

Well, we know that there could be ash clouds there, and that we couldn't measure the altitude using weather radar. There's another means for measuring the altitude of these clouds there—lightning, which increases in frequency as the altitude increases. However, the conclusion is that this is the "only reliable" indicator of altitude. That's assuming there aren't alternatives to lightning discharge and weather radar to detect ash cloud altitude. Any answer choice that hints at ruling out an alternative means of measuring altitude will be necessary to reach this conclusion.

Answer Explanation:
The argument requires that the areas without weather radar can still detect these discharges. If these discharges can't be detected in regions without weather radar, then they can't be used to detect the altitude of the ash cloud, and the argument falls apart. (Note this answer doesn't match our anticipation, showing how important it is to stay flexible!)

Key Takeaway:
"It doesn't match my anticipation," isn't a sufficient reason to eliminate an answer choice. Sometimes, your anticipation will be wrong—either because you messed up the logic or because you are right in your analysis but the correct answer focuses on something else. Stay flexible in the answer choices!

Incorrect. Relative accuracy compared to regular clouds is out of scope. While the argument does require a certain level of accuracy, it doesn't need to be as accurate as it is in detecting regular clouds, especially since we don't know how accurate it is in measuring those.

Incorrect. The stimulus establishes that, where present, weather radar can measure the height of ash clouds, but that it's not present in all parts of the world. It very well could measure ash clouds up as high as they go in the parts of the world where it is present.

Incorrect. Tempting trap answer! It would be easy to think that if the frequency of lightning changes with ash cloud size, then it's not a reliable indicator of height. However, it's possible that that could be accounted for—maybe areas without weather radar can't measure altitude, but they can measure the size of the ash cloud, and then account for the different discharge rate for these clouds to measure their altitude.