Q17 - Discharges of lightning from a volcanic ash cloud

[ohthatpatrick]

Question Type:
Necessary Assumption

Stimulus Breakdown:
Conclusion: Sometimes, lightning discharge data is our only reliable indicator of ash cloud altitude.
Evidence: Some parts of the world don't have weather radar to measure ash cloud altitude. Lightning discharge data would kick in at an altitude of 5km and become progressively more frequent as the ash cloud climbs.

Answer Anticipation:
Are we convinced that sometimes LDD is our only hope of measuring ash altitude?

We know in some parts of the world they don't have weather radar to measure the altitude. Do they have any other means besides LDD? The author assumes that it's either weather radar or LDD. Furthermore, do ash clouds ever actually occur in those weather radar 'deserts'? If we never have ash clouds in those parts of the world, we don't have to worry about finding another means to measure ash altitude. The author assumes that ash clouds DO sometimes occur in those parts of the world that don't have weather radar.

Are we convinced that LDD is itself a reliable indicator of ash altitude?

Not quite. The author is assuming that a measuring system that only kicks in at 5km and has some form of positive correlation with greater altitude could be considered 'a reliable indicator'.

Correct Answer:
B

Answer Choice Analysis:
(A) "Any" is dangerously strong. The author does need to assume that ash clouds AT LEAST SOMETIMES exceed an altitude of 5km. If they never did, then LDD would never be a useful indicator. But the author isn't claiming that we need to or can use LDD for "any" ash cloud ... just that "sometimes" we would need to and could use LDD.

(B) YES! In order for LDD to come to our rescue when we don't have the weather radar option, we have to assume that LDD is sometimes available when weather radar isn't.

(C) We don't need to assume anything about regular clouds. They are totally out of scope.

(D) We never said that weather radar was limited by altitude. We just said it was limited in certain parts of the world.

(E) New Comparison between "large" vs. "small", which is totally out of scope.

Takeaway/Pattern: This Assumption has a Fill-in-the-Blank feel. "We can't use weather radar in area Z, so sometimes LDD is our only hope." (assumes that we CAN use LDD in area Z.)

(C) and (E) should be spotted quickly for introducing New Comparisons. (A) should be red flagged for its strong language.

#officialexplanation

[AlexK795]

I was between A and B here and fortunately selected B because I felt that its negated version more directly weakened the argument. I am struggling, quite severely, however, to understand why the negated version of A would not weaken the argument as well - perhaps its because I am negating the statement incorrectly.

Negated A: the highest point of any volcanic ash cloud will not eventually exceed an altitude of 5 km. This would mean that lightning discharges would not occur given the altitude necessary for them was not met, and in turn make lightning discharge data impossible to collect - because the discharges would not occur - ultimately meaning that data about the discharges could not be "a" indictor, much less the only indicator, of the altitude of ash clouds.

Please explain why my thought process is wrong here.

Thanks!