#17 - Discharges of lightning from a volcanic ash

[Administrator]

Please post your questions below! Thank you!

[BostonLawGuy]

Greetings!

I am not so sure how the author assumes "lighting discharges can be detected" even in the absence of weather radar.

I am not sure what weather radar has to do with detecting lighting discharges as it seems only to be able to measure the altitude of "ash clouds" in which lighting takes place. But there is not mention that they can also detect lighting.

I'm seriously missing something in this question.

[Malila Robinson]

Hi BostonLawGuy,
The argument is that:

Lightning from ash clouds discharges only when the ash clouds's highest point hits an altitude of 5km.

Radar can measure the altitude of the ash clouds.

Radar is not available everywhere.

(Conclusion)- So, sometimes the only way you can determine the altitude of ash clouds is through lightning discharge data.

To get to this conclusion the "sometimes" (in bold above) must be referring to the times when there is no radar data avilable, otherwise the discharge data would not be the "only" (in bold above) method, you would be also able to use the radar data. This makes answer B correct.

Hope that helps!
-Malila

[mluskey15]

Hi! I sort of understand the explanation above, but was wondering if you can clarify where the assumption that "Lightning discharges can be detected in some regions in which weather radar is unavailable" fits in? Thanks!

[Christen Hammock]

Hey Mluskey!

The conclusion here is that we can sometimes use lightning discharge data to measure ash cloud altitudes. The Assumption Negation Technique works great to clarify the right answer here: If lightning discharges could not be detected in some regions in which weather radar is unavailable, it wouldn't work as a substitute for measurement, and the conclusion would fall apart!

Christen

[Chantal]

Hi all: I was stuck between (A) and (B) for this question, but unfortunately chose the wrong answer (A). Could someone explain where I went wrong in my thinking?

My negation of AC (A): "The highest point of any volcanic ash cloud will NOT eventually exceed an altitude of 5km."
If that's true, then doesn't that mean we can't use lightning discharge data to measure the altitude of ash clouds? Since discharges of lightning only occurs when the cloud's highest point exceeds 5km, if the highest point of ANY volcanic ash cloud will NOT eventually exceed an altitude of 5km, then we won't have any discharges of lightning to collect data from.

But since (A) is wrong, my negation must be wrong too because I feel like what I have above does destroy the conclusion, when it shouldn't.

Thank you in advance!
Chantal

[Adam Tyson]

Good question, Chantal, and you're right that the problem is in your negation. Here's a trick to help with more accurate negations of assumption answers: start with the thought "that's not true."

Answer A's uses the word "any," which is another way of saying "all" or "every." It means that every volcanic ash cloud must at some point rise above 5km, with no exceptions. To negate that, we say "that's not true! They don't ALL have to rise that high. Some of them do not!" So we don't say "they all do not," but just "some of them do not," because the logical opposite of "all" is "not all," aka "some don't."

Does the author have to assume that ALL volcanic ash clouds get that high? Nope, because even if some do not, it still could be the case that in some places that don't have weather radar, lightning discharge might be our only reliable indicator of altitude. Answer A is too extreme, and the author isn't committed to believing that any volcanic ash cloud will reach that altitude, just that at least some will do so in places where weather radar is not available.

Be sure your negations are based on logical opposites rather than polar opposites to avoid this problem! Pairs like "All" and "not all"; "some" and "none"; "must" and "doesn't have to"; etc.

[BMM2021]

Hi,

Answer B makes sense to me, but I actually read the answers from the bottom up and liked E initially. Since lightning frequency is positively related to ash cloud altitude, I figured if there is some lightning-frequency discrepancy between large and small ash clouds, that would undermine the reliability of discharge data in indicating cloud altitude (i.e. a cloud with more frequent lightning would be considered to be relatively higher in altitude compared to another cloud with less frequent lightning; however they could both be at the same altitude but simply differ in size, if E is negated.) Thus I thought the negation of E undermined the "reliable indicator" portion of the conclusion, and moved on.

What did I get wrong here?

Thanks

[Adam Tyson]

What you're missing there, BMM, is that the conclusion is not about lightning discharge always being reliable, but about it sometimes being the only reliable method. The argument is based on the premise that in some places, weather radar isn't available. The author has to assume that in those places where there is no radar, lightning discharge data IS available. Otherwise, there would be no place in which lightning discharge data is the only reliable indicator. Any place where that method works, weather radar would also work!

The negation of answer E has no impact on that argument. If we say lightning discharges ARE more frequent for large clouds than small ones, that does nothing to tell us whether there are places in the world where lightning discharge is the only method we could use. It says nothing about what might occur in those places in which weather radar is not available. Since that negation doesn't harm that conclusion, it's a loser! The correct answer will, when negated, ruin the argument.

[lsatstudent99966]

Does lightning data tell us anything when volcanic clouds are below 5 km?

At first, I thought it might still be useful—like if there’s no lightning, that could mean the cloud is below 5 km. But after rethinking, I’m not sure that makes sense because even when clouds are above 5 km, lightning isn’t guaranteed. So, the absence of lightning doesn’t necessarily indicate anything about cloud height, and therefore lightning data is useless when the cloud is under 5 km?

(Asking this only to confirm if my understanding of the stimulus is correct!)

Thanks in advance!