LSAT 149 | Section 1 | Logical Reasoning: Q17

LR Question 17 Explanation

QUESTION TEXT: Discharges of lightning from a volcanic ash cloud occur…

QUESTION TYPE: Necessary Assumption

CONCLUSION: Sometimes, lightning discharge is the only way we can tell how high an ash cloud is.

REASONING: Ash clouds from volcanos only start making lightning at a height of 5 km. Then the higher they get, the more lightning there is. In some parts of the world, there is no weather radar available to measure ash cloud height.

ANALYSIS: This argument shows that weather radar won’t work. But, how do we know that lightning measurement will work? 5km above a volcano is quite high up, and we’re not told whether these “lightning discharges” are as bright as regular lightning. It might be more like little sparks in an ash cloud.

So, the author has assume the lightning discharges are detectable.

___________

1. This isn’t necessary. The argument only said that lightning can sometimes let us see how high an ash cloud is. It doesn’t have to work every time, so it’s possible for some ash clouds not to get high enough.
    
   This answer would have been right had it said “The highest point of some volcanic ash clouds….”
2. CORRECT. If this isn’t true, then we can’t use lightning discharges where there is no radar.
   Negation: Without weather radar, we can’t detect lightning discharges.
3. This actually doesn’t matter. The argument is talking about what we can do without weather radar. So what weather radar does do when it measures altitude is not relevant. And we don’t care about regular clouds, we only care about volcanic ash clouds.
4. There are two problems with this. First, we’re talking about what we can do without weather radar. So it doesn’t really matter how well weather radar works when we can use it.
    
   Second “partly beyond” doesn’t necessarily matter. As long as radar can measure the bottom of the cloud, it can say how high off the ground it is. Doesn’t matter if it can’t reach the top. Some measurement is better than none.
5. The stimulus made no distinction between large and small ash clouds. This isn’t relevant.

Recap: The question begins with “Discharges of lightning from a volcanic ash cloud occur”. It is a Necessary Assumption question. Learn how to master LSAT Necessary questions on the LSAT Logical Reasoning question types page.