#18 - In the troposphere, the lowest level of the earth’s

[Administrator]

Complete Question Explanation

Must Be True-#%. The correct answer choice is (D)

In this stimulus, we are bombarded with scientific facts: In the troposphere, the lowest level of the earth's atmosphere, the temperature decreases steadily as you go up, with temperatures ranging significantly from poles to equator. In the layers of the stratosphere, which is warmed by ozone, the temperature increases steadily as you go up.

As long as we can keep the layers and their attributes distinct, we can effectively attack this Must Be True question regardless of its scientific and numerical nature.

Answer choice (A): There is no discussion of the relative thickness of the various areas of the troposphere, so this answer choice is unsupported by the information in the stimulus.

Answer choice (B): Although the author discusses the variation in temperature, no information is provided regarding this specific comparison, so this choice fails the Fact Test and is incorrect.

Answer choice (C): This answer choice is incorrect for the same reason that answer choice (B) above is incorrect: we don't have specific information to assess this comparison.

Answer choice (D): This is the correct answer choice. We know that at the point where the stratosphere begins, the temperature immediately starts to increase, so the statement provided by this answer choice must be true.

Answer choice (E): The author does not provide sufficient information in this short stimulus to allow us to assess the effect of ozone depletion on the air temperature in the troposphere and stratosphere, respectively, so this answer choice cannot be correct.

[halisyaka]

Hi there,

I am having a hard time with this question. I picked B over D and here is my reasoning:

When we reach top of the troposphere, temperature is -50C over the poles and -85C over the Equator. Then the stratosphere starts where temperature gets warmer. So if we progress straight upwards, temperature would be warmer at the poles than at the Equator. For instance, if temperature increases by 10 degrees, then it would be -40C and -75C at poles and Equator, respectively.

What am I misunderstanding here? Also, could you please elaborate on answer choice D as its language is a bit confusing?

Thank you in advance.

[Luke Haqq]

Hi Halisyaka!

The explanation that you provided seems to make sense to me. The reason it wouldn't be the right answer choice, though, is that it's something that's likely to be true based on the given information. However, this is a must-be-true question, so an answer can't be correct if it's only likely to be true rather than necessarily being true.

Answer (B) states, "It is warmer at the top of the stratosphere over the poles than it is at the top of the stratosphere over the equator." We don't know from the stimulus how the temperatures differ between the Equator and poles for the stratosphere. The only information we have on that point concerns the troposphere. You're right to point out that the top of the troposphere would have the -50C/-85C differential between the poles and Equator--so it would seem likely that a differential would persist in the stratosphere. All that we know about the stratosphere is "the temperature stops decreasing and instead increases as one progresses straight upward through the stratosphere." It might increase dramatically and uniformly once one enters the stratosphere--for example, it's possible that the temperature is the same in the stratosphere on any part of the globe, whether the poles or Equator. Or it might increase non-uniformly, with the poles being warmer, as you suggest. We don't know that either of these is the case, which is why (B) can't be true.

Answer (D) states, "The temperature at any point at the top of the stratosphere is at least as great as the temperature at the top of the troposphere directly beneath that point." In other words, this is making a claim about the "top of the stratosphere," which we don't have any direct, specific information on. Yet while we don't know specific information about the top, we know from the passage that anything in the stratosphere must be warmer than anything in the troposphere. We do have information about the top of the troposphere (-50C/-85C), and we know that the temperature starts increasing as "one progresses straight upward through the stratosphere." The "directly beneath that point" language is reflecting this, and takes out the variable of poles vs. Equator. (D) is saying the troposphere at a given point is not hotter than a second point--if one moves straight up from that same point to the top of the stratosphere. We know that has to be true because the temperature increases in the stratosphere.

[T.B.Justin]

This is how I am understanding the wording of answer choice D:

If at a certain spot at the top of the troposphere the temperature is -60C, directly above that spot in the top of the stratosphere is -59C, then its at least as great as and cannot be lower, so D must be true.


Eliminating the incorrect answer choices was more effective for me than parsing out the language for answer choice D in a way that I could understand.

[Adam Tyson]

Good analysis, T.B.! Since the temp increases as you rise through the stratosphere, the temp at the top of that layer must be higher than the temp at the bottom of that layer where it meets the troposphere. Well done!