PrepTest 120, Section 1, Question 18
In the troposphere, the lowest level of the(72 more words not shown)
If the statements above are(10 more words not shown)
- AThe troposphere over(10 more words not shown)
- BIt is warmer(21 more words not shown)
- CThe temperature in(28 more words not shown)
- DThe temperature at(26 more words not shown)
- EDepletion of the(19 more words not shown)
Reading the argument
First the air gets colder as we move upward. Then it starts getting warmer. Keep those two stretches separate.
The first stretch is the troposphere. At its top, the temperature is −50°C over the poles and −85°C over the equator. The equatorial figure is colder at that height, even if that surprises us based on temperatures we associate with the ground.
The next stretch is the stratosphere. It begins where the troposphere ends, and temperature increases as we continue straight upward. Ozone absorbing ultraviolet sunlight supplies the explanation for that warming. So the picture is cooling up to the boundary, then warming above it.
What we’re looking for
We need a statement that must be true. I don't have a precise choice predicted, but I want to be careful about what the description actually gives us. It tells us the direction of temperature change along an upward path. It doesn't give every temperature or tell us how fast warming occurs in different parts of the world.
Working the answers
- A
We have temperatures, not measurements of the layer's thickness. The difference between −50°C and −85°C doesn't tell us how thick the troposphere is over either location. We'd need more information to connect these facts with relative thickness.
- B
The comparison given is at the top of the troposphere. B moves it to the top of the stratosphere. Although both upward paths get warmer above the boundary, we aren't told how much either warms. The initially colder equatorial path could gain more heat, so its final temperature needn't remain lower.
- C
Moving the comparison to the middle of the stratosphere has the same problem. Warming on both paths doesn't tell us which path reaches the higher temperature there. We can't carry the pole-versus-equator ranking to a new height without knowing the changes along each path.
- DCorrect
Stay on one vertical path. At the top of the troposphere, the stratosphere begins; from there, the temperature increases as we go upward. The top of the stratosphere therefore cannot be colder than the top of the troposphere directly below it.
For example, a path starting at −50°C at the boundary then goes to temperatures above −50°C. We don't need to know the final number to know it is at least as great. D compares the two endpoints on that same path, avoiding the unsupported comparisons between different locations.
- E
Ozone is described as generating heat when it absorbs ultraviolet sunlight. That doesn't establish that removing ozone would raise the stratosphere's temperature. We also aren't told what effect depletion would have on the troposphere. E adds a two-layer prediction that the stated mechanism doesn't support.
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