Which of the following best explains why atmospheric pressure decreases as altitude increases on Earth?

• A. Gravity is stronger at higher altitudes, pulling air molecules away from the surface.
• B. The temperature increases with altitude, causing the air to expand and increase pressure.
• C. There are fewer air molecules above you at higher altitudes, resulting in less weight pressing down.
• D. The composition of the atmosphere changes drastically at higher altitudes, increasing pressure.

Answer: (C)

Verified step by step guidance

1. Understand that atmospheric pressure is the force exerted by the weight of air molecules above a given point on Earth's surface.
2. Recognize that as altitude increases, the number of air molecules above that point decreases because the atmosphere becomes less dense.
3. Recall that pressure is related to the weight of the air column above, so fewer air molecules mean less weight pressing down, resulting in lower pressure.
4. Note that gravity actually decreases very slightly with altitude but not enough to cause an increase in pressure; instead, gravity pulls air molecules toward Earth, keeping most of them near the surface.
5. Understand that temperature and composition changes can affect pressure locally but do not explain the general decrease in atmospheric pressure with altitude.