Atmospheric pressure decreases with an increase in altitude because:

Analyze the question: We are asked why atmospheric pressure drops as we go higher above the earth's surface.

Explain the concept: Atmospheric pressure at any point is caused by the weight of the air column above that point. It is also related to how often air molecules collide with a surface and how hard they hit it.

• At sea level, a tall column of air lies above you, so the pressure is high.
• As you climb, less air remains above you, so there is less weight pressing down.
• The air also becomes less dense at higher altitude. There are fewer molecules per unit volume, so there are fewer collisions and less interaction among them.

Check each option:

• A. Acceleration due to gravity increases with altitude. False. Gravity actually decreases very slightly with altitude, and that is not the main reason for the drop in pressure.
• B. Temperature increases with altitude. False. In the lower atmosphere (the troposphere, where we live and climb mountains), temperature decreases with altitude. In any case, temperature is not the reason for the pressure drop.
• C. Internal energy increases with altitude. False. This has no basis as the cause of lower pressure.
• D. Interaction among air molecules decreases due to less quantity of air. This is the best option. Higher up, there is less air, so fewer molecules and fewer collisions, and the pressure is lower.

Answer: D. It is the only option that describes the real cause: there is less air at higher altitude.

Tip: Remember "higher up, less air above, less pressure." For any WASSCE question on pressure variation, think of the weight or amount of fluid above the point. The same idea explains why pressure increases with depth in liquids. Also remember that gravity decreases slightly with height, so any option claiming it increases is wrong.