Explain why atmospheric pressure varies with height above the surface of…

The idea

This separate-science item asks you to explain, not merely state, why atmospheric pressure changes with height. As you move higher above the surface, there is less air above you, so the weight of the air column pressing down is smaller and the atmospheric pressure is lower. Near the surface the column of air is longest, so pressure is greatest; at high altitude the column is short and pressure is reduced. The explanation must connect height to the amount (and therefore weight) of air above the point, and hence to the pressure exerted. A useful check is to compare a point at sea level with a point high on a mountain: the mountain-top point has less air above it, so the pressure there is lower.

What earns the marks

• Explains that a greater height means less air above the point.
• Links the reduced amount of air above to a smaller weight of air column.
• Concludes that atmospheric pressure is lower at greater height.
• States that pressure is greatest at the surface where the air column is longest.

Common mistakes and how to put them right

• States only that pressure decreases with height without explaining the reduced weight of air above.

  Put it right: Give the causal link to the air column.

• Claims pressure increases with height because air is 'thinner'.

  Put it right: Explain that less air above means lower pressure.

• Confuses atmospheric pressure with temperature changes with altitude.

  Put it right: Focus the explanation on the weight of air above the point.

Exam tips

• Use the phrase 'less air above' to show the cause of the change in pressure.
• Chain the reasoning: greater height, less air above, smaller weight, lower pressure.
• Compare two heights explicitly to make the explanation clear and complete.