So atmospheric pressure decreases with an increase in height.

The idea

Fewer molecules above a surface means a smaller weight of air pressing down, and fewer molecules around it means fewer collisions each second. Therefore, atmospheric pressure falls as height rises. For example, pressure at sea level is about 100 kPa, but at an airliner's cruising altitude, it drops to near 20 kPa. This is why cabins are pressurised to around 70 kPa. Each window experiences a resultant outward force calculated by multiplying the pressure difference by its area (e.g., 4 kN for an 810 cm² window). The decrease is non-linear: pressure roughly halves for every 5.5 km climbed, resulting in a curved graph. Altimeters utilise this relationship by measuring atmospheric pressure and converting it into an altitude reading.

What earns the marks

• State that there are fewer air molecules above a surface at a greater height.
• Explain that this results in a smaller weight of air acting on the surface.
• Identify that fewer molecules lead to fewer collisions with the surface each second.
• Conclude that atmospheric pressure is therefore lower at greater altitudes.

Common mistakes and how to put them right

• Stating only that there is 'less air', without linking this to weight or collision frequency.

• Failing to convert kilopascals into pascals before using pressure values in calculations.

• Assuming the graph of pressure against altitude is a straight line, rather than a curve.

Exam tips

• Extract the pressure value from the graph and write it down before starting any calculation.
• Always check units; expect to convert kilopascals to pascals and square centimetres to square metres.