Go up a mountain and two things change together: the air gets thinner and it gets colder. Both come from simple physics, and both are examinable as calculations. This is one of the few genuinely numerical sections in the topic, so it is worth easy marks.
📚 What you need to know
Gravity holds the atmosphere in place, pulling molecules towards the surface. That makes air denser at lower altitudes.
Gravitational force is inversely proportional to distance from Earth, so its pull weakens with height.
As a result, pressure and density fall with altitude — the atmosphere gets thinner.
Atmospheric thinning means fewer molecules in a given volume of air, so less oxygen per breath and a risk of altitude sickness.
The standard lapse rate: temperature falls by about 1 °C for every 100 m gained in altitude.
The lapse rate applies within the troposphere, up to around 10 km.
Why the air thins out
Gravity pulls every gas molecule towards Earth. Near the surface the molecules are pressed together by the weight of all the air above them, so air there is dense. Higher up there is less air pressing down, and the gravitational pull is slightly weaker, so molecules are further apart.
This is the point students miss: air at 8,000 m is still about 21% oxygen. There is simply far less air, so each breath delivers much less oxygen to the blood.
High-altitude climbers carry oxygen not because the air composition changed, but because the air pressure is too low to push enough oxygen into the bloodstream. Saying that correctly is worth a mark on its own.
The standard lapse rate
Learn this rate
Temperature falls by about 1 °C for every 100 m of altitude gained
(the same as 10 °C per km, within the troposphere)
Real lapse rates vary with humidity and weather, which is why this one is called the standard rate. For exam calculations, use it as given.
WORKED EXAMPLE
A weather station at the base of a mountain sits at 200 m and records 18 °C. The summit is at 3,400 m. Use the standard lapse rate to estimate the summit temperature. [3]
Step 1: find the height gained3400 – 200 = 3200 mStep 2: how many 100 m steps is that?3200 ÷ 100 = 32Step 3: apply 1 °C per steptemperature drop = 32 × 1 = 32 °C18 – 32 = -14Summit temperature is about -14 °CUse the height gained, not the summit height. Starting from 3,400 instead of 3,200 is the standard error here.
Why this shapes ecosystems
The lapse rate is not just a physics fact — it explains why a single mountain can hold several different habitats.
Two places can be a few kilometres apart horizontally and have completely different climates, purely because of height.
Lower slopes may support forest, while higher ground is snow-covered or supports only low-growing plants adapted to cold and wind.
Species distribution changes with altitude in much the same way it changes with latitude — a useful comparison in essay answers.
Falling temperature and thinning air together limit which organisms can survive high up.
Exam boundary: you do not need to memorise specific pressures or gas volumes at particular altitudes. You do need the lapse rate, the reason for thinning, and the 10 km limit of the troposphere.
💡 Exam tip
Write the lapse rate as 1 °C per 100 m and convert carefully. Most errors are unit errors, not concept errors.
Give gravity as the reason for density change, and mention the inverse relationship with distance.
Say the percentage of oxygen stays roughly constant — it is the density that drops.
State that the lapse rate applies within the troposphere. Extending it to 50 km is wrong.
Bring in altitude zonation of vegetation if a question links atmosphere to ecosystems.
⚠️ Common mix-up
Saying there is less oxygen as a proportion. There is less air, so fewer oxygen molecules per breath.
Using the summit height instead of the height gained in lapse rate calculations.
Thinking it gets colder because you are closer to space. The surface is the heat source; you are moving away from it.
Applying the lapse rate above the troposphere. In the stratosphere temperature rises with height because ozone absorbs UV.
Confusing pressure and density with composition. The mix of gases is much the same; the amount is not.
Up next: Milankovitch Cycles (HL) — the slow orbital changes that drive ice ages, and why they cannot explain what is happening now.
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