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Lessons / Reading the weather

Reading the weather · 4 min · 4 steps

Why mountains are colder

Plays here

Goal. Use two cities' typical years to show how temperature falls with height.

Steps

  1. Predict

    Quito and Guayaquil in Ecuador are both close to the equator, about 270 km apart. Quito is 2,850 m up in the Andes; Guayaquil is on the coast. Which is colder?

    • Guayaquil
    • Quito
    • Both are the same
    Show the answer

    Quito. Quito: in the app's typical year it averages about 12 °C, against about 25 °C at Guayaquil.

    Quito's typical year: cool, and almost the same in every month, because it is near the equator but high in the Andes.

  2. Now compare Guayaquil, on the coast below: about 12 °C warmer, all year round.

  3. Air cools as it rises because the pressure on it falls and it expands. In the open atmosphere the temperature drops by about 6.5 °C for every kilometre on average; between places on the ground the drop is often smaller, about 4 to 5 °C per kilometre, as here.

  4. The same happens everywhere. In the app's typical year Lhasa, 3,650 m up on the Tibetan Plateau, averages about 7 °C, and Chengdu, at almost the same latitude but about 500 m up, about 17 °C. In Taiwan, snow falls on Yushan most winters while Taipei stays mild.

The common mistake

Myth

Mountain tops are closer to the Sun, so they should be warmer.

Fact. A few kilometres make no difference to the distance from the Sun, 150 million km away. The air is warmed mainly by the ground below it, and rising air expands and cools, so higher places are colder.

Check yourself

Question 1

On average, how much colder does the air get for each kilometre you climb?

  • About 0.5 °C
  • About 6.5 °C
  • About 65 °C
Show the answer

About 6.5 °C. About 6.5 °C per kilometre in the open atmosphere, which is why snow lies on high mountains even in the tropics.

Question 2

Why does rising air cool down?

  • It moves away from the Sun
  • The pressure falls, so it expands and cools
  • It mixes with snow
Show the answer

The pressure falls, so it expands and cools. Air that rises meets lower pressure, expands and uses up some of its energy, so it cools.

Question 3

Two cities lie at the same latitude, one at sea level and one 3 km up. Which is warmer on average?

  • The one at sea level
  • The one 3 km up
  • They are the same
Show the answer

The one at sea level. Three kilometres of height can make the average temperature well over 10 °C lower.

Words in this lesson

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Curriculum

Taiwan 12-year curriculum (108 課綱), natural science
Fa-Ⅳ-4
US Next Generation Science Standards
MS-ESS2-6
England national curriculum
KS3 Geography: weather and climate