Explain a rainfall mechanism with a diagram

To explain rainfall, write the chain: air rises, cools, water vapour condenses, cloud forms, rain falls. The only thing that changes between rainfall types is what makes the air rise.

This lesson sits in the weather and climate module and builds on reading the rainfall values in read a climate graph correctly.

What makes air rise?

• Convectional: the ground is heated by the sun, warms the air above it, and the warm air rises.
• Relief (orographic): moving air meets high ground and is forced upward.
• Frontal: a warm air mass meets a cold one and rides up over it.

After that, the sequence is the same: cooling, condensation, cloud, rain.

Convectional rain, step by step

1. The sun heats the ground strongly.
2. The ground warms the air touching it.
3. Warm air rises.
4. Rising air cools as it climbs.
5. Water vapour condenses into droplets at the dew point.
6. Cumulonimbus cloud forms, and heavy rain falls, sometimes with thunder.

Here is a simple diagram you can redraw in an exam.

          cloud (condensation)
        ~~~~~~~~~~~~~~~~~~~
          ^   ^   ^   rain falls
          |   |   |   / / /
   warm air rises, cooling
          |   |   |
  ======= heated ground =======
          sun's heat

Worked example

Question: Explain how relief rainfall forms. Use a diagram.

Diagram (labels): a hill with moist air arriving from the sea on the windward side, an arrow up the slope, a cloud near the summit, rain on the windward side, and a drier area on the leeward side (the rain shadow).

Answer:

1. Moist air blows in from the sea and meets a range of hills.
2. The hills force the air to rise.
3. As it rises, the air cools.
4. Cooler air cannot hold as much water vapour, so the vapour condenses into cloud droplets.
5. Rain falls on the windward slope.
6. The air descends on the leeward side. It has lost much of its moisture, so that side is drier. This is a rain shadow.

Each sentence leads to the next with a cause, which is what earns marks.

The mistake to watch for

Mistaken answer: “Hot air rises so it rains.”

The student skipped cooling and condensation, the two steps that actually make water droplets.

The correction is to add the missing links: “Hot air rises, cools, and the water vapour condenses to form cloud, then rain.” A useful check is to count your causes. If one sentence jumps from the start to the rain, an examiner cannot give credit for the steps you did not write.

Check yourself

1. Name the process that turns water vapour into cloud droplets.

2. State what causes the air to rise in relief rainfall.

3. Explain in two sentences why the leeward side of a mountain can be dry.

Where this leads next

Next, see why measurements of rain and temperature can mislead in interpret weather observations with instrument limits. The statistics and distribution explorer helps you compare rainfall totals. Writing explanations in clear steps is something our teachers practise with students in online one-to-one Geography tuition.