What this lesson teaches
I can explain how the water cycle, humidity and rising air cause different kinds of rain.
Syllabus K329, Topic 3.1, Key Question 3. Why does precipitation vary across Earth's surface?
Make a guess
Moist wind from the sea blows into a mountain range. Which side of the mountains is likely to be drier?
- The top, because mountains squeeze water out of clouds.
- The far side, away from the wind.
- The side facing the wind.
- Neither side, because mountains do not affect rain.
Show the answer
The far side, away from the wind.
The air rises and rains on the windward side. On the leeward side it sinks and warms, and it has already lost moisture, forming a rain shadow.
In both types of rain, moist air rises, cools and condenses; the difference is what makes the air rise.
When air rises, pressure drops, so the air expands and cools. At the dew point, water vapour, which is invisible, condenses on tiny particles like salt or dust. These are condensation nuclei. This forms a cloud of visible droplets or ice crystals. Cloud droplets then bump together and join, which is called coalescence. Once they are heavy enough, they fall as rain, though some evaporate on the way down.
Convectional rain starts with strong heating of the ground. Warm air rises fast, cools and condenses. Tall clouds build and heavy showers fall. This explains many afternoon showers in places near the equator, like Singapore.
Relief rain starts when moist wind is forced up a hill or mountain. The windward side, facing the wind, gets rain. On the leeward side, the air sinks, is compressed and warms, so its relative humidity falls; it has also lost moisture. This creates a drier area called a rain shadow. Which side is windward depends on the wind direction.
- Convectional rain
- Heated air rises by itself.
- Relief rain
- A hill or mountain forces air to rise.
- Rain shadow
- The drier leeward side of a mountain.
Step by step
Air rises
Heating lifts it, or a mountain forces it up.
Air cools
At the dew point, vapour condenses on tiny particles.
Drops grow and fall
Droplets join until they are heavy enough to fall.
Worked example: A complete explanation
Moist sea air is forced up a mountain. It expands and cools. At the dew point, vapour condenses and droplets grow and fall as rain. On the far side, the air sinks and warms, so that side is drier.
Watch out for this
Mountains squeeze water out of clouds.
Mountains lift the air. The rain comes from cooling, condensation and droplets growing.
Check your understanding
Which step happens in both convectional and relief rain?
- Sinking air cools and makes rain clouds.
- Moist air rises, expands and cools as it goes.
- A mountain or hill forces the air to rise.
Show the answer
Moist air rises, expands and cools as it goes.
Yes. Only the cause of rising is different.
Check your understanding
Why is the leeward side of a mountain often dry?
- The leeward side is hotter because it faces the sun for more of the day.
- Clouds cannot cross the mountain top, so they stay on the windward side.
- Sinking air warms, so humidity falls, and it has already lost moisture.
Show the answer
Sinking air warms, so humidity falls, and it has already lost moisture.
This creates a rain shadow.