Moist air rises, expands, cools and condenses in both types of rainfall. The difference is what lifts the air.
Rising air meets lower pressure, so it expands and cools. At its dew point, water vapour condenses on tiny particles such as salt or dust. These particles are condensation nuclei. The resulting cloud contains visible droplets or ice crystals; water vapour itself is invisible.
Cloud droplets must grow before they can reach the ground as rain. In warm clouds, droplets collide and join: coalescence. Large enough drops fall through the air and may reach the ground as precipitation. Some evaporate before arriving. Ice processes also produce precipitation in colder clouds.
Convectional rainfall starts with strong surface heating. Warm, buoyant air rises, cools and condenses. Continued rising air can build tall clouds and heavy showers. This helps explain afternoon showers in equatorial places, although other weather systems can bring rain at other times.
Relief rainfall starts when moist wind is forced up a hill or mountain. This windward side receives rain as air cools and condenses. On the leeward side, descending air is compressed and warms, lowering its relative humidity. It has also lost some moisture. These processes can create a drier rain shadow. Wind direction determines which side is windward; it is not always the same compass side.
Step by step
Air rises
Surface heating lifts air in convectional rainfall; a mountain forces it up in relief rainfall.
Air expands and cools
At the dew point, water vapour condenses on tiny particles.
Drops grow and fall
Droplets join through coalescence. Large enough drops can reach the ground as rain.
Worked example: A complete explanation
Moist air from the sea is forced up the windward mountain slope. Lower pressure allows it to expand and cool. At the dew point, vapour condenses on nuclei; droplets grow and fall as rain. The descending leeward air warms, giving lower relative humidity and a drier side.
Watch out for this
Mountains squeeze water directly out of clouds.
The barrier lifts moist air. Expansion, cooling, condensation and droplet growth explain the rain.
Check your understanding
Which step is shared by convectional and relief rainfall?
- Moist air rises, expands and cools towards the dew point.
- A mountain must force the air to rise.
- Descending air cools until rain forms.