What this lesson teaches
I can explain how the water cycle, humidity and rising air cause different kinds of rain.
Syllabus K226, Topic 3.1, Key Question 3. Why does precipitation vary across Earth's surface?
Make a guess
Some air cools down at night. No water is added to it. What happens to its relative humidity?
- It stays the same, because the vapour is unchanged.
- It drops to zero once the air is cold.
- It falls, because no water is added.
- It rises towards 100%.
Show the answer
It rises towards 100%.
Cool air needs less water vapour to be saturated. The vapour stays the same, so the air becomes closer to saturation.
Air becomes saturated when it cools enough, and on the ground, soil and surfaces decide where rainwater goes.
The water cycle moves water between stores. The Sun's energy evaporates water from seas, lakes, soil and other wet surfaces. Plants release water vapour through transpiration. Together, these are called evapotranspiration. Water vapour can then condense into droplets and fall back as precipitation.
On the ground, rain takes several routes. Infiltration is water soaking into the soil. Runoff flows over the surface. Groundwater moves slowly through soil and rock. Open, connected pores let water soak in, but packed soil and paving stop it. Runoff is usually much faster than groundwater flow, though speed also depends on slope. Plants catch rain, slow surface flow and help keep paths into the soil open.
Relative humidity shows how close air is to saturation, being as full of water vapour as it can be at that temperature. It is the actual amount of water vapour divided by the saturation amount at the same temperature, times 100. Warmer air needs more water vapour to be saturated. So if the amount of vapour stays the same, warming lowers relative humidity, and cooling raises it. Adding water vapour also raises it. Air is not a box that holds water; this is just a way to describe saturation.
The dew point is the temperature at which cooling air becomes saturated, without any water being added. At about 100% relative humidity, further cooling makes vapour condense, usually on tiny particles. But rain needs one more step: the droplets must grow big enough to fall to the ground.
- Evapotranspiration
- Evaporation plus water released by plants.
- Relative humidity
- How close air is to saturation, in %.
- Dew point
- Temperature at which cooling air reaches 100% humidity.
Step by step
Cool the air
The amount of water vapour stays the same.
Reach the dew point
Relative humidity rises to 100%.
Form droplets
Vapour condenses on tiny particles. Drops must grow before rain falls.
Worked example: Work out relative humidity
In a simple example, the air has 15 units of vapour, and would be saturated at 30 units at its temperature. Relative humidity is 15 / 30 x 100 = 50%. If the air cools until saturation is 15 units, humidity reaches 100%, without adding any water.
Watch out for this
Relative humidity can only rise if more water evaporates into the air.
It also rises when air cools, even if no water is added.
Check your understanding
Some air cools, with no water added or removed. What happens to its relative humidity?
- The vapour turns straight into rain as it cools.
- It falls, as cool air holds less.
- It rises towards 100%, as the air cools.
Show the answer
It rises towards 100%, as the air cools.
Yes. Cooler air is closer to saturation.