Air can reach saturation simply by cooling. On the ground, soil and surface cover determine where rainwater goes next.
The water cycle moves water between stores. Solar energy drives evaporation from oceans, lakes, soil and other wet surfaces. Plants release water vapour through transpiration. Together, evaporation and transpiration are called evapotranspiration. Vapour can condense into droplets and return to the surface as precipitation.
Rainwater takes several routes. Infiltration is water entering the soil; runoff flows over the surface; groundwater flows through soil and rock. Open, connected pores let water infiltrate. Compacted soil and impermeable paving restrict it. Runoff is often faster than groundwater flow, but speed also depends on slope and the route taken. Vegetation intercepts rain, slows surface flow and helps keep pathways into the soil open.
Relative humidity tells you how close air is to saturation. More precisely, it is actual water-vapour pressure divided by saturation vapour pressure at the same temperature, multiplied by 100. The saturation value rises with temperature. If the amount of vapour stays the same, warming lowers relative humidity and cooling raises it. Air is not a physical container that holds water.
The dew point is the temperature at which cooling air reaches saturation without changing its water-vapour content. At about 100% relative humidity, further cooling allows vapour to condense, usually on tiny particles. Rain still needs another step: the droplets must grow large enough to fall and reach the ground.
Step by step
Cool the air
Keep the amount of water vapour unchanged as temperature falls.
Reach saturation
The saturation value falls until relative humidity reaches about 100% at the dew point.
Form droplets
Further cooling allows condensation on tiny particles. The droplets must still grow before rain falls.
Worked example: Calculate a relative humidity
In a simplified example, actual vapour pressure is 15 units and the saturation value is 30 units: relative humidity is 15 / 30 x 100 = 50%. Cooling until the saturation value becomes 15 raises it to 100%, without adding vapour. The units are illustrative, not a temperature table.
Watch out for this
Relative humidity can rise only when evaporation adds more water vapour.
It can also rise when temperature falls while water-vapour content remains the same.
Check your understanding
An unsaturated sample cools with no water vapour added or removed. What usually happens first?
- Its relative humidity falls.
- All its water vapour immediately becomes rain.
- Its relative humidity rises towards saturation.