1. WATER CYCLE 4: precipitation and runoff
Air uplift
Condensation
Cloud droplets
Precipitation
Excess runoff
We are grateful to Hodder for their permission to use the diagrams in this resource.
WJEC Focus Box 3.1.4
Eduqas Focus Box 2.1.4

2. Water Cycle 4 Air uplift, condensation, clouds
Air uplift leads to cooling and condensation because of the fall in pressure with altitude
This results in of the expansion of the air
As a result, there are fewer collisions between air molecules: this reduces the amount of heat energy per unit volume and air temperature falls
Cloud formation occurs when temperature is low enough for vapour to condense into water droplets
Inquiry question: How do condensation and clouds form?
Independent reading for students:
Class activity
Why does condensation occur on windows? Why is your breath visible when you exhale on cold days?

3. Water Cycle 4 Air uplift
Inquiry question: How do condensation and clouds form?
Orographic rainfall is the result of uplift related to relief features, augmented by the so-called feeder-seeder mechanism. The effects are most obvious when warm maritime air encounters a mountainous coastal margin (for example, along the West coast of Scotland or the Cumbrian coastline). As a consequence, the area beyond a relief barrier may suffer from a ‘rain shadow’ effect with much drier conditions. 
Convectional rain results from intense daytime heating of the land. Air parcels adjacent to the ground heat up by conduction; they rise, cool and may form cumulus or cumulonimbus cloud.
Frontal rain forms when two surface air streams meet. For instance, when polar (cold) and tropical (warm) air masses meet over the North Atlantic ocean in the mid-latitudes, the latter will rise up over the former under low pressure conditions (which result from divergent air flow in the upper atmosphere).
Feeder-seeder mechanism A process that increases levels of orographic rainfall. Water droplets or ice particles from high altitude ‘seeder’ clouds fall through a lower-level orographic stratus cloud (the ‘feeder’ cloud), collecting more cloud water as they do so, which results in heavier rainfall.
Independent reading for students:
(c) Hodder Education [Edexcel A-level Geography Book 2 (Dunn)]

4. Water Cycle 4 Air uplift mechanisms
The feeder-seeder mechanism involves water droplets from high altitude ‘seeder’ clouds falling through a lower-level orographic stratus cloud (the ‘feeder’ cloud), collecting more cloud water as they do so, which results in heavier rainfall
Orographic rainfall is related to relief features, augmented by the feeder-seeder mechanism
Convectional rain results from intense daytime heating of the land
Frontal rain forms when two surface air streams meet e.g. when polar (cold) and tropical (warm) air masses meet over the North Atlantic ocean
Inquiry question: How do condensation and clouds form?
Independent reading for students:
Class activity
Think about how the three mechanisms could interact, e.g. when a weather front crosses a relief barrier

5. Water Cycle 4 Precipitation formation
Air uplift and condensation does not always lead to precipitation; further physical processes must operate
Air uplift
Condensation
Cloud droplets
Precipitation
Inquiry question: What are the mechanisms of precipitation formation?
This is a complex area of science. A relatively simple account is found at whereas more detailed tutorials are also available online, such as:
Class activity
Is it possible to tell which clouds will produce rain or snow? What evidence do you look for?

6. Water Cycle 4 Bergeron-Findeisen process
Clouds at high altitude contain a mixture of water droplets and ice crystals; the ice crystals grow rapidly by attracting vapour from the water droplets
Eventually, the ice crystals become too large to be held aloft; in falling to the ground, they pass through warmer air and melt to produce rain
Class activity
Watch this short film:
Inquiry question: What are the mechanisms of precipitation formation?
A video showing the Bergeron process:

7. Water Cycle 4 Collision process
A second theory explains rainfall in the warm tropics
‘Super-sized’ condensation nuclei, e.g. large sea salt particles, provide ‘seeds’ around which very large water droplets form
The larger ‘super’ droplets fall and collide with smaller droplets, absorbing them  
In support of this theory, unexpected downpours and flash flooding in arid areas demonstrate how high numbers of large raindrops can be generated quickly
Inquiry question: What are the mechanisms of precipitation formation?
The collision theory is outlined in this short film:
Class activity
Watch this short film:

8. Water Cycle 4 Excess runoff generation (1)
Runoff is all of the rainwater that contributes to river discharge following a rainfall event
Three drainage basin water flows contribute to runoff: overland flow, throughflow and groundwater flow
The balance between these three water pathways determines how flashy the river response is; and thus any likelihood of flooding
Inquiry question: Why is excessive runoff generated in certain climatic zones?
Group reading and discussion of library or online materials dealing with monsoon rainfall, prolonged and intense rainfall (eg hurricanes) and ice melting. Students working in pairs or groups can each present a short PowerPoint dealing with one of these three scenarios. These topics are covered in most A-level textbooks, for both past and present courses. Online support can be found at:
Class activity
Research how excess runoff results from (1) monsoon rainfall (2) UK storm events and (3) snowmelt (show your results in a table)

9. Water Cycle 4 Excess runoff generation (2)
Storm
Precipitation
Prior discharge
Peak discharge
Storm amount (mm)
Average intensity (mm / hr)
Maximum intensity (mm / hr)
Litres per second A 12 3 10 57
1034 B 11 2 70
694 C 30 4 9
1019 D 16 1 79
665
Answers to Question 1
Storm A generates of the highest peak discharge despite its relatively low amount of rainfall. This is explained by the high maximum intensity of 10 mm per hour. Most likely, this generated infiltration-excess overland flow resulting in a very short lag time. Storm B was the smallest storm in terms of total rainfall. However, proceeding discharge was relatively high, suggesting the soil may have been partly saturated. As a result, some throughflow or overland flow may have been generated, and so peak discharge was not the very lowest of the four storms. Storm C was the largest in terms of total amount. However it lasted for a long time - 10 hours - and occurred at a time of year when the preceding discharge was extremely low i.e. most likely summer. Therefore we can assume that most of the rainfall was able to infiltrate dry soil during the first hours of the storm. Only much later did the soil become saturated, resulting in saturation-excess overland flow and a high peak discharge. Storm D lasted for 16 hours. Despite having the highest preceding discharge, it resulted in the lowest peak discharge. Most likely, the extremely low intensity allowed all of the rainwater to infiltrate the soil or evaporate from vegetation surfaces.
Class activity
1. Explain the variations in peak discharge for each of the four storm events  
2. State and comment on three factors affecting run-off

10. Water Cycle 4 Land use changes and runoff (1)
Urbanisation renders previously permeable ground surfaces impermeable
Surfaces like concrete and tarmac increase overland flow generation and decrease the effectiveness of infiltration, throughflow and soil storage
The more ground that is covered by impermeable hard surfaces such as concrete or paving slabs, the less rainfall will soak into the ground and the more will flow over the surface into drains and sewers
Inquiry question: How and why is excessive runoff generated by certain human activities?
This topic (and the associated topic human causes of flooding) is covered in most A-level textbooks, for both past and present courses. Land use and river basin mismanagement should be focused on.
Class activity
Vegetation is removed to make room for urban areas. Why does this also contribute to increased runoff?

11. Water Cycle 4 Land use changes and runoff (2)
In England, Wales and Scotland, much of the forest which once covered the land has been removed
Flood risk in some parts of the UK is undoubtedly higher as a result of these changes
One  study in mid-Wales found that rainwater’s infiltration rate into the soil was 67 times higher under trees than under sheep pasture
Inquiry question: How and why is excessive runoff generated by certain human activities?
Excessive runoff and flooding in Wales:
Class activity
What other examples are there of recent floods in the UK? Might greater forest cover have prevented them?

12. Water Cycle 4 River basin mismanagement
(c) Hodder Education [Edexcel A-level Geography Book 2 (Dunn)]
Inquiry question: How and why is excessive runoff generated by certain human activities?
Class activity
Describe and explain the hydrological changes resulting from river basin mismanagement as shown above

13. Water Cycle 4 Plenary
There are three main causes of air uplift, condensation and cloud formation: they are the orographic, frontal and convection mechanisms
There are two recognized theories of precipitation formation: the Bergeron-Findeisen process and the collision process
Excess runoff generation occurs naturally in many parts of the world for climatic reasons. These include monsoon rainfall and annual snowmelt
Increasingly, human factors are to blame for overland flow generation in many drainage basins. Two important causes are urbanisation and forest removal