Precipitation And Excess Runoff Within The Water Cycle

Question 1

Name the three types of precipitation

Answer 1

Orographic
Frontal
Conventional

Question 2

Name the two competing theories on precipitation formation

Answer 2

The collision mechanism

The Bergeron-Findeisen process

Question 3

How is conventional precipitation formed?

Answer 3

1. Sun heats ground and warm air rises
2. As air rises it begins to cool and water vapour condenses
3. When saturation point is reached, large cumulonimbus clouds are formed
4. Heavy rainfall occurs

Question 4

How is frontal precipitation formed?

Answer 4

1. Area of warm air meets area of cold air
2. Warm air is forced over cold air due to different densities, creating the ‘front’
3. The warm, less dense, air cools and water vapour condenses
4. Clouds form and precipitation occurs

Question 5

How is orographic precipitation formed?

Answer 5

1. Moist air is lifted as it moves over a mountain range
2. As air rise it cools and water vapour condenses forming clouds
3. Most rainfall falls on mountain ridge, on Lee side of mountain rainfall is low (rain shadow)
4. Alternating bands of cloud and rain shadow is known as orographic bounce

Question 6

How do clouds form?

Answer 6

1. Cloud droplets get head start by forming on a particle e.g. dust, soot or sea salt. These are cloud condemnation nuclei
2. Cloud droplets grow - either by more water condense ms into each droplet or by several droplets coming together and coalescing
3. Continues up to 9mm in diameter
4. They fall as rain, snow or hail

Question 7

Name the three types of cloud

Answer 7

Stratus
Cumulus
Cirrus

Question 8

What is cloud seeding?

Answer 8

By changing the number of cloud condensation nuclei in the atmosphere, you can control how many cloud droplets there are
Adding nuclei to clean atmosphere could cause a cloud to form
Substances used in cloud seeding included silver iodide, frozen CO² and salt
They are dispersed from aircrafts or fired into atmosphere

Question 9

What is air uplift?

Answer 9

Uplift occurs when hot air rises due to warm air expanding
As air expand it becomes less dense than cold air around it
Less dense hot air floats in denser cold air

Question 10

Outline the Bergeron-Findeisen process

Answer 10

1. Clouds at high altitude contain a mixture of water droplets and ice crystal
2. Ice crystal grow rapidly at expense of water droplets due to rapid flux of water vapour from droplets to ice crystals
3. Ice crystals fracture due to fast high altitude air currents. Hexagonal ice shapes develop with larger surface areas. More water vapour condenses around larger snowflakes accelerating growth
4. Eventually snowflakes become too large and fall under gravity. As they warm they melt to produce rain

Question 11

How has the Bergeron-Findeisen process proved and disproved?

Answer 11

Proved - Cloud seeding with dry ice can generate rainfall

Disproved - Cannot explain rainfall formation in warm tropics

Question 12

Outline the collision process

Answer 12

1. ‘Super sized’ condensation nuclei provide ‘seeds’ around which droplets form. They are far larger and heavier than normal droplets
2. These ‘super sized’ droplets collide with smaller droplets sweeping them along into their wake

Question 13

How has the collision process been proved?

Answer 13

Langmuir carried out experiments
He argued higher terminal velocity of large droplets allows them to absorb smaller droplets, causing rapid fusion and raindrop growth

Question 14

What are the three key physical causes of excess runoff?

Answer 14

Prolonged rainfall
Snowmelt
Monsoon

Question 15

How does prolonged rainfall affect surface runoff?

Answer 15

In the UK storms and intense rainfall are the most important contributors to excess runoff
Ground cannot hold anymore water and becomes saturated

Question 16

How does snowmelt affect excess runoff?

Answer 16

Increases global temperatures has reduced accumulation and increased ablation
Large volumes of summer meltwater and reduction in winter accumulation led to lower storage levels in ice sheets and glaciers
Seasonal river levels dropped by due to lack of meltwater

Question 17

How do monsoons affect excess runoff?

Answer 17

Monsoon rainfall would reduce in duration and there will be a 4°C increase
Fine aerosol particles and fossil fuels could stop monsoon rains in south central China and northern India

Question 18

What is the key human cause of excess runoff?

Answer 18

Urbanisation

Question 19

How does urbanisation affect excess runoff?

Answer 19

• Deforestation (Decrease evapotranspiration and interception, increased sedimentation)
• Houses (Decreased infiltration, increased storm flows and decreased base flows during dry periods)
• Residential/Commercial/Industrial areas (Decreased porosity, reducing time of runoff, increasing peak discharge and greatly increased volume of runoff and flood damage)
• Storm drains (Local flooding relief and concentration of flood water may cause flooding downstream)