Water Cycle 4: precipitation and runoff

Question 1

What leads to cooling and condensation because of the fall in pressure with altitude?

Answer 1

Air uplift

Question 2

Air uplift results in what?

Answer 2

Expansion of the air

Question 3

Because of the expansion of the air from air uplift, there are fewer what which reduces what?

Answer 3

There are fewer collisions between air molecules: this reduces the amount of heat energy per unit volume and air temperature falls

Question 4

When does cloud formation occur?

Answer 4

When temperature is low enough for vapour to condense into water droplets

Question 5

What are the three air lift mechanisms?

Answer 5

Orographic rainfall
Convectional rain
Frontal rain (cyclonic rainfall)

Question 6

Explain orographic rainfall

Answer 6

When air is forced to rise over a barrier (e.g. a mountain), it cools and condenses, forming rain. The leeward (downwind) slope receives relatively little rain, which is known as the rain shadow effect.

Question 7

What is orographic rainfall related to?

Answer 7

Related to relief features, augmented by the feeder-seeder mechanism

Question 8

Explain the feeder-seeder mechanism

Answer 8

Involved 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

Question 9

Explain convectional rain

Answer 9

Common in tropical areas, & in UK during summer
When land becomes hot, air above it expands and rises
As air rises, it cools and condenses, if it continues to rise, rain will fall

Question 10

Convectional rain results from what?

Answer 10

From intense daytime heating of the land

Question 11

Explain frontal/cyclonic rainfall

Answer 11

Happens when warm air, which is lighter and less dense, is forced to rise over cold denser air. It cools and condenses as it rises, forming rain

Question 12

When does frontal rain form?

Answer 12

When two surface air streams meet, e.g. when polar and tropical air masses meet over the North Atlantic ocean

Question 13

Air uplift and condensation does not always lead to what?

Answer 13

Precipitation

Question 14

What evidence should you look for to tell which clouds will produce rain or snow?

Answer 14

Thickness of clouds, the thicker the higher chance of precipitation
Looks gray - blocking more sunlight so looks more gray
Temperatures e.g. temps below 4 degrees has a greater chance of snow

Question 15

Explain the Bergeron-Findeisen process

Answer 15

Clouds at high altitude contain a mixture of water droplets and ice crystals; the ice crystals grow rapidly by attracting vapour from water droplets
Eventually, the ice crystals become too large to be held aloft; falling to the ground, they pass through warmer air and melt to produce rain

Question 16

What second theory explains rainfall in the warm tropics?

Answer 16

Collision process

Question 17

What is the process of ice crystal growth that occurs in mixed phase clouds?

Answer 17

Bergeron-Findeisen process

Question 18

Explain the collision process

Answer 18

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

Question 19

What is runoff?

Answer 19

All of the rainwater that contributes to river discharge following a rainfall event

Question 20

What three drainage basin water flows contribute to runoff?

Answer 20

Overland flow
Throughflow (through soil)
Groundwater flow (through rocks)

Question 21

The balance between the three water pathways determines what?

Answer 21

How flashy the river response is and thus any likelihood of flooding

Question 22

What does urbanisation do to previously permeable ground?

Answer 22

Renders previously permeable ground surfaces impermeable

Question 23

Surfaces like concrete and tarmac increase what and decrease what?

Answer 23

Increase overland flow generation and decrease the effectiveness of infiltration, through flow and soil storage

Question 24

The more ground that is covered by impermeable hard surfaces, the less what?

Answer 24

The less rainfall will soak into the ground and the more will flow over the surface into drains and sewers

Question 25

Why does the removal of vegetation to make room for urban areas also contribute to increased runoff?

Answer 25

Less interception storage
Less water absorbed by vegetation
Less infiltration - roots break up soil so water can run through easier, so instead ground gets baked and it’s harder for water to go in

Question 26

Flood risk in some parts of the UK are undoubtedly higher as a result?

Answer 26

Much of the forest which once covered the land has been removed

Question 27

Name the process of ice crystal growth

Answer 27

Bergeron–Findeisen process