Water Cycle Pack D

Question 1

What causes clouds?

Answer 1

• Air rises and cools to saturation points (when the air can hold no more moisture)
• There is a fall in pressure as altitude increases, so air expands
• There are fewer collisions between air molecules, so the amount of heat energy falls and therefore temperature falls
• Clouds form as water vapour condenses into water droplets
• Dew point must be reached for condensation to occur

Question 2

What causes rain in general?

Answer 2

• Cloud droplets have to grow or fuse with
• There are two theories about this

Question 3

What is the Bergeron-Findeisen theory of rain droplet formation?

Answer 3

• Clouds at high altitudes contain a mix of very cold water droplets and ice crystals (as temperature is just below 0)
• The ice crystals grow rapidly because every water droplet that comes into contact freezes and adds to the crystal
• They eventually become too large and dense to be held up and fall to the ground due to gravity
• If the air between the clouds and ground is <2°C, snowflakes remain as snow
• If the air temperature is warmer, they turn into raindrops as they fall
• We can generate rainfall by adding dry-ice to the top of clouds, so this supports the theory
• Theory does not explain the generation of rainfall in the Tropics, where temperatures are too high

Question 4

What is the collision theory of rain droplet formation?

Answer 4

• Super sized condensation nuclei (e.g. salt particles) provide ‘seeds’ around which water can condense
• Much larger and heavier than normal droplets
• Large droplets fall and collide with smaller droplets, absorbing them
• Super-droplets move faster than others as they are larger, enabling them to overtake and absorb them
• Theory can explain how rainfall forms in the Tropics, unexpected downpours and flash floods

Question 5

What causes orographic rain?

Answer 5

• Air is forced to rise over a barrier
• it cools and condensation occurs
• Rain forms
• The leeward slope receives relatively little rainfall (the rain shadow effect)
• Augmented by the feeder-seeder mechanism

Question 6

What is the seeder-feeder effect?

Answer 6

• The fallout of precipitation from a seeder cloud into a lower-level cloud generated by relief
• Can result in the growth of ice crystals or rain that originated in the seeder cloud as they fall through the feeder cloud
• Leads to precipitation enhancement over the mountains

Question 7

Where in the UK does orographic rainfall occur?

Answer 7

• North and western UK
• West cost of Scotland
• Cumbrian coastline

Question 8

What two cells from the tricellular model is the UK between?

Answer 8

• Polar cell bringing cold air
• Ferrel cell bringing warm air

Question 9

What causes frontal rain?

Answer 9

• Warm air is lighter and less dense
• It is forced to rise over cold, denser air
• As it rises, the air cools and its ability to hold water vapour decreases
• Condensation occurs which forms clouds and rain
• Forms where two surface air streams meet

Question 10

Where in the UK does frontal rain occur?

Answer 10

All over the UK

Question 11

What causes convectional rain?

Answer 11

• When the land becomes hot, the air above it rises as it becomes warmer and expands
• As it rises, it cools and condensation occurs so its ability to hold water vapour decreases
• Further ascent causes more cooling
• Cool air descends and replaces warm air
• Results from intense daytime heating of the land

Question 12

When and where is there convectional rain in the UK?

Answer 12

• Southern UK
• In summer