What drives the water cycle at Higher Level: The same ideas as SL with different examples: India's monsoon, Typhoon Haiyan and the Blue Nile. At HL, expect to explain how latent heat and the oceans shape the climate in a seven-mark answer.
Practise this as you read
- Link every flow to the energy that drives it.
- Trace heat from the tropics to the poles through the cycle.
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Two engines: The hydrological cycle never stops. Two things keep it moving: energy from the sun and gravity.
The points to remember
- Solar radiation heats water so it evaporates; it also melts ice and drives transpiration.
- The sun heats the Earth unevenly, which makes winds that carry vapour and clouds (advection).
- Gravity pulls water down: rain falls, water drains through soil, rivers flow to the sea.
- Name two energy sources when asked: solar (thermal), kinetic (wind) or gravity.
- The sun lifts water up (energy in); gravity brings it back down.
Remember it as: The sun lifts water up; gravity brings it back down.
Real example: from June to September the sun heats the Indian Ocean. The summer monsoon carries the vapour north over India, and it falls as rain on the Himalayas. Gravity drains it down the Ganges, about 2,500 km, to the Bay of Bengal.
Name two, not one: 'State two sources of energy' needs two: the sun and gravity (or wind). One source alone gets no mark.
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Water can be ice, liquid water or invisible water vapour. Changing from one to another is a phase change, and it needs or releases energy called latent heat.
The points to remember
- A phase change is water changing state: liquid to gas, gas to liquid, solid to liquid.
- Latent heat is the energy taken in or given out in a phase change.
- The temperature does not change while it happens: the energy breaks or makes bonds.
- Breaking bonds takes energy in (evaporation, melting); making bonds gives it out (condensation, freezing).
Remember it as: Breaking bonds takes energy in; making bonds gives it out.
Takes heat in (cools)
- Evaporation: liquid to gas
- Melting: ice to liquid
- Sublimation: ice to vapour
Gives heat out (warms)
- Condensation: gas to liquid
- Freezing: liquid to ice
- Deposition: vapour to ice
Real example: a pan of pasta water reaches 100 °C and starts to boil. Turn the gas up and it boils faster, but it gets no hotter than 100 °C: the extra energy goes into turning liquid into steam. That energy is latent heat.
Evaporation is the sun's way of lifting water into the air. Because it takes latent heat in, it cools whatever the water evaporates from.
The points to remember
- Evaporation needs energy to break the bonds between water molecules.
- It takes this latent heat from the surroundings.
- The surroundings lose energy, so they cool down.
- Plants do the same through transpiration: forests cool the air around them.
Your body
- Sweat evaporates and takes heat from your skin.
- You feel cold after a swim, even in the sun.
- A dog pants and wet clothes chill you for the same reason.
Forests
- A large tree can transpire up to 400 litres of water a day.
- Forests can be 2-8 °C cooler than nearby cities.
Whole regions
- Up to half of the Amazon's rain comes from its own transpiration.
- Clear the forest and that cooling, and that rain, are lost.
Real example: in Phoenix, Arizona, many homes use evaporative coolers. As the water in the pads evaporates it takes heat from the air, which can come out more than 10 °C cooler. They work best in dry air, where evaporation is fast.
'But a pan on the stove gets hotter': The stove gives far more heat than the evaporating water takes. Evaporation is still taking heat; the stove just wins. With no stove, as on your skin, you feel the cooling.
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Condensation is evaporation in reverse. The latent heat that went into the vapour comes back out, and it warms the air around the cloud.
The points to remember
- Condensation turns vapour back into liquid droplets: clouds, dew, mist.
- Making the bonds releases the latent heat to the surroundings.
- The surroundings gain energy, so they warm up; the warm air rises.
- This is what powers hurricanes over warm seas, and why steam burns are so bad.
- A steamy bathroom feels warm: vapour condensing on the cold walls gives out its heat.
Evaporation
- Liquid to gas
- Latent heat taken in
- Surroundings cool
Condensation
- Gas to liquid
- Latent heat given out
- Surroundings warm
Real example: Typhoon Haiyan grew over the warm western Pacific in November 2013. Condensation in its clouds released so much latent heat that its winds passed 300 km/h before it struck the Philippines. Storms that move over cold water or land lose this fuel and weaken.
Three steps for the marks: Condensation releases energy as bonds form; the energy passes to the surroundings; the surroundings warm up. One step per sentence.
The sun lifts water up, but everything that comes back down is pulled by gravity. Winds, made by the sun heating the Earth unevenly, do the sideways moving: advection.
The points to remember
- Gravity makes precipitation fall from clouds to the ground.
- It pulls water down through the soil (infiltration) and deeper rock (percolation).
- It makes surface run-off and rivers flow downhill to the sea.
- It moves groundwater slowly towards rivers and the sea.
- Wind (kinetic energy from uneven heating) carries vapour and clouds over the land.
Moved by gravity
- Precipitation
- Infiltration and percolation
- Surface run-off and streamflow
- Groundwater flow
Moved by the sun's energy
- Evaporation and transpiration
- Melting and sublimation
- Winds: advection of vapour and clouds
Real example: summer rain on the Ethiopian Highlands runs off the steep slopes into the Blue Nile. Gravity carries it more than 4,000 km north, through Sudan and Egypt, to the Mediterranean Sea.
Say what gravity does: 'Infiltration' alone gets no mark. Write 'gravity pulls water down through the soil' or 'gravity makes rivers flow downhill to the sea'.
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Evaporation and condensation do more than move water. They carry the sun's heat around the planet, and the oceans store it, so the water cycle shapes the climate.
How water shapes climate
- Evaporation is greatest where sunlight is strongest (the tropics) and takes in heat.
- Winds and storms carry the vapour to higher latitudes and altitudes.
- There it condenses, releases the heat and forms clouds and rain: the tropics-poles gap shrinks.
- Oceans store heat in summer and by day, and release it in winter and at night: milder coasts.
- Warm currents carry heat towards the poles; cold currents cool and dry the coasts.
- Ice and clouds reflect sunlight (cooling); melting ice reflects less, so it warms further.
- Water vapour is a greenhouse gas; the oceans also absorb carbon dioxide.
Real example: the cold Peru Current flows up the coast of South America. Air over it is cool and holds little moisture, so Lima, Peru, gets less than 20 mm of rain a year and is a desert by the sea.
In a [7], one mechanism per point: Each point names a process and its effect on climate: 'condensation releases latent heat at higher latitudes, warming them'. Global warming points count for at most two marks.
How this comes up: Paper 2, Section B (b): explain how the hydrological cycle influences climate [7]. Seven separate mechanisms, each linked to its effect.
India's summer monsoon brings about 80% of the country's yearly rain, and coastal cities such as Mumbai have far milder winters than inland Delhi.
Explain how the movement and storage of water in the hydrological cycle influence climate around the Earth.
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