What drives the water cycle

Two engines: The hydrological cycle never stops. Two things keep it moving: energy from the sun and gravity.

Remember it as: The sun lifts water up; gravity brings it back down.

Real example: sunlight evaporates water from the Atlantic Ocean. The trade winds carry the vapour west over the Amazon rainforest, where it falls as rain. Gravity then drains it down the Amazon River back to the Atlantic: about a fifth of all the river water that reaches the world's oceans.

Name two, not one: 'State two sources of energy' needs two: the sun and gravity (or wind). One source alone gets no mark.

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.

Remember it as: Breaking bonds takes energy in; making bonds gives it out.

Real example: heat a kettle and the water warms from 20 °C to 100 °C. Then it boils, and although the heater is still on, the temperature stays at 100 °C until the water has turned to steam. That energy is latent heat: it is 'hidden' in the vapour, not shown on a thermometer.

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.

Real example: in northern Nigeria and Sudan, farmers keep vegetables fresh in a zeer pot. Water in the sand evaporates through the outer pot, taking heat with it, so the inner pot stays far cooler than the hot air outside.

'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.

Condensation is evaporation in reverse. The latent heat that went into the vapour comes back out, and it warms the air around the cloud.

Real example: in August 2005 Hurricane Katrina passed over the very warm Gulf of Mexico. Huge amounts of water evaporated; as the vapour rose and condensed into storm clouds it released its latent heat, which warmed the air, made it rise faster and strengthened the storm before it hit New Orleans.

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.

Real example: rain and meltwater in the Swiss Alps run into the Rhine. Gravity carries the water downhill for 1,230 km, through Germany and the Netherlands, to the North 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'.

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.

Real example: the Gulf Stream, a warm ocean current, carries tropical heat to north-west Europe. London's January average is about 5 °C; St John's in Canada, at a similar latitude but with a cold current offshore, averages about -5 °C.

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 (a): outline how energy drives the water cycle [4]. One process and its energy per point.