The movement of water across Earth’s surface and atmosphere is governed by two fundamental forces: solar energy and gravity, which together drive the hydrological cycle globally.

Solar Radiation and Evaporation

Solar radiation provides the energy input necessary for water movement. The Sun’s energy drives evaporation, where liquid water is converted into water vapour.

Solar radiation powers evaporation and releases heat on condensation; gravity moves water through soils and rivers to the sea.

Evaporation as an Energy-Driven Process

When water molecules absorb solar radiation, they gain kinetic energy and escape from the liquid surface into the atmosphere as vapour. This process is crucial because:

• It transfers water from oceans, lakes, rivers, and soils into the atmosphere.

• It forms the primary input of atmospheric water vapour, a critical component of the hydrological cycle.

• It drives atmospheric processes, including cloud formation and weather patterns.

After evaporation, the vapour remains suspended in the atmosphere until it undergoes condensation.

Condensation and Latent Heat

When water vapour cools in the atmosphere, it undergoes condensation, returning to liquid form and forming droplets within clouds.

As water vapour condenses into cloud droplets, latent heat is released to the surrounding air.

This diagram links solar energy to evaporation and shows how condensation releases latent heat into the atmosphere, while gravity routes water by surface runoff, streamflow and groundwater flow to the ocean. Arrows for infiltration and percolation connect surface and subsurface movement. Extra items (e.g., “boundary layer”, “water management”) extend beyond syllabus depth but help situate the core processes. Source.

Energy Release in Condensation

Unlike evaporation, condensation releases energy. The heat absorbed during evaporation is stored as latent heat of vaporisation and is released back into the atmosphere when vapour condenses. This energy transfer:

• Helps power atmospheric circulation, such as storms and convection currents.

• Balances the Earth’s energy budget, distributing heat around the planet.

This exchange of energy between evaporation and condensation ensures that the hydrological cycle is not only a transfer of water, but also a redistribution of energy.

Role of Gravity in Water Movement

Gravity ensures that water, once returned to Earth’s surface, flows through various pathways.

Surface Flows

Gravity drives streamflow in rivers and channels water downslope toward seas and oceans. This process is responsible for:

• The transport of freshwater across landscapes.

• The shaping of landforms through erosion and deposition.

• The delivery of sediments and nutrients to aquatic ecosystems.

Subsurface Flows

Water infiltrates soils and moves downwards due to gravity through percolation, eventually reaching groundwater stores such as aquifers. Gravity-driven flows include:

• Infiltration: Water entering soil pores.

• Percolation: Downward movement through soil and rock layers.

• Groundwater flow: Slow movement of water through saturated rock beneath the surface.

These processes connect terrestrial, atmospheric, and oceanic water stores.

Interplay Between Energy and Gravity

The hydrological cycle is sustained by the interaction of solar radiation and gravity, working together to move water continuously.

• Solar energy initiates movement through evaporation and sustains atmospheric processes.

• Gravity ensures the downward transfer of water once it condenses and precipitates.

This combination ensures that water:

• Circulates between atmosphere, land, and oceans.

• Maintains balance across different water stores.

• Provides the basis for ecosystems and human societies dependent on freshwater flows.

Key Processes Linking Energy and Gravity

• Evaporation (energy-driven): Converts liquid to vapour.

• Condensation (energy release): Forms clouds and redistributes heat.

• Precipitation (gravity): Water falls back to Earth as rain, snow, or hail.

• Runoff (gravity): Water flows overland into rivers and seas.

• Infiltration and percolation (gravity): Water moves into and through soils.

Between these processes, the water cycle operates as a closed system globally, with inputs and outputs balanced over long timescales.

Importance for the Hydrological Cycle

The combined forces of solar radiation and gravity ensure that:

• Water is cycled continuously, maintaining the availability of freshwater.

• Heat energy is redistributed across the globe, influencing climate and weather.

• Ecosystems and societies have sustained access to water resources.

Without the energy of the Sun and the force of gravity, the hydrological cycle would not function, leaving water stagnant and unevenly distributed across Earth.