You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
Spec mapping (AQA 7037): Paper 1, §3.1.1 — Processes driving change in the magnitude of the water-cycle stores over time and space: evaporation, condensation, precipitation (and its types), cryospheric processes, and the role of energy and runoff. Synoptic links: the energy (latent-heat) transfers here underpin atmospheric processes in weather/hazards, and cryospheric processes connect to glaciated landscapes and sea-level change. AOs: AO1 (the physics and mechanisms of each process), AO2 (explaining spatial variation, e.g. rain shadow), AO3 (interpreting a soil-moisture / lapse-rate resource quantitatively).
This lesson examines the physical processes that drive the water cycle in greater depth than earlier lessons, which introduced them in the context of the global cycle and the drainage basin. Here we unpack the underlying physics — latent-heat exchange, adiabatic cooling, vapour-pressure gradients — the spatial variation each process exhibits, and the factors that modify it. Because so much of the water cycle is really a story of energy being absorbed and released as water changes state, a secure grasp of these processes is essential for explanation at the highest bands.
Before the individual processes, it is worth stating the unifying principle: every change of state moves energy. Water has an unusually high specific heat capacity and very large latent heats, so the water cycle is one of the planet's most important mechanisms for redistributing energy.
| Phase change | Direction of energy | Latent heat |
|---|---|---|
| Evaporation (liquid → vapour) | Absorbs energy from surroundings (cooling) | ~2,260 kJ kg⁻¹ |
| Condensation (vapour → liquid) | Releases energy to surroundings (warming) | ~2,260 kJ kg⁻¹ |
| Melting (solid → liquid) | Absorbs energy | ~334 kJ kg⁻¹ |
| Freezing (liquid → solid) | Releases energy | ~334 kJ kg⁻¹ |
| Sublimation (solid → vapour) | Absorbs energy | ~2,838 kJ kg⁻¹ |
Evaporation at the tropical ocean surface absorbs latent heat; that heat is carried poleward as water vapour and released as the air rises, cools, and condenses — powering storms and warming the upper troposphere. The water cycle is therefore inseparable from the energy budget, a point examiners reward when explaining why condensation aids cloud growth or why evaporation cools a surface. This latent-heat transport is one of the principal ways the planet moves energy from the hot tropics to the cooler high latitudes: every gram of water that evaporates in the tropics and condenses elsewhere delivers ~2,260 J of heat to that location. In this sense the water cycle is not just a mover of water but a mover of energy, and the two roles are inseparable — which is why understanding the latent heats above underpins explanation throughout this lesson.
graph LR
OCN["Tropical ocean surface"] -->|"evaporation absorbs latent heat (~2260 kJ/kg)"| VAP["Water vapour in air"]
VAP -->|"advection poleward and upward"| RISE["Air rises and cools adiabatically"]
RISE -->|"condensation releases latent heat"| CLD["Cloud forms; troposphere warms"]
CLD -->|"precipitation"| SURF["Water returns to surface"]
SURF -->|"runoff and re-evaporation"| OCN
Key Definition: Evaporation is the conversion of liquid water to water vapour (a gas) at a temperature below boiling point. It occurs from open water, wet soil, and intercepted water on surfaces.
Evaporation requires energy to break the hydrogen bonds holding water molecules together — the latent heat of vaporisation, ~2,260 kJ per kg of water evaporated. Because this energy is drawn from the surroundings, evaporation produces a cooling effect (the principle behind sweating and behind the cooling of a moist surface on a breezy day). Only molecules with sufficient kinetic energy escape the surface, so warmer water — with more high-energy molecules — evaporates faster. This makes evaporation a major mechanism of heat transfer in the climate system, moving energy from the surface into the atmosphere as latent heat.
| Factor | Effect | Explanation |
|---|---|---|
| Temperature | Higher → faster evaporation | More molecules have sufficient kinetic energy to escape the liquid surface |
| Humidity | Lower → faster evaporation | The vapour pressure gradient between the surface and the air is steeper |
| Wind speed | Higher → faster evaporation | Wind removes saturated air from above the surface, maintaining the vapour pressure gradient |
| Surface area | Greater → more evaporation | More area for molecules to escape from |
| Salinity | Higher → slower evaporation | Dissolved salts reduce the vapour pressure of water (Raoult's Law) — ocean water evaporates ~5% slower than pure water |
| Solar radiation | More → faster evaporation | Provides the energy required for phase change |
Global evaporation is estimated at approximately 577,000 km³/year (Trenberth et al., 2007). Oceanic evaporation (~505,000 km³/year) dominates because:
The subtropical oceans (around 15°–30° latitude) have the highest evaporation rates — high solar input combined with relatively low humidity and persistent trade winds. The equatorial zone has slightly lower evaporation because humidity is very high, reducing the vapour pressure gradient.
Key Definition: Transpiration is the loss of water vapour from plant surfaces, primarily through stomata (microscopic pores on the underside of leaves).
Transpiration is driven by the soil-plant-atmosphere continuum (SPAC), a concept developed by Philip (1966):
| Factor | Effect |
|---|---|
| Temperature | Higher temperatures increase the rate of evaporation from leaf cell surfaces |
| Humidity | Low humidity increases the diffusion gradient from leaf interior to atmosphere |
| Wind | Increases removal of humid air from the leaf boundary layer |
| Light intensity | Stomata open in response to light (for photosynthesis), allowing transpiration |
| Soil moisture availability | Low soil moisture → stomata close to conserve water → transpiration decreases |
| Leaf area index (LAI) | Higher LAI → more leaf surface area → greater total transpiration |
Transpiration deserves emphasis because it is the dominant route by which water returns from the land to the atmosphere, and because it is the process most directly controlled by vegetation — and therefore most altered by land-use change. When a plant transpires, it is not "wasting" water: the transpiration stream is what draws water and dissolved nutrients up from the roots, and the evaporative cooling at the leaf protects the plant from overheating. At the landscape scale, transpiration is the engine of rainfall recycling, returning moisture to the atmosphere where it can fall again downwind. This is why vegetation is an active participant in the water cycle, not a passive recipient of rain — and why removing it (deforestation) does not merely reduce local interception but can suppress regional rainfall hundreds of kilometres away, as the Amazon "flying rivers" demonstrate.
Key Definition: Evapotranspiration (ET) is the combined water loss from a surface through evaporation and transpiration. Potential evapotranspiration (PE) is the maximum ET that would occur if water supply were unlimited. Actual evapotranspiration (AE) is the real-world ET limited by available moisture.
Howard Penman (1948) developed an equation to estimate PE from meteorological data:
PE is calculated using:
The Penman-Monteith equation (Monteith, 1965) extended this to include vegetation resistance to water loss (stomatal conductance), making it applicable to vegetated surfaces. It is the standard method recommended by the FAO for estimating crop water requirements.
| Month | Typical PE (mm) | Typical Rainfall (mm) | Water Balance |
|---|---|---|---|
| January | 10 | 80 | Surplus (+70 mm) |
| April | 45 | 55 | Surplus (+10 mm) |
| July | 95 | 50 | Deficit (−45 mm) |
| October | 30 | 75 | Surplus (+45 mm) |
Approximate values for SE England
From April to September, PE typically exceeds rainfall, creating a soil moisture deficit. From October to March, rainfall exceeds PE, allowing soil moisture recharge and groundwater recharge.
Key Definition: Condensation is the process by which water vapour changes to liquid water. It releases latent heat — the same 2,260 kJ/kg absorbed during evaporation — warming the surrounding air.
The concept of relative humidity is central here. Warm air can hold more water vapour than cold air, so relative humidity (the actual vapour content as a percentage of the maximum the air can hold at that temperature) rises as air cools even if no water is added — because the maximum it can hold falls. When cooling brings relative humidity to 100%, the air is saturated and further cooling forces condensation. This is why the mechanism of cooling (adiabatic ascent, contact with a cold surface, mixing of air masses) is the key control on where and when clouds and precipitation form, and why simply having humid air is not enough — the air must be cooled to its dew point and nuclei must be present. Understanding this distinction between absolute moisture content and relative humidity is essential for explaining everything from morning dew and radiation fog to the cloud that caps a mountain.
Air cools primarily through adiabatic cooling as it rises and expands:
| Mechanism | Process | Example |
|---|---|---|
| Orographic uplift | Air forced upward over a topographic barrier | Western Highlands of Scotland: >3,000 mm/year rainfall as Atlantic air rises over mountains |
| Convective uplift | Surface heating creates rising thermals | Summer thunderstorms over SE England |
| Frontal uplift | Warm air rises over denser cold air at a front | UK winter depressions — warm front produces steady rain, cold front produces heavier showers |
| Convergence | Air streams meet and are forced upward | ITCZ — convergence of trade winds produces heavy equatorial rainfall |
Exam Tip: If asked to explain why the SALR is lower than the DALR, always refer to latent heat release during condensation. This warming partially compensates for the cooling due to expansion, so the net cooling rate is lower.
| Type | Mechanism | Characteristics | Typical Regions |
|---|---|---|---|
| Convective | Surface heating → rising thermals → cumulonimbus clouds | Intense, short-duration, localised; often with thunder and lightning | Tropics; mid-latitude summers |
| Orographic (relief) | Air forced over mountain barrier | Persistent on windward slopes; rain shadow on leeward side | Western UK, Scandinavian mountains, Himalayas |
| Frontal (cyclonic) | Warm air rises over cold air at weather fronts | Widespread, moderate intensity, prolonged | Mid-latitudes (UK depressions) |
| Convergence | Air streams meet and rise at ITCZ | Heavy, persistent, often diurnal pattern | Equatorial belt |
When moist air crosses a mountain range, it rises on the windward side, cools adiabatically, and produces precipitation. By the time the air descends on the leeward side, it has lost much of its moisture and warms as it descends (the Foehn effect). This creates a rain shadow — a region of significantly reduced rainfall.
UK example: The western Lake District receives >2,000 mm/year, while Penrith (on the eastern, leeward side of the Cumbrian Mountains) receives ~800 mm/year.
Global example: The Atacama Desert in Chile is one of the driest places on Earth (<15 mm/year in parts) due to its position in the rain shadow of the Andes — reinforced by the cold offshore Humboldt Current (which stabilises the air and suppresses convection) and by subsidence in the descending limb of the Hadley cell. The Atacama is a textbook reminder that aridity usually has several reinforcing causes, not one.
Why does some cloud produce rain and some not? Cloud droplets are tiny (~0.01 mm) and fall negligibly slowly, held aloft by updraughts; they must grow roughly a million-fold in volume to fall as raindrops (~1–2 mm). Two growth mechanisms achieve this:
Subscribe to continue reading
Get full access to this lesson and all 10 lessons in this course.