The hydrological cycle

The hydrological cycle (recap)

From Unit 2: You learned about the water cycle in Unit 2. Here's a quick reminder before we look at the energy side of the cycle.

Big idea: Water is constantly moving around Earth in a giant recycling system called the hydrological cycle. The same water dinosaurs drank is still here today!

Quick recap: stores and flows

What's new in Unit 4?

In Unit 2, you learned WHAT the water cycle is. Now we focus on the energy that makes it work:

The energy driver: Energy from the sun drives the entire hydrological cycle. Water is held in different places called stores. Without solar energy, water would not move between these stores.

What is latent heat?

Phase change: Water can be a liquid (like in a lake) or a gas (invisible water vapour). Changing between these is called a phase change.

Latent heat definition

Latent heat: Latent heat means "hidden heat." It is energy used to break or form bonds between water molecules — NOT to change temperature.

The word "latent" comes from Latin meaning "hidden." You cannot measure this energy with a thermometer — the temperature stays the same during a phase change.

Why does latent heat exist?

Water molecules are held together by bonds:

Memory trick: Breaking up is hard to do! Breaking bonds needs energy. Forming bonds releases energy.

The boiling kettle example

During a phase change, energy goes into breaking or forming bonds — NOT into changing temperature. That's why it's "hidden" heat.

Latent heat absorbed (evaporation)

Why evaporation causes COOLING: When water evaporates (liquid → gas), it needs energy to break molecular bonds. It takes this energy FROM THE SURROUNDINGS. When surroundings lose energy, they get COOLER.

Think of evaporation as "stealing" heat. The water takes the energy it needs, leaving less energy behind — so everything nearby cools down.

Everyday examples

Why forests cool the climate

Forests as natural air conditioners: Trees release huge amounts of water vapour through transpiration. This absorbs latent heat from the surroundings, cooling the local area. A single large tree can transpire up to 400 litres of water per day — that's a LOT of cooling!

Climate change link: Deforestation = less transpiration = less local rainfall = warming.

Forests recycle water — trees transpire, rain forms locally, falls on the same forest, and the cycle repeats. Without trees, this recycling stops. Rain falls elsewhere (often over oceans), the deforested area dries out, and the natural cooling is lost.

Common confusion

"But my kitchen gets hot when I boil water!": The stove releases FAR more heat than the evaporating water absorbs. The evaporation IS absorbing heat from the pot — but the stove overpowers this. In the examples above, there's no stove adding heat, so you feel the cooling effect.

Exam answer: evaporative cooling (3 marks): (1) Evaporation requires energy to break molecular bonds (2) This energy is absorbed from the surroundings (3) Surroundings lose energy → temperature decreases

Exam answer: forests and cooling (4 marks): (1) Trees release water vapour through transpiration (2) Transpiration absorbs latent heat from surroundings (3) This cools the local area (4) Deforestation removes this cooling effect, contributing to local and global warming

Latent heat released (condensation)

Why condensation causes WARMING: When water vapour condenses (gas → liquid), bonds form between molecules. Forming bonds RELEASES energy TO THE SURROUNDINGS. When surroundings gain energy, they get WARMER.

Think of condensation as "giving back" the heat that was taken during evaporation. The water vapour releases energy as it becomes liquid.

Everyday examples

Why this matters globally

Energy redistribution: Latent heat moves energy around the planet. Heat is absorbed at Earth's surface (evaporation over oceans) and released high in the atmosphere (condensation in clouds). This transfers energy from the equator towards the poles.

Exam answer: condensation warming (3 marks): (1) Condensation releases energy when molecular bonds form (2) This energy is transferred to the surroundings (3) Surroundings gain energy → temperature increases

Evaporation vs transpiration

These two terms are often confused in exams. Here's the difference:

Evaporation

Evaporation: Evaporation is water turning to vapour from non-living surfaces — oceans, lakes, rivers, puddles, wet soil.

Transpiration

Transpiration: Transpiration is water vapour released from living plants through tiny pores called stomata on leaves.

This is why forests cool local climate — all that transpiration absorbs heat!

What about humans and animals?: Humans and animals also lose water vapour — through sweating and breathing out moist air. This also absorbs latent heat, which is why sweating cools you down! However, we don't call it transpiration — that term is only used for plants.

Side-by-side comparison

The key difference: Evaporation = NON-LIVING. Transpiration = LIVING (plants only — not animals!). Both absorb latent heat. Both driven by solar energy.

Model exam answer: Q: Explain the difference between evaporation and transpiration.

Evaporation is the loss of water vapour from non-living surfaces such as oceans and lakes. Transpiration is the loss of water vapour from living plants through stomata in their leaves. Both absorb latent heat and are driven by solar energy.

Evapotranspiration

Evapotranspiration: Evapotranspiration = evaporation + transpiration added together.

Scientists use this term when measuring TOTAL water loss from a landscape. Both processes happen at the same time and are hard to measure separately.

Factors affecting evapotranspiration

Evapotranspiration rates vary depending on conditions:

Quick summary table

Visual summary

Evapotranspiration = both combined. Examiners love this word!