The water cycle and weather
FoundationSolar energy drives evaporation; water vapour cools and condenses into clouds before falling as precipitation. Weather describes short-term atmospheric conditions.
- Solar energy drives evaporation
- Cooling causes condensation and cloud formation
- Precipitation returns water to the surface; weather is short-term atmospheric conditions
§Text Passage
1Where does rain come from? The answer is a grand cycle driven by the Sun. Seawater evaporates into water vapour, which rises, cools and condenses into tiny droplets that gather as clouds; inside the clouds the droplets keep merging and growing until they fall as rain or snow, then return to the sea through rivers and groundwater. Evaporation, condensation and precipitation go round and round without end.
2Condensation is easy to spot in daily life: the droplets on a chilled soft-drink can are water vapour from the air condensing on the cold surface, and morning dew on grass or fog in late autumn forms by the same process. The amount of vapour in the air is called humidity; when it is high, sweat evaporates slowly and we feel especially muggy and hot.
3The water cycle shapes the weather everywhere. In Hong Kong, spring is humid and foggy, summer typhoons bring heavy rain and winter is dry and sunny, all closely tied to the moisture in the air and the movement of winds. By watching the shape of clouds, the amount of cloud cover and changes in humidity, we can make simple forecasts — and the Observatory's daily reports are built on exactly such observations.
4The water cycle constantly renews the Earth's freshwater and decides harvests and water supplies. Hong Kong relies on rainfall collected locally together with water from the Dongjiang River, which shows why protecting water resources matters; with extreme weather growing more frequent, rainstorms and droughts are becoming fiercer. Learning to save water and plan well is the long-term task of living with the water cycle.
1Summary 重點摘要
Solar energy drives evaporation; water vapour cools and condenses into clouds before falling as precipitation. Weather describes short-term atmospheric conditions.
2Vocabulary cards Vocabulary
Liquid water absorbing heat at its surface and turning into water vapour.
Water vapour cooling and changing into tiny liquid droplets.
Water falling from clouds as rain, snow, hail or similar forms.
Short-term conditions of temperature, wind, clouds and precipitation in a place.
The continuous movement of water between oceans, the atmosphere and land.
The gaseous form of water, present in the air.
A visible mass of countless tiny water droplets or ice crystals.
The long-term average pattern of weather in a place over many years.
3Step by step Steps
- EvaporationThe water cycle is driven by the Sun: solar heat warms the water at the surface of oceans, lakes, rivers and soil, and once the molecules gain enough energy they leave the surface as water vapour and rise with the air; moisture evaporating from plants adds to the vapour in the air. A shirt drying on the line works the same way — in sunshine, the water in the fabric quietly turns to vapour and slips away.
- Cooling and condensationRiding up with the warm air, water vapour reaches cooler heights and condenses into countless tiny droplets or ice crystals, which hang together in the air as clouds. Dust motes in the air act as the nuclei on which the vapour clings and takes shape; the thick fogs of Hong Kong's spring are exactly warm, moist air condensing on meeting cooler surroundings. Condensation turns invisible vapour into visible droplets — the shared cause of clouds, fog and dew.
- Precipitation returns waterInside the cloud, droplets keep merging and crystals keep growing until the updraughts can no longer support them, and they fall as rain, snow or hail. On reaching the ground, part of the water soaks underground to join the groundwater, and part flows through rivers back to the sea, where the cycle begins anew. The water cycle constantly replenishes and renews the Earth's fresh water — reservoirs and rivers alike depend on it to sustain all living things.
- Weather versus climateKeep weather and climate apart: weather is the state of the air at a place over a short time, such as rain tomorrow or heat today, while climate is the average pattern over many years — Hong Kong's summer typhoons and its humid, foggy springs are climate features. Watching cloud cover, humidity and wind direction allows simple short-term forecasts, and once the distinction is clear, reading weather reports and climate news becomes far less confusing.
- What speeds up evaporationEvaporation follows clear rules: it quickens with higher temperature, a larger liquid surface, stronger wind and drier air. Spreading the washing out and hanging it in a breezy, sunny spot uses several of these factors at once; on a still, humid day, by contrast, vapour struggles to leave the surface and the clothes stay damp for hours. Knowing these factors helps you arrange drying and storage far more wisely in daily life.
- What weather watchers measureDescribing weather rests on instrument readings: thermometers measure air temperature, hygrometers measure the moisture in the air, wind vanes and anemometers record where the wind comes from and how strong it is, rain gauges collect precipitation, and cloud cover and pressure complete the picture. The daily reports and forecasts issued by the observatory are simply these observations gathered and analysed together — accurate observation is the foundation of a reliable forecast.
4Diagrams Diagrams
5Worked example Example
Example: why do water droplets form on the outside of a cold drink?
- The surrounding air contains water vapour
- The cold cup cools nearby air
- Vapour condenses into liquid droplets
- The droplets come from air, not through the cup wall
- Answer: It is condensation. Warm air around the cup holds invisible water vapour; touching the icy wall it cools, and the vapour condenses into liquid droplets clinging outside. The droplets come from the surrounding air, not through the cup wall — a dry wall at the start proves it — which is why fresh beads soon appear after wiping, the very principle that builds clouds.
6Common mistakes Mistakes
7Quick check Quick check
Q1What is liquid water changing into vapour called?
Q2How do clouds mainly form?
Q3What is the main difference between weather and climate?
Q4What is the energy source of the water cycle?
Q5Name three factors that affect how fast water evaporates.
8Recap Recap
- Solar energy drives evaporation
- Cooling causes condensation and cloud formation
- Precipitation returns water to the surface; weather is short-term atmospheric conditions
- The water cycle constantly renews Earth's freshwater
- Climate is the long-term average pattern, unlike short-term weather