Exploring the Water Cycle
FoundationThe sun evaporates sea and lake water into vapour; rising vapour condenses into clouds; drops grow heavy and fall as rain; water flows back to the sea and evaporates again — the water cycle never stops. A sealed-bag experiment recreates it in class. Hong Kong gets about 70 to 80 percent of its water from the Dongjiang and the rest from reservoirs, so saving water is everyone's duty.
- The sun powers the cycle of evaporation, condensation and precipitation; the Earth's total water hardly changes.
- Clouds, fog and dew are condensed vapour; rain, snow and hail are different forms of precipitation.
- Sealed-bag and saucer-lid experiments recreate 'rise — cool — condense — fall' in class.
文體小知識：水循環科普說明文跟隨一滴水的旅程，解釋蒸發、凝結和降水的循環，聯繫雲、雨、霧的形成。
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1The sun powers the water cycle. Sunshine heats oceans, lakes and rivers, turning liquid water into invisible vapour that rises into the air — evaporation; the warmer it is, the windier and the wider the surface, the faster it goes. Leaves also release water vapour, called transpiration. A wet shirt drying in the sun is evaporation at work in daily life.
2As vapour rises, the cold upper air makes it condense on tiny specks of dust into countless suspended droplets and ice crystals — clouds. Condensing in cool air near the ground, vapour becomes fog; dew is the beads on morning grass. In clouds the droplets collide and merge until air can no longer hold them, and they fall as rain; in very cold heights vapour turns straight into snow or hail. Precipitation partly runs to the sea in rivers, partly soaks into groundwater, and partly evaporates again — an unending cycle. The Earth's total water hardly changes; it merely keeps changing form and place.
3The water cycle can be recreated in class. Sealed-bag experiment: put about fifty millilitres of blue water in a clear zip bag, seal it, and tape it to a sunny window. Soon vapour rises inside, condenses on the cooler upper part into beads, and the beads slide down the wall back to the bottom — evaporation, condensation and precipitation in one bag. The lid experiment covers a cup of hot water with a cold saucer; within minutes beads gather under the saucer, showing 'rise — cool — condense' again.
4Hong Kong lacks large rivers and lakes. About 70 to 80 percent of our water comes from the Dongjiang: in the great drought of 1963 water was supplied only once every four days; in 1965 the Dongjiang-Shenzhen supply scheme was completed and solved the water shortage. The rest comes from reservoirs collecting rain — Plover Cove, the first reservoir formed by damming a sea inlet, and High Island in a deep valley — while seawater flushes most toilets. Fresh water is hard won, so saving it is everyone's duty: shower instead of bathe, fit water-saving showerheads, reuse rice-washing water on plants, and turn off the tap while soaping — dozens of litres saved per person each day.
1Summary 重點摘要
The sun evaporates sea and lake water into vapour; rising vapour condenses into clouds; drops grow heavy and fall as rain; water flows back to the sea and evaporates again — the water cycle never stops. A sealed-bag experiment recreates it in class. Hong Kong gets about 70 to 80 percent of its water from the Dongjiang and the rest from reservoirs, so saving water is everyone's duty.
2Vocabulary cards Vocabulary
Water turning from liquid into vapour and rising into the air when heated.
Vapour cooling back into droplets or ice crystals — the making of clouds, fog and dew.
Water falling from the sky as rain, snow or hail.
Water in its gas form, invisible and mixed into the air.
Nature's endless loop of evaporation, condensation, precipitation and return flow.
Leaves releasing water into the air as vapour.
A huge basin storing fresh water, such as Plover Cove and High Island reservoirs.
Water supplied to Hong Kong from the Dongjiang by the Dongjiang-Shenzhen scheme — about 70 to 80 percent of our supply.
Water with little salt that we can drink, such as rain and river water; the Earth's fresh water is very limited.
3Step by step Steps
- Step 1: Learn evaporation from drying clothesExample: a wet shirt hung in the sun dries within a few hours, because the water leaves the cloth as invisible vapour — a process called evaporation. The higher the temperature, the drier the air and the stronger the wind, the faster it dries, which is why clothes dry quicker in summer than in winter. Rivers, lakes and seas on the Earth are evaporating all the time, sending vapour upwards as the raw material of clouds. A cup of water left uncovered for a few days also shrinks through evaporation, and coastal salt farms use sunshine and wind to evaporate seawater, leaving grains of salt behind — a practical use of evaporation.
- Step 2: Learn condensation from a cold drinkExample: take a chilled soft drink out of the fridge and a layer of beads soon covers the bottle. The water did not leak from inside; it is vapour from the air condensing on the icy bottle. Clouds, fog and morning dew are all products of condensation: vapour high up forms clouds, vapour near the ground forms fog, and vapour on grass forms dew. Condensation is the opposite of evaporation — the two work as a pair.
- Step 3: See how rain formsExample: countless tiny droplets inside a cloud bump into and merge with each other, growing until the air can no longer hold them up, and they fall as rain. If the high air is extremely cold, vapour turns directly into ice crystals and falls as snow; in strong up-and-down convection, ice builds up in layers and becomes hail. Rain, snow and hail together are called precipitation, which returns the water of the sky to the ground and completes the lower half of the water cycle. Cloud droplets need tiny particles such as dust in the air to condense on, which is why hazy skies often carry many clouds — a detail that often appears in examinations.
- Step 4: Run the sealed-bag experimentExample: seal about fifty millilitres of blue water in a clear plastic bag and tape it to a sunny window; within half an hour small beads appear on the upper part of the bag, grow bigger and slide down to the bottom. Inside the bag the water evaporates, condenses and falls again — a perfect repeat of the water cycle in nature. The experiment needs only simple, safe materials and suits daily observation, making it a fine classroom way to learn the cycle.
- Step 5: Run the saucer-lid experimentExample: cover a cup of hot water with an ice-cold saucer, and within a few minutes beads gather under the saucer. The hot water keeps giving off invisible vapour, which rises, meets the cold underside and condenses into little droplets. The result shows within a minute and clearly demonstrates the two stages of 'evaporate and rise, cool and condense'; it also explains why the bathroom mirror fogs up.
- Step 6: Learn Hong Kong's water sourcesExample: about seventy to eighty percent of Hong Kong's fresh water comes from the Dongjiang, imported from the mainland; the remaining twenty to thirty percent is rainwater collected locally, mainly by the two large reservoirs, Plover Cove and High Island. Seawater is used for flushing toilets to save precious fresh water. Knowing where our water comes from shows how hard-won it is, and saving water and keeping catchment areas clean is everyone's duty. The government also enforces catchment regulations that restrict dumping rubbish or keeping livestock inside catchment areas, so the law and the reservoir system work hand in hand to protect our water.
- Step 7: Practise saving waterExample: take a shower instead of a bath to save tens of litres each time; turn off the tap while soaping your hands or brushing your teeth to save about ten litres a day per person; reuse vegetable-washing water on the plants. Although the Earth has plenty of water, fresh water is only a tiny fraction, and usable fresh water is smaller still. Building water-saving habits lightens the load on the reservoirs and shows respect for the regions that supply us. Fitting water-saving showerheads and dual-flush toilet cisterns also cuts the water used each time, and such fittings are becoming ever more common in newly built flats in Hong Kong.
4Diagrams Diagrams
5Worked example Example
Example: Chi ran the sealed-bag water cycle experiment: about fifty millilitres of blue water in a bag taped to a sunny window. After an hour beads appeared on the upper bag; after three hours they slid down to the bottom. Explain what happened and which natural processes each step models.
- In the first hour the sun's heat evaporated the water, and vapour gathered upwards — modelling evaporation in nature.
- The upper bag is cooler, so the vapour condensed into beads — modelling vapour condensing into clouds high up.
- The beads merged and grew until they could no longer hang on, sliding down the wall — modelling precipitation.
- Water back at the bottom evaporated again in the sun, an endless loop — the cycle runs as long as the sun supplies energy.
- The total water never changed, only its form and position — just as the Earth's water keeps circulating.
- Answer: Evaporation, condensation and precipitation happened in turn inside the bag, recreating the water cycle; the total water stayed the same, changing only between vapour and droplets, powered by the sun.
6Common mistakes Mistakes
7Quick check Quick check
Q1What are the three main processes of the water cycle?
Q2How do clouds form?
Q3Where does Hong Kong's water mainly come from?
Q4What powers the endless water cycle?
Q5Name two ways to save water at home.
8Recap Recap
- The sun powers the cycle of evaporation, condensation and precipitation; the Earth's total water hardly changes.
- Clouds, fog and dew are condensed vapour; rain, snow and hail are different forms of precipitation.
- Sealed-bag and saucer-lid experiments recreate 'rise — cool — condense — fall' in class.
- About 70 to 80 percent of Hong Kong's water comes from the Dongjiang, the rest from reservoir rain, with seawater flushing toilets.
- Fresh water is hard won: shower instead of bathe, tap off while soaping, and reuse rice water to save it.