12.2 Weather, Clouds & the Water Cycle
Key Takeaways
- Weather is the day-to-day state of the atmosphere in one place; climate is the average pattern of weather over many years
- The water cycle - evaporation, condensation, precipitation and collection or runoff - is a closed system powered entirely by energy from the Sun
- Cloud type is a forecast tool: cumulus means fair weather building, stratus means grey drizzle, cirrus means change is coming, and cumulonimbus means thunderstorms - a named Paper C topic
- Coastal regions have milder, wetter weather than inland regions, and mountains create dry rain shadows on their sheltered side - the reason weather differs between regions, a named Paper A topic
- A weather hypothesis must be testable: predict what you should observe, collect the data, and keep or reject the hypothesis based on the evidence - a named Paper G skill
Weather versus Climate
Weather is what the atmosphere is doing right now in one place - the temperature, wind, cloud and rain you experience today. Climate is the average pattern of weather in a place over many years, usually at least thirty. Darwin has a wet-season climate, but any single January day in Darwin has its own weather. Exam questions love the swap: a week of cold days in Brisbane is weather, not a change of climate.
How weather affects different regions is a named topic on Paper A, and everything in this section builds toward explaining those regional differences.
The Water Cycle: A Sun-Powered System
Earth's water is constantly moving through the water cycle, a closed system - the water in your drink bottle has been around for billions of years. The whole cycle is driven by energy from the Sun.
- Evaporation - the Sun heats water in oceans, lakes and soil, turning liquid water into invisible water vapour that rises into the air. Plants also release water vapour from their leaves, called transpiration.
- Condensation - high up, where the air is cold, the water vapour cools and turns back into tiny liquid droplets. Billions of droplets clumped together are what we see as a cloud. Fog is simply condensation happening at ground level.
- Precipitation - when droplets grow too heavy to stay aloft, water falls as rain, hail, sleet or snow.
- Collection and runoff - fallen water soaks into the ground, flows across the surface as runoff into creeks and rivers, and eventually returns to the ocean, ready to evaporate again.
Two ideas earn marks. First, the Sun is the engine: no solar energy, no evaporation, and the cycle stops. Second, water is never created or destroyed in the cycle - it only changes state and changes location, which is why the total amount of water on Earth stays essentially constant.
Cloud Patterns: Reading the Sky
Cloud patterns are a named topic on Paper C, and the good news is that four cloud types tell you most of the story:
| Cloud | What it looks like | What it signals |
|---|---|---|
| Cumulus | Puffy white heaps with flat bases, like cotton wool | Fair weather while small; can grow into storm clouds |
| Stratus | A flat grey sheet covering the whole sky | Overcast conditions, light drizzle or mist |
| Cirrus | Thin, wispy streaks very high up, made of ice crystals | Fine now, but a change in weather is often on the way |
| Cumulonimbus | A towering dark giant reaching high into the sky | Thunderstorms, heavy rain, hail and lightning |
The linking idea is altitude and shape: the higher and wispier the cloud, the colder and more stable the air; the taller and darker the cloud, the more violent the updrafts inside it.
Why Weather Differs Between Regions
- Coastal versus inland. Water heats up and cools down far more slowly than land, so the ocean acts like a hot-water bottle in winter and a cool towel in summer. Coastal cities such as Sydney have milder temperatures and more reliable rain than inland towns such as Broken Hill, which swing from scorching days to cold nights.
- Mountains and rain shadows. When moist air from the ocean meets a mountain range, it is forced upward, cools, and drops its moisture as rain on the coastal side. By the time the air crosses the range it is dry, leaving a rain shadow - a dry region on the sheltered side. This is why country just west of the Great Dividing Range is wetter than the interior further west.
- Latitude. Places near the equator receive more direct sunlight all year and are hot, while places closer to the poles receive sunlight at a slant and are cold.
Weather maps compress all of this into symbols: cold and warm fronts, pressure systems, wind arrows and rain shading. Low-pressure systems generally bring cloud and rain; high-pressure systems generally bring clear, settled skies. A forecast is simply a scientist's best prediction from this data - which is why forecasts improve as more measurements come in.
Forming and testing hypotheses about weather - Paper G
A hypothesis is a testable explanation or prediction. Worked example. A student notices that the school's asphalt basketball court feels much hotter than the grass oval at lunch and hypothesises: dark surfaces heat up faster in sunlight than light-coloured surfaces. To test it, she places a thermometer on the asphalt and one on the grass at the same time each day for a week, keeping everything else the same. Five days out of seven the asphalt reads higher, so the evidence supports the hypothesis. If the readings had been equal, she would reject it - and that would still be a successful experiment, because a hypothesis is a tool for learning, not a guess that must be defended. ICAS Paper G asks exactly this: given a hypothesis, choose the observation that would test it, or given the data, decide whether the hypothesis is supported.
Which statement describes climate rather than weather?
Moist air blows in from the ocean and rises over a mountain range. Town W sits on the ocean side of the range and Town E sits on the far side, at the same latitude and the same altitude above sea level. Which prediction is best supported?
A Year 7 student hypothesises that dark-coloured roofs make a house hotter than light-coloured roofs on a sunny day. Which test would give the fairest evidence for or against this hypothesis?