Weather Extremes Lab

The machine, and the year it moves

Weather extremes are not scattered at random. There is a machine driving them, it runs on the same principle everywhere, and once you have it you can say why drought belongs in some places and storms in others. That much you may already have met. Here is the part that earns the higher marks on this specification. ⚠️ THE CIRCULATION EXPLAINS WHERE EXTREMES NORMALLY HAPPEN. AND "NORMALLY" IS DOING A LOT OF WORK. Every few years the machine shifts. The winds that hold the system in place weaken, warm water slides across an ocean, and the rain that a region depends on falls somewhere else entirely. That is El Nino, and it is not a separate topic. It is the same system, running differently for a year or two. Your specification asks for three things about each hazard, and they are three different questions. DISTRIBUTION is where it happens. FREQUENCY is how often. ⚠️ CHANGE OVER TIME asks whether either of those is moving - and most answers give one sentence that tries to cover all three. So the module runs in that order: the machine first, then what it produces, then what happens when it shifts.

Terms for pressure and circulation

Six terms. What each one means, with nothing about where it sits.

Put the labels on the three-cell model

A cross-section from the equator on the left to the pole on the right. Tap a label, then tap the pin it belongs to. Two labels have no pin here.

Three latitudes, three kinds of weather

The same loop, read at three different places. Each column is an outcome you can point to on a map.

Why sinking air brings no rain

A student can state that deserts sit under sinking air but cannot say why sinking air produces dry conditions. Which explanation is right?

• Sinking air warms as it descends, and warmer air can hold more water vapour, so its moisture stays as vapour instead of condensing into cloud
• The air is dry to begin with, because there is no water beneath a desert to evaporate
• The air sinks too quickly for rain to have time to form
• High pressure physically pushes clouds away to lower-pressure regions

How to answer a question about distribution

Your specification asks for the distribution, frequency and change over time of a hazard. Those are three separate questions, and one sentence cannot answer all three. ⚠️ DISTRIBUTION is WHERE. Answer it with locations and, better, with a reason for the pattern. A distribution answer that only lists places has described a map rather than explained one. ⚠️ FREQUENCY is HOW OFTEN. Answer it with a rate: several a year, one in a decade. Not "a lot", which is not a frequency. ⚠️ CHANGE OVER TIME is whether either of those is MOVING, and it is where the marks concentrate because it is the one most often skipped. Is the hazard becoming more common? Is it appearing where it did not before? Say which of the two you mean. And there is a trap in the evidence itself. ⚠️ More RECORDED events is not the same as more events. Detection has improved enormously, so a rising count may reflect better observation as much as a changing world. An answer that notices that is doing the analysis the question wants. One more, because it costs marks every year: a single dramatic event is not evidence about a trend. You need a record long enough for a pattern to be visible.

Match each latitude band to the hazard it favours

True about where tropical storms form

Select the TWO statements that are true.

• They form over ocean water warmer than about 26.5 degrees Celsius
• They do not form directly on the equator
• They can form over any ocean at any latitude, given enough wind
• They form over land as readily as over sea

The year the rain moved

Across the Pacific, the trade winds normally blow from east to west. They drag warm surface water with them, piling it up in the west near Indonesia and Australia. Warm water means rising air, and rising air means rain. Meanwhile cold water wells up off South America, and the eastern Pacific stays dry. That is the normal state, and whole economies are built on it. ⚠️ Every few years the trade winds weaken, and can even reverse. THE WARM WATER SLIDES BACK EASTWARDS, AND THE RISING AIR GOES WITH IT. This is an El Nino year. The rain does not disappear. It moves. The South American coast, normally dry, may receive months of it. And Australia and Indonesia, normally wet, get drought - which is why your specification lists El Nino under the CAUSES of drought rather than as a curiosity. ⚠️ LA NINA is the same machine pushed the other way. The trade winds strengthen, warm water piles even further west, and Australia and Indonesia get more rain than usual while the eastern Pacific becomes drier still. ⚠️ Notice what this does to the word NORMALLY. The circulation tells you where drought belongs in an average year. El Nino tells you that an average year is a statistical creature, and that the pattern itself has a rhythm. Which is exactly why "change over time" is a question worth asking of any hazard, and why one dry year proves nothing on its own.

Order the conditions that let a storm form

Put the stages of tropical storm formation into the order they occur.

• Ocean water is heated to above about 26.5 degrees Celsius
• Warm, moist air rises rapidly from the sea surface
• Rising air cools, its vapour condenses, and vast thunderclouds build
• Condensation releases heat, which drives yet more air upwards
• The Coriolis effect sets the whole rising column spinning
• The storm moves and strengthens over warm water, weakening once it crosses land

The circulation run

Five questions on the machine and what it produces. Three lives.

Spot the true circulation facts

Tap the TWO statements that are true.

• Air sinking at about 30 degrees is why the great desert belts sit where they do
• In an El Nino year, drought becomes more likely in Australia and Indonesia
• Tropical storms form most readily directly on the equator
• A rise in the number of recorded storms proves on its own that more storms are occurring

Complete the weather hazard facts

Where air rises, surface pressure is _____, and the rising air cools until its vapour condenses into cloud and rain. Where air sinks, surface pressure is _____, and the descending air warms so that cloud does not readily form. The loop nearest the equator is called the _____ cell, and beneath the sinking air at its poleward edge, at about _____ degrees, lie the world's great deserts.

Distribution, frequency, or change

A candidate writes: "Tropical storms happen in hot places near the sea and there seem to be more of them these days." Which of the three things the specification asks for has actually been answered?

• None of them properly: the distribution has no reason attached, the frequency is an impression rather than a rate, and the change over time offers no evidence and no timescale
• Distribution only, and fully
• Change over time only, and fully
• All three, briefly but adequately

Three claims about where hazards happen

Three pieces of student reasoning to judge. Each answer has to carry the explanation, not just the verdict.

• A student writes that "the air above the Sahara sinks because the desert below it is so dry". What has gone wrong?
• A student explains that in an El Nino year Australia gets heavy rainfall because the warm water moves towards it. Is that right?
• A student uses a graph showing far more recorded tropical storms in 2020 than in 1920 to argue that storms are becoming much more frequent. How should the argument be improved?

Explain why the hazards sit where they do

A classmate can label the three-cell model perfectly and still writes that droughts and storms happen "in hot countries". Write them the explanation that turns the diagram into an answer.

• Explain what happens to air at the equator and at about 30 degrees, and what each produces at the surface
• Use that to explain the distribution of the world's great deserts and of prolonged drought
• Explain where tropical storms form and why they do not form on the equator itself
• Explain what changes during an El Nino year, and which regions become more likely to suffer drought as a result
• Finish by explaining why a rise in the number of recorded hazards is not on its own proof that hazards are becoming more frequent