Anatomy of a Storm

The calm at the centre of the worst

A tropical storm seen from above is a spiral several hundred kilometres across, and at its heart is a hole. The eye is typically 30 to 50 km wide, and inside it the air is descending rather than rising, so the wind drops, the rain stops and the sky can be blue. People who have been through one describe stepping outside into sunshine. Then the other side of the storm arrives. That is because the eye is ringed by the eyewall, a wall of towering cloud where air is spiralling upwards fastest and where the strongest winds and heaviest rain in the entire system are found. Cloud tops there can reach 15 km. Beyond the eyewall, curved rainbands spiral outwards with gaps of calmer weather between them, so a storm passing overhead delivers bands of violent weather separated by lulls, then the eerie quiet of the eye, then the eyewall again from the opposite direction. Storms are graded on the Saffir-Simpson scale by sustained wind speed, starting at category one at 119 km/h and reaching category five above 252 km/h. Typhoon Haiyan, which struck the central Philippines on 8 November 2013, made landfall with one-minute sustained winds of around 315 km/h and a central pressure near 895 hPa, putting it among the most powerful storms ever recorded to hit land. This module covers the structure, the named example and the question of what a warming world does to both.

Five storm terms

The first two are the pair that answers most often reverse. The third is what actually kills people. The last two are how storms are measured and where the damage happens.

Order the life of a storm

These six stages describe a tropical storm from formation to the end. Put them in order.

• Air is heated over an ocean surface above about 27 degrees Celsius and rises rapidly
• The rising air cools and its moisture condenses, releasing heat that drives it upwards harder
• Air drawn in beneath is deflected by the rotation of the Earth, so the system begins to spin
• An eye forms at the centre and the system intensifies while it remains over warm water
• The storm is steered westwards and then polewards, and crosses a coast
• Cut off from the warm ocean that fuelled it, the system weakens and breaks up

Where is the worst of it?

A family shelters as a tropical storm passes directly overhead. Which answer correctly describes what they experience and why?

• Violent wind and rain as the eyewall arrives, then an abrupt calm with clearing sky as the eye passes over, then violent weather again from the opposite direction as the far side of the eyewall reaches them. The lull happens because air descends inside the eye, warming as it sinks so that cloud cannot form, while in the eyewall air is spiralling upwards fastest of anywhere in the storm. The dangerous mistake is treating the calm as the end, because the second half of the eyewall is still to come.
• The worst conditions occur as the eye passes overhead, since that is the centre of the storm.
• Conditions worsen steadily to a single peak and then ease steadily away.
• There is heavy rain throughout but the wind stays constant across the whole storm.

Haiyan: effects and responses

The named example needs six to eight quantified details, sorted the way the mark scheme sorts them: effects into primary and secondary, responses into immediate and long-term. Typhoon Haiyan struck the central Philippines on 8 November 2013.

Where do the strongest winds blow?

The image shows a tropical storm from directly above, moving in the direction of the arrow. Tap the part of the storm where the winds are strongest.

Match each part to what it does

Sorting, and the careful climate sentence

Two things decide the grade here, and the second is where most candidates overreach. Sort what you know. A named-example question is marked on organised knowledge, so never deliver impacts in a heap. Split effects into those the storm caused directly and those that followed from the damage, and split responses into what happened in the first hours and days and what took months or years. The sorting is worth marks in itself, because it demonstrates that you understand the difference between a wall collapsing and a disease outbreak that follows because the water supply failed. Be careful about climate change. The tempting sentence is that a warming world simply means more tropical storms, and it is not what the science says. The expected changes are not all in the same direction. A warmer ocean surface holds more energy, so the proportion of storms reaching the highest categories is expected to rise and the rainfall they deliver is expected to increase. Rising sea level makes any given surge reach further inland, which matters enormously since surge is the biggest killer. But the total number of storms forming each year is not expected to rise in step, and some projections have it falling. The creditworthy claim is about intensity and impact rather than count, and writing that distinction into a sentence marks out a strong answer immediately. Finally, remember what the category scale leaves out. It grades sustained wind only, so a slow-moving lower-category storm that sits over a region for two days can kill far more people through flooding than a faster, fiercer one. Quoting a category is a start, not a description.

Complete the tropical storm facts

At the centre of a tropical storm is the eye, typically 30 to 50 km across, where air _____ and so no cloud forms and the wind drops. Around it stands the eyewall, where the strongest winds and heaviest rain occur. Storms are graded by sustained wind speed on the Saffir-Simpson scale, which begins at _____ km/h for category one. Typhoon Haiyan crossed the central Philippines on 8 November _____ with sustained winds near 315 km/h, and the roughly 5 m storm _____ it drove ashore destroyed something like 90 per cent of Tacloban. A storm weakens after landfall because it is cut off from the warm _____ that had been fuelling it.

Spot the three accurate storm claims

Select the THREE statements that are true.

• The expected effect of a warming climate is a greater proportion of very intense storms and heavier rainfall from them, together with worse surge because sea level is higher, rather than simply a larger number of storms each year.
• The Saffir-Simpson scale grades sustained wind speed only, so a slow-moving storm of a lower category can cause more deaths through prolonged flooding than a faster storm of a higher one.
• Storm surge, caused by wind driving water ashore and by very low central pressure letting the sea bulge upwards, is usually the biggest killer, and it drove roughly 5 m of water into Tacloban in 2013.
• The eye of a tropical storm contains the most violent conditions in the system.
• Climate scientists expect the total number of tropical storms forming each year to rise sharply.
• Tropical storms strengthen once they move over land, because there is less to slow them down.

A named-example answer, marked

Use a named example of a tropical storm to describe its effects and the responses to it. [9 marks] Typhoon Haiyan struck the central Philippines on 8 November 2013 with one-minute sustained winds of around 315 km/h and a central pressure near 895 hPa, which put it among the most powerful storms ever recorded making landfall. The primary effects came directly from the wind and the water. A storm surge of roughly 5 m drove into Tacloban, where something like 90 per cent of the city was destroyed and the airport was wrecked, and across the region around 6,300 people were killed and about 28,000 injured. Roughly 1.1 million houses were damaged or destroyed and around 600,000 hectares of farmland were hit, displacing some 4 million people and affecting around 14 million in total. Secondary effects followed from that damage rather than from the storm itself: power, water and communications were cut, blocked roads meant aid could not reach survivors for several days, and shortages of food and clean water raised the risk of disease and led to looting, with the overall bill of the order of 2 billion US dollars. The responses divided the same way. Immediately, around 800,000 people had been evacuated on official warnings before landfall, although some evacuation centres were themselves flooded by the surge; international appeals were launched within days and foreign governments, aid agencies and military units brought water, food, shelter and field hospitals once the airport reopened. In the longer term the Philippines rebuilt under a building back better programme with stronger construction standards and restrictions on rebuilding on the most exposed shoreline, replanted coconut and rice, and rewrote its warning language after it became clear that the phrase storm surge had not conveyed to many residents what was actually coming. What earned the marks: a named storm with a precise date and measurements; effects sorted into primary and secondary and responses into immediate and long-term, which is how the mark scheme is organised; figures attached to almost every point rather than to one or two; and the evacuation and the warning-language lesson used to show that a response can be both substantial and imperfect.

Find the two secondary effects

Five things that happened because of Typhoon Haiyan. Two of them are secondary effects, meaning consequences of the damage rather than of the wind and water themselves. Tap those TWO.

• Clean water supplies failed across the region, and the risk of disease rose sharply among survivors in crowded shelters.
• A wall of seawater around five metres deep swept through Tacloban as the storm came ashore.
• Blocked and broken roads meant relief supplies could not be driven to many communities for several days after the storm passed.
• Roughly 1.1 million houses were damaged or destroyed by the wind and the water.
• Sustained winds of around 315 km/h were recorded as the system made landfall.

Write the climate change sentence

Assemble a sentence that states the expected changes without overclaiming.

Four decisions around a landfall

Each stage is a real choice before, during or after a storm like Haiyan.

• Two days out. Forecasts show a category five system heading for a densely populated coast, and the projected surge is several metres. Your warnings have gone out using the standard technical vocabulary.
• You are choosing evacuation centres. The largest and sturdiest building in the city is a stadium two hundred metres from the shoreline at only a few metres above sea level.
• The wind has dropped abruptly, the rain has stopped and there is blue sky overhead. People around you are leaving shelter to inspect the damage.
• Six months on. Families want to rebuild where their homes stood, which in many cases is directly on the shoreline that the surge swept clean.

Explain the structure

Describe the structure of a tropical storm and explain how climate change may affect tropical storms in the future. [9 marks]

• Describe the structure from the centre outwards, giving the eye, the eyewall and the rainbands with a measurement for at least one of them
• Explain what the air is doing in each part, since the sinking air in the eye is why it is calm and the rising air in the eyewall is why it is violent
• Then turn to climate change, and separate intensity from frequency rather than treating them as one claim
• Include sea level rise, because it worsens storm surge, which is the part of a storm that kills most people
• Use cautious wording such as "expected to" and "projected", since the honest position is that the changes are not all in the same direction