What you'll learn

• How extreme weather becomes a natural weather hazard.
• Why tropical storms form over some warm oceans but not others.
• Why droughts develop, especially in high-pressure and monsoon-margin regions.
• How the distribution and frequency of these hazards vary globally, and how they may be changing.

1. Starting point: weather, extremes and hazards

Weather means the day-to-day condition of the atmosphere, such as temperature, rainfall, wind speed, cloud cover and pressure.

Extreme weather is weather that is unusual or severe compared with the normal conditions for that place. This is important: 35°C may be normal in parts of the Sahara, but extreme in the UK.

Extreme weather only becomes a serious hazard when it affects people or valuable environments. For example, a powerful storm over the open ocean may be less damaging than a weaker storm hitting a densely populated coast.

2. The atmospheric background: why air rises or sinks

At a global scale, weather hazards are linked to how the Sun heats the Earth unevenly.

The Equator receives more concentrated solar energy than the poles, so warm air near the Equator tends to rise. Rising air creates low pressure, which often brings clouds and rainfall.

At around 30° north and south of the Equator, air often sinks. Sinking air creates high pressure, which usually brings clear skies, dry conditions and low rainfall.

This rising and sinking pattern helps explain both hazards in this topic:

• tropical storms need warm, moist, rising air over oceans
• droughts often develop where air sinks and rainfall is suppressed

3. Tropical storms: what they are

A tropical storm is an intense low-pressure weather system that forms over warm tropical oceans. At GCSE, this usually refers to severe tropical cyclones that can produce destructive winds, torrential rain, storm surges and coastal flooding.

They have different names in different ocean basins:

• hurricanes in the North Atlantic and eastern North Pacific
• typhoons in the western North Pacific
• cyclones in the Indian Ocean and around Australia

4. How tropical storms form

Tropical storms need several conditions to happen at the same time.

First, the sea surface usually needs to be about 27°C or warmer. Warm water increases evaporation, where liquid water changes into water vapour. This loads the air with moisture.

As warm, moist air rises, it cools and condenses into clouds. Condensation releases latent heat, which is stored heat energy. This extra heat makes the air rise even more strongly, deepening the low pressure.

The storm also needs the Coriolis effect, which is the apparent turning of moving air caused by the Earth’s rotation. This helps the storm spin. The Coriolis effect is too weak right at the Equator, so tropical storms usually form at least about 5° north or south of it.

Finally, the storm needs low vertical wind shear. Vertical wind shear means a change in wind speed or direction with height. If wind shear is too strong, it can tear the storm apart before it organises.

The diagram below shows how these processes connect inside a developing tropical storm.

5. Drought: what it is

A drought is a long period when rainfall is below normal, causing a shortage of water. Droughts are usually slow-onset hazards, meaning they build up over weeks, months or even years.

There are different types:

• meteorological drought: rainfall is below the long-term average
• agricultural drought: soil moisture is too low for crops and pasture
• hydrological drought: rivers, reservoirs and groundwater stores fall below normal

6. What causes drought?

A major cause is persistent high pressure. In a high-pressure system, air sinks. As it sinks, it warms and becomes less likely to condense into clouds, so rainfall is limited.

A long-lasting high-pressure system is sometimes called a blocking anticyclone. It can divert rain-bearing weather systems away from an area.

Drought can also be linked to the Intertropical Convergence Zone, or ITCZ. This is a seasonal belt of low pressure near the tropics where warm air rises and rainfall is common. In regions such as the Sahel in Africa, people rely on the ITCZ moving northwards to bring seasonal rain. If it does not move far enough, or the rains are weaker than usual, drought can develop.

Another cause is ENSO — the El Niño-Southern Oscillation. This is a natural pattern of changing sea temperatures and air pressure in the Pacific Ocean. El Niño and La Niña events can shift rainfall patterns around the world, contributing to drought in places such as Australia, Indonesia, southern Africa or the Horn of Africa, depending on the event.

Evapotranspiration also matters. This is water lost from the land through evaporation from surfaces and transpiration from plants. Hotter conditions increase evapotranspiration, so drought can become more severe even if rainfall only falls slightly.

7. Global distribution of tropical storms and drought

Tropical storms are mainly found over warm tropical oceans between about 5° and 30° north and south of the Equator. They are common in the North Atlantic and Caribbean, western North Pacific, north Indian Ocean, southwest Indian Ocean, around northern Australia, and parts of the South Pacific.

They are rare at the Equator because the Coriolis effect is too weak. They are also rare in the South Atlantic and eastern South Pacific because ocean conditions are usually less favourable, including cooler water and stronger wind shear.

Drought-prone areas include the subtropical high-pressure belts around 30° north and south, desert margins, continental interiors, rain-shadow areas and places with unreliable seasonal rainfall. Examples include the Sahel, the Horn of Africa, Australia, the Middle East, the Mediterranean, southwest USA and northern Mexico, and parts of southern Africa.

The map below brings these global patterns together.

8. Frequency: how often do these hazards happen?

Frequency means how often an event occurs in a given time period. For tropical storms, frequency is often measured as the number of named storms per year in an ocean basin.

Globally, around 80 to 90 named tropical storms form in a typical year, although the exact number varies by dataset and year. Roughly half may reach hurricane, typhoon or cyclone strength. The western North Pacific is usually the most active basin.

Tropical storms also have seasonal patterns. In the Northern Hemisphere, activity is often highest in late summer and autumn, when oceans have stored the most heat. In the Southern Hemisphere, the main season is usually around January to March.

Drought frequency is harder to measure because drought has no single start date or end date. It depends on whether you are measuring rainfall, soil moisture, river flow, reservoir storage or impacts on people.

9. Have tropical storms and droughts changed over time?

This is where you need to be careful and balanced.

For tropical storms, the total global number each year has not shown a simple, clear increase. However, there is stronger evidence that the proportion of very intense storms has increased in some basins, and warmer air can hold more moisture, making extreme rainfall more likely. Rising sea level can also make storm surges more damaging.

Satellite monitoring since the 1970s has improved storm detection, especially over oceans. This means older records may have missed some storms, so long-term trends must be interpreted carefully.

For droughts, climate change can increase risk because warmer temperatures raise evapotranspiration. This can turn a dry period into a more severe hot drought. Some regions, such as the Mediterranean, southern Africa, western North America and parts of the Horn of Africa, have experienced increased drought stress or more severe drought impacts. But drought is not increasing everywhere in the same way.

Named examples you could use include Typhoon Haiyan in the Philippines in 2013, Cyclone Idai affecting Mozambique, Zimbabwe and Malawi in 2019, and the Horn of Africa drought affecting Ethiopia, Somalia and Kenya in the early 2020s. Exact impact figures vary by source, so use the figures your teacher or case-study sheet gives you.