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Hydro-meteorological hazards: cyclones, floods, drought and ENSO

Edexcel IAL Geography · WGE01 Global Challenges

Hydro-meteorological hazards are caused by atmospheric and water processes. Short-term weather causes cyclones and floods; medium-term rainfall trends cause drought; and the ENSO cycle links floods and droughts across the Pacific.

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Tropical cyclones and floods

Tropical cyclones (hurricanes, typhoons) are intense low-pressure systems that form over warm tropical oceans. Their hazards are:

The Saffir-Simpson scale rates cyclones 1 to 5 by maximum sustained wind; it does not account for storm surge, rainfall flooding or tornadoes (NOAA NHC).

Floods result from intense or prolonged rainfall, snowmelt, storm surge or blocked drainage. Flash floods rise within hours on steep or urban catchments.

Drought

Drought is a medium-term hazard: a long period of below-average rainfall.

It develops slowly, can last years and affects huge areas, so it is hard to define when it starts and ends.

ENSO: El Nino and La Nina

Under normal conditions trade winds blow west along the equatorial Pacific, piling warm water near Asia (about 8 C warmer than the eastern Pacific). Cold, nutrient-rich water upwells off South America. Rain falls over the warm western Pacific (NOAA PMEL).

During El Nino the trade winds weaken, warm water spreads east, upwelling off South America weakens and rainfall shifts east: flooding in Peru and drought in Indonesia and Australia (linked to bush fires).

La Nina is the opposite: unusually cold water in the central and eastern equatorial Pacific, with stronger normal patterns.

The 1982 to 1983 and 1997 to 1998 El Ninos were among the largest on record.

Worked example

Explain how El Nino can cause drought in eastern Australia.

  1. The trade winds weaken, so warm surface water moves east, away from Australia.
  2. The area of rising air and rainfall shifts east with the warm water.
  3. Sinking, drier air dominates over eastern Australia, so rainfall falls below average for months.

Answer: Weaker trade winds move warm water and rainfall east, leaving Australia under drier air.

Key idea: Cyclones: wind, storm surge, rain. Drought: medium-term rainfall deficit. El Nino: weak trade winds, warm water east, floods in Peru, drought in Australia and Indonesia. The June 2025 examiners noted many answers explained only floods or only droughts.

Check yourself

  1. Which cyclone hazard is NOT included in a Saffir-Simpson category?
    • Maximum sustained wind speed
    • Wind damage to roofs
    • Storm surge
    • Wind strength over five categories
    Show answer

    Storm surge. The scale is based only on sustained wind; surge and rain are excluded.

  2. Using the NHC scale (Category 4: 130 to 156 mph), a hurricane with sustained winds of 140 mph is
    • Category 4
    • Category 3
    • Category 5
    • Category 2
    Show answer

    Category 4. 140 mph lies between 130 and 156 mph.

  3. During an El Nino event, which change is typical on the coast of Peru?
    • Stronger upwelling of cold water
    • More severe drought
    • Heavier rainfall and flooding
    • Stronger trade winds
    Show answer

    Heavier rainfall and flooding. Warm water and rainfall shift east towards South America.

  4. La Nina is best described as
    • unusually warm water off Peru
    • unusually cold water in the central and eastern equatorial Pacific
    • a tropical cyclone in the Pacific
    • the reversal of the Gulf Stream
    Show answer

    unusually cold water in the central and eastern equatorial Pacific. It is the cool counterpart of El Nino.

  5. Why is drought called a medium-term hazard?
    • It lasts only a few hours
    • It is caused by plate movement
    • It only affects one river basin
    • It develops from below-average rainfall over months or years
    Show answer

    It develops from below-average rainfall over months or years. Drought builds up slowly from a long rainfall deficit.

  6. When rivers, reservoirs and groundwater fall to low levels, the drought is described as
    • meteorological
    • hydrological
    • agricultural
    • geophysical
    Show answer

    hydrological. Hydrological drought refers to water stores.

  7. Explain how El Nino changes rainfall patterns on the two sides of the Pacific.
    Show answer

    The trade winds weaken so warm water moves east; rainfall shifts east, bringing floods to Peru, while Indonesia and Australia have drought.. Weaker trade winds, warm water east, more rain in the east (Peru floods), drought in the west (Australia, Indonesia).

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Last updated 8 November 2026. Spotted a mistake? Tell us.