6.4 Drought

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Describe and explain the causes of drought.
  2. Explain how prolonged high pressure leads to a lack of rain.
  3. Explain the effect of El Niño Southern Oscillation (ENSO) and La Niña on ocean temperatures and evaporation, and how these cause drought.
  4. Explain how climate change contributes to drought.

Main Content

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What is a Drought?

A drought is a long period of time when an area receives much less rainfall than normal. This shortage of water affects people, animals, plants, and the environment. Droughts are natural hazards — events caused by nature that can seriously harm communities.

Droughts do not happen suddenly. They build up slowly, sometimes over months or even years. Understanding why they happen helps us manage them better.

Cause 1: Prolonged High Pressure

To understand this cause, you first need to understand what air pressure is.

Air pressure is the weight of air pushing down on the Earth's surface. There are two types:

  • Low pressure — air rises upward. As it rises, it cools and forms clouds, which produce rain. Low pressure = wet weather.
  • High pressure — air sinks downward. Sinking air warms up and prevents clouds from forming. High pressure = dry, sunny weather.

How does high pressure cause drought?

When a region experiences prolonged high pressure (meaning high pressure that stays in place for a very long time — weeks, months, or even longer), the following happens step by step:

  1. Air over the region sinks downward continuously.
  2. Sinking air warms up as it descends.
  3. Warm, sinking air cannot hold moisture in the form of clouds.
  4. No clouds form, so no rain falls.
  5. The longer the high pressure stays, the drier the land becomes.
  6. Eventually, the region suffers a drought.

Think of it this way: high pressure acts like a "lid" that traps dry conditions and prevents rain-making clouds from developing.

Example: The Sahara Desert in North Africa sits under near-permanent high pressure, which is why it receives almost no rainfall year-round.

Cause 2: El Niño Southern Oscillation (ENSO) and La Niña

This is one of the most important — and most commonly examined — causes of drought. Let's build up the explanation step by step.

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