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How does a heat wave happen?

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Quick answer

A heat wave forms when a slow-moving high-pressure system traps air over a region. The sinking air is compressed and warmed, clouds are suppressed, and dry ground stops evaporation from cooling the surface. With little wind to move the mass, heat accumulates day after day until temperatures far exceed normal.

A heat wave happens when a large, slow-moving high-pressure system parks over a region and traps hot air near the ground for days or weeks. The sinking air compresses and warms, skies stay clear, and the ground keeps heating until temperatures climb far above normal.

Step by step

  1. A large high-pressure system builds and settles over a region, often linked to a stalled jet stream.
  2. Air inside the system sinks toward the ground, and the increasing pressure compresses and heats it.
  3. Sinking air dries out, so clouds cannot form and the sun heats the ground all day.
  4. Dry soil means little water evaporates, so solar energy warms the air instead of being used for evaporation.
  5. The slow-moving system keeps the hot air mass in place, and heat accumulates over several days.
  6. Temperatures peak until the high-pressure system weakens or moves away and cooler air arrives.

Key things to understand

  1. A strong high-pressure system, often called a heat dome, settles over a region and pushes air downward toward the ground.
  2. As that air sinks it is compressed by the higher pressure below, and compression raises its temperature.
  3. The sinking air also suppresses cloud formation, so sunlight reaches the ground almost unchecked all day.
  4. Dry soil and low vegetation mean less water evaporates, so more of the sun's energy goes into heating the air instead of cooling it.
  5. Because the high-pressure system moves slowly, the same hot air mass sits in place for days, letting heat build up.

Labelled diagram

  1. High pressure forms

    A slow-moving high-pressure system settles over the region and begins to push air downward.

  2. Air sinks and warms

    Compression from sinking raises the air temperature and dries it out.

  3. Clouds disappear

    Dry sinking air suppresses cloud formation, leaving clear skies and strong sunlight.

  4. Ground heats up

    Sun-baked dry soil releases little moisture, so heat stays in the air.

  5. Heat accumulates

    The stalled system holds the hot air for days, pushing temperatures higher.

  6. System breaks down

    When the high pressure weakens or moves, cooler air returns and the heat wave ends.

How a heat dome builds and traps heat

Real-world examples

  • The 2003 European heat wave pushed temperatures above 40°C (104°F) in France and killed tens of thousands of people, many of them elderly.
  • In May 2022, Jacobabad in Pakistan recorded around 51°C (124°F), among the highest temperatures ever measured in South Asia.
  • The 2021 Pacific Northwest heat wave sent Lytton, Canada, to 49.6°C (121°F), a national record, before a wildfire destroyed the town.
  • In 2015, a heat wave in Andhra Pradesh and Telangana, India, killed more than 2,000 people during a prolonged spell of 45°C-plus days.

Common mistake

✗ Myth: A heat wave is just a few unusually hot summer days.

✓ Fact: A heat wave is defined by unusually high temperatures relative to the local normal, sustained over several days and often nights. The duration matters because heat accumulates and the body gets no overnight recovery, which is what makes it dangerous.

People also ask

What causes a heat wave?
A heat wave is caused by a large high-pressure system that stalls over a region. Air sinks inside it, compresses and warms, clouds are suppressed, and dry ground stops evaporation from cooling the surface. Because the system moves slowly, heat builds up over many days.
How long can a heat wave last?
Most heat waves last from a few days to a couple of weeks, depending on how long the high-pressure system stays in place. The record-breaking European heat wave of 2003 lasted around two weeks in some areas. The longer it lasts, the more heat accumulates and the greater the health risk.
Why are heat waves worse at night?
At night the ground normally radiates heat away and the air cools. During a heat wave, high humidity and a warm air mass slow that cooling, and city surfaces release stored heat. Without cooler nights, the body cannot recover, which raises the risk of heat illness.
Why do heat waves last so long?
The high-pressure system responsible is large and slow-moving, so it can sit over one region for days or weeks. Each sunny day adds more heat, and without wind or rain to mix and cool the air, temperatures keep climbing until the pattern finally shifts.
Are heat waves becoming more common?
Yes, climate scientists report that heat waves are becoming more frequent, longer and more intense as global average temperatures rise. A warmer baseline means ordinary summer weather starts from a higher starting point, so hot spells push further into record territory.
Why are cities hotter during a heat wave?
Cities trap heat in asphalt, concrete and brick, which absorb sunlight and release it slowly overnight. This urban heat island effect can make a city several degrees warmer than nearby countryside, and it also stops nights from cooling down enough for relief.
What makes a heat wave dangerous?
The body relies on sweating and evaporation to shed heat, and prolonged high temperatures plus humidity can overwhelm that cooling system. This can lead to heat exhaustion or heatstroke, especially for older people, outdoor workers and those without air conditioning; anyone unwell should see a doctor.

Part of the guide: Water & Weather Phenomena →