Heat Wave Definition

by Laurent Cousineau

Heat Wave Across Europe in 2003

Heat Wave Across Europe in 2003

A heat wave is a prolonged period of extremely high temperatures for a particular region.

A Relative Term

However, there exist no universal definitions for a heat wave as it is relative to a specific area and to a certain time of year.

In fact, average temperatures in one region may be considered heat wave conditions in another. For instance, an average day in the Mediterranean would be regarded as heat wave conditions in Northern Europe.

When temperatures soar to about 10 degrees Celsius above the normal for several days, it is considered to be a heat wave.

When and Where does it Occur?

Often, these unusually high temperatures are combined with humidity and frequently take place between early July and early September.

Regions that are more susceptible to heat waves include:
  • Inland deserts
  • Semi deserts
  • and Mediterranean-type climates


The main causes of heat waves are:
  • Climate Change
  • The position of the jet stream (it causes air on one area to be significantly warmer than another)
  • El Niño and La Niña (opposite reaction to El Niño) which could disrupt jet streams


Heat waves could have dire effects including:
  • Disastrous crop failures,
  • Deaths from hyperthermia
  • Widespread power outages due to increased use of air conditioning
  • Wildfires (often when a heat wave is combined with a drought)
  • Other fires as power transformers explode under the heat and humidity
  • Damage to infrastructure (roads, highways and water lines)

Notable Examples

In 2003, over 70,000 people died during an unprecedented heat wave in Europe, France being the most affected (see image above). It was Europe's hottest summer on record since at least 1540. In addition, the heat wave combined with drought to cause a crop shortfall in Southern Europe.

In 1936, the North American heat wave caused over 5,000 deaths and devastated incredible amounts of crops. In the United States, heat waves are the deadliest weather phenomenon as they cause more deaths than floods, hurricanes, lightning and tornadoes. In the 1995 Chicago heat wave, 600 people died in nearly five days.

Worrisome Predictions

Sadly, the World Meteorological Organization predicts that in less than twenty years, the number of heat-related deaths will double due to global warming.

As climate change ensues, more frequent and powerful heat waves can be expected in the future.

However, heat waves are but one of many effects of climate change.

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