Heat wave

A heat wave is a period of abnormally hot weather that lasts for several days or even weeks. These events are often described as natural disasters because of their intense impact on the environment. Scientists usually measure a heat wave by comparing current temperatures to the usual climate of a specific area. This makes defining a heat wave difficult because what is normal in a desert is different from a cool region.

Because temperatures vary by location, different countries use different rules to decide when a heat wave is happening.

Heat waves form through a specific process in the upper atmosphere. It begins when a high-pressure area strengthens at an altitude of 5,000 meters. This area stays over a region for several days or even weeks. Under high pressure, the air sinks toward the Earth's surface. As this air sinks, it undergoes adiabatic warming, which means it warms and dries out.

This sinking air prevents convection, a process that helps clouds form. Without clouds, more shortwave radiation from the sun reaches the ground, increasing the heat.

There are different ways to classify these events depending on where they occur. In the ocean, scientists study marine heatwaves. These are prolonged periods of unusually warm sea surface temperatures in a specific region. They are often triggered by high-pressure systems that reduce cloud cover over the water. These marine events can cause mass mortality in benthic communities and lead to coral bleaching.

On land, definitions vary significantly by country and local climate.

Different nations use specific numbers to define their own heat waves. For example, Greece defines a heat wave as three consecutive days with temperatures at 39°C or higher. In Sweden, a heat wave requires five days of temperatures exceeding 30°C. The Netherlands and Belgium use a threshold of 30°C for at least five days. In the United States, the Northeast often looks for temperatures of 32°C or higher for three days. These varying thresholds allow local authorities to respond to the specific needs of their climate.

In the United Kingdom, the Met Office uses a Heat Health Watch system to manage risk. This system uses four different levels to categorize the danger. Level 1 represents normal summer conditions for the region. Level 2 occurs when there is a 60% or higher risk of temperatures staying above the threshold. Level 3 is triggered when there is a 90% or higher chance of continued extreme heat.

Level 4 involves the most severe conditions and requires a widespread response from social and health services.

Since the 1950s, heat waves have become more frequent and intense over land. This trend is caused by climate change. According to the World Meteorological Organization, heat waves continued to intensify in 2024. Record-breaking temperatures were reported across Europe, North America, and China.

In many regions, temperatures rose above 45°C for several days in a row. One study of 13,115 cities found that extreme heat exposure tripled between 1983 and 2016.

Extreme heat has serious consequences for human health and the global economy. It can overwhelm human thermoregulation, the body's way of controlling its internal temperature. This can lead to hyperthermia and thousands of deaths.

Heat waves also damage the economy by reducing labor productivity and disrupting industry. They can cause catastrophic crop failures and increase the risk of wildfires during droughts. Furthermore, the high demand for air conditioning can lead to widespread electricity outages.