Heat wave

A heat wave is defined as a prolonged period of excessively hot weather, representing a significant deviation from the typical climate of a region for that time of year. It is primarily characterized by abnormally high maximum daytime temperatures and, critically, is often accompanied by elevated minimum nighttime temperatures. This sustained heat, even during the night, is a critical factor in amplifying its impacts.

Specific definitions of heat waves vary considerably depending on geographical location, local climate, and the intended purpose of the definition (for example, for public health warnings versus meteorological records). However, a common meteorological threshold requires temperatures to remain significantly above the local seasonal average for a duration of at least 2 to 5 consecutive days. What constitutes "significantly above average" can be quantified by:

• Exceeding a certain percentile of historical temperatures (such as the 90th or 95th percentile).
• Surpassing specific absolute temperature thresholds that are locally relevant.

Heat waves are most typically associated with stationary high-pressure systems in the atmosphere, sometimes referred to as "heat domes." These systems induce the sinking of air. As this air descends, it is compressed and warms. 

Additionally, high-pressure systems actively suppress the formation of clouds and precipitation, leading to uninterrupted and intense solar radiation reaching the surface, which further heats the ground and the air above it. The stagnant nature of the high-pressure system also limits horizontal air circulation, preventing cooler air masses from entering the region and effectively trapping the heat near the surface for an extended period.

In urban areas, this situation is often intensified by the urban heat island effect. Built environments, such as asphalt and concrete, absorb and retain heat more effectively than surrounding natural landscapes. This results in higher local temperatures, particularly noticeable during the night, which further elevates health risks and increasing cooling demands.

How heat waves manifest

The experience and impact of a heat wave are not uniform but vary significantly based on specific meteorological conditions and the local environment:

Daytime intensity

The most evident manifestation is the experience of exceptionally high maximum temperatures during the day. These temperatures can soar far beyond typical summer highs, sometimes reaching 40°C (104°F) or even higher in many temperate and subtropical regions. 

The intensity of direct solar radiation also plays a major role, especially under the clear skies characteristic of high-pressure systems.

Nighttime heat

A critical, and often underestimated, aspect of a dangerous heat wave is elevated minimum temperatures. When nighttime temperatures remain high, both the human body and many ecosystems are deprived of adequate relief. 

This reduces the body’s ability to cool down and recover from the daytime heat stress, substantially increasing the risk of heat-related illnesses and mortality, particularly during prolonged events. Tropical nights, where the temperature does not drop below 20°C (68°F), can be especially stressful.

Humidity

The level of atmospheric moisture profoundly influences how heat is perceived and its overall impact:

• In humid climates: Heat waves are often characterized by high dew points. High humidity hinders the body’s ability to cool itself through sweating and evaporation, making the conditions feel significantly hotter and more oppressive than the air temperature alone suggests. This effect is often quantified using the heat index or "feels like" temperature.
• In dry regions: While humidity is low, heat waves are dominated by intense sunshine and very high air temperatures. Low humidity can lead to rapid evaporation from surfaces and bodies, but the sheer intensity of the heat can still be extremely dangerous, and the risk of dehydration is high. Dry heat also significantly increases the risk of wildfires.

How heat waves are classified

Given the regional variations, there is no single, universally agreed-upon definition or classification system for heat waves. However, meteorological and public health agencies commonly classify or describe heat waves using a combination of the following criteria:

1. Maximum temperatures:
   
   • Intensity is often defined by how much the daily maximum temperature exceeds a certain percentile of the historical temperature distribution for that specific date or period (e.g., exceeding the 90th or 95th percentile over a 30-year climatological reference period). This relative threshold helps account for regional climate differences.
   • Some definitions also employ absolute temperature thresholds (e.g., exceeding 35°C, 40°C, or even higher for a specified number of days), particularly in regions where such temperatures are relatively rare.
   • The specific threshold chosen often depends on the vulnerability of the local population and infrastructure.
2. Minimum temperatures:
   
   • Increasingly, heat wave definitions, especially those focused on public health outcomes, include criteria for high overnight minimum temperatures.
   • Persistent warm nights are a critical factor in exacerbating heat stress on vulnerable populations by preventing physiological recovery during sleep. Metrics like the number of consecutive nights above a certain temperature threshold (e.g., 20°C or 25°C) are often used.
3. Duration:
   
   • A heat wave typically requires a minimum number of consecutive days where the temperature criteria are met. This commonly ranges from 2 to 5 consecutive days, but some definitions may require a longer minimum duration.
   • Longer-duration events are generally more dangerous and impactful. Some heat waves, particularly those linked to persistent atmospheric blocking patterns, can last for a week, two weeks, or even longer, leading to cumulative stress on systems and populations.
4. Compound indicators:
   
   • To provide a more comprehensive picture of heat stress, some agencies utilize indices that combine multiple factors like temperature, humidity, and duration. Examples include:
     
     • The heat index: Combines air temperature and relative humidity to estimate the "feels like" temperature for the human body.
     • The wet-bulb globe temperature (WBGT): A measure used to estimate the effect of temperature, humidity, wind speed, and radiant heat on humans, often used in occupational health and sports.
     • The excess heat factor (EHF): Considers both the intensity of the heat and the acclimatization of the population to recent temperatures.

Impacts of heat waves

Heat waves have widespread and often severe consequences across various sectors:

Human health

The most immediate and serious impact is on human health. Exposure to extreme heat can lead to a range of heat-related illnesses, from milder conditions like heat rash and cramps to more severe and potentially fatal conditions such as heat exhaustion and heatstroke. 

Heat stress can also worsen pre-existing health conditions, particularly cardiovascular, respiratory, and renal diseases, leading to increased hospitalizations and mortality.

Vulnerable groups are disproportionately affected, including:

• The elderly
• Infants and young children
• Pregnant women
• Individuals with chronic illnesses
• Outdoor workers and athletes
• Low-income populations without access to adequate cooling
• Those who are socially isolated

As previously highlighted, prolonged nighttime heat is a major contributing factor to heat-related mortality as it prevents the body from recovering overnight.

Agriculture

Heat waves pose significant threats to agricultural productivity:

• Crop damage: Heat stress can damage crops at various stages of development, reducing yields and impacting quality. It can disrupt crucial processes like pollination, seed set, and fruit development.
• Livestock stress: Livestock suffer from heat exposure, which can lead to reduced weight gain, decreased milk or egg production, reproductive problems, and increased mortality.
• Timing is critical: Heat occurring during sensitive growth stages (such as flowering or grain filling for crops) can be particularly damaging and lead to substantial economic losses. Heat also significantly increases the demand for water for irrigation and livestock.

Infrastructure

Heat waves place significant strain on critical infrastructure:

• Power grids: Face immense pressure due to the surge in demand for electricity for air conditioning and cooling, increasing the risk of brownouts and blackouts.
• Transportation: Roads can buckle and melt, railway tracks can warp due to thermal expansion leading to speed restrictions and delays, and airport runways can be impacted, affecting flight operations.
• Water supply: Systems may be strained as demand for water increases for drinking, irrigation, and cooling, especially when heat waves coincide with or exacerbate drought conditions.

Wildfires and ecosystems

High temperatures significantly increase evaporation rates, drying out soil moisture and vegetation. This creates tinderbox conditions and dramatically increases the risk and intensity of wildfires. 

The fire weather index is often used to monitor and forecast wildfire risk during hot and dry conditions, as it integrates factors like temperature, wind speed, humidity, and fuel availability.

• Aquatic ecosystems: Are vulnerable as water temperatures rise, leading to reduced dissolved oxygen levels, which can stress or kill fish and other aquatic life.
• Terrestrial ecosystems: Experience heat stress on plants and animals, potentially leading to vegetation dieback and changes in species behavior and distribution.

Heat waves and droughts

Heat waves and droughts are often interconnected, creating dangerous compound extreme events that are more impactful than either phenomenon occurring in isolation:

Drought intensifies heat waves

When the ground is dry due to drought, less solar energy is used for evaporation (latent heat), and more energy is transferred into heating the air (sensible heat). 

This lack of evaporative cooling allows surface temperatures to rise much higher than they would over moist soil, amplifying the intensity of the heat wave.

Heat waves can intensify droughts

Conversely, the high temperatures and increased solar radiation during a heat wave significantly increase evaporation from land surfaces and water bodies and increase plant transpiration. 

This leads to a more rapid depletion of soil moisture and surface water resources, exacerbating existing drought conditions or even triggering the onset of a drought.

Feedback loops and compounding impacts

The interaction between heat and drought can create positive feedback loops. A heat wave dries out the landscape, which makes the subsequent heat wave even hotter. 

This cycle is a growing concern under climate change. The combination of extreme heat and water scarcity is particularly dangerous for agriculture, water resources, and ecosystems, potentially leading to widespread crop failure, severe water shortages, and increased tree mortality, with significant economic and environmental consequences. 

In some regions, prolonged and intense heat waves—particularly those associated with persistent blocking high-pressure systems—can be a primary driver in the initiation or worsening of drought conditions by overwhelming the system’s ability to cope with increased evaporative demand.

Understanding and addressing heat wave impacts

In summary, a heat wave is far more than simply a few hot days. It is a complex meteorological phenomenon primarily driven by persistent high-pressure systems, characterized by prolonged periods of abnormally high temperatures, including crucial elevated nighttime minimums. 

Its definition varies regionally but is typically classified by temperature thresholds (absolute or relative), duration, and sometimes compound indices. The impacts of heat waves are significant and far-reaching, affecting human health, agriculture, critical infrastructure, and natural ecosystems. 

Furthermore, heat waves are intrinsically linked with droughts, often amplifying each other’s effects in dangerous compound events. 

Understanding the multifaceted nature of heat waves and their interactions with other climate extremes is essential for effective forecasting, public health preparedness, infrastructure planning, and building climate resilience in a warming world.