Heatwave glossary: Urban heat island, wet-bulb, heat index, and more

From wet-bulb temperature to heat dome, here are some key heatwave terms everyone should understand during extreme summers

  Summers are getting hotter, longer and more unpredictable, while heat-related warnings are also growing more complex. Weather departments and climate experts now frequently use terms such as severe heatwave or heat index to explain rising risks linked to extreme temperatures.

 

However, many people may not fully understand what these phrases actually mean or why they matter.

 

Here are some key heatwave terms that can help understanding forecasts, assess personal risk and take timely precautions during extreme heat conditions.

 

Severe heatwave

 

A heatwave is declared when the maximum temperature reaches at least 40°C in plains and 30°C in hilly regions, or when temperatures remain significantly above normal for several days.

 

 

A severe heatwave refers to more intense and prolonged heat conditions. According to the India Meteorological Department (IMD), a heatwave becomes 'severe' when temperatures reach 47°C or higher in plains, or when departures from normal temperatures become exceptionally high over a sustained period.

 

In simple terms, this means the heat is not only extreme but also prolonged, which increases the risk of dehydration, heatstroke and heat-related deaths.

 

Heat index or real-feel temperature

 

The heat index is the “feels like” temperature calculated using both air temperature and humidity. It is also commonly called the real-feel or apparent temperature.

 

For example, if the actual temperature is 38°C but humidity is high, the body may experience it as 45°C because sweat cannot evaporate easily and cool the body effectively.

 

Warm nights or night-time heat

 

Warm nights refer to unusually high temperatures during the night that prevent the body from cooling down and recovering from daytime heat exposure.

 

Hot nights can be especially dangerous because the body remains under stress for longer periods, which may lead to fatigue, disturbed sleep and increased health risks for the elderly and those with medical conditions.

 

Urban heat island

 

An urban heat island refers to a city area that becomes much hotter than surrounding suburban regions because of concrete buildings, roads, traffic and limited greenery.

 

Metro cities like Delhi, Mumbai and Bengaluru often remain warmer than nearby rural areas since concrete absorbs and traps heat during the day and slowly releases it at night.

 

Red alert and orange alert

 

The IMD issues colour-coded heat alerts to warn people about rising temperatures and related risks.

 

An orange alert is issued when severe heat conditions persists for 2 days and advises people to stay cautious. Meanwhile a red alert is issued when severe heat wave persists for more than 2 days and calls for immediate precautions, especially for vulnerable groups.

 

El Niño

 

El Niño is a natural climate pattern that occurs when surface waters in the central and eastern Pacific Ocean become unusually warm, which then disrupts weather patterns across different parts of the world. It typically develops every few years and can influence rainfall, temperatures and storms globally.

 

El Niño intensifies heatwaves and raises global temperatures because warmer oceans add more heat and moisture to the atmosphere. In India, El Niño years are commonly linked to hotter summers, weaker monsoons and dry conditions in several regions.

 

Heat dome

 

A heat dome is a high-pressure weather system that traps hot air over a region and prevents it from escaping. This creates a “lid effect” in the atmosphere, causing temperatures to remain unusually high for days or even weeks.

 

As the trapped air continues to heat up under clear skies and strong sunlight, temperatures keep rising and nights also remain unusually warm. Heat domes can intensify heatwaves, increase wildfire risks and place added stress on health systems and power supplies. Recent heat domes across parts of Europe pushed temperatures 12°C to 16°C above normal levels.

 

Wet-bulb temperature

 

Wet-bulb temperature measures the combined effect of heat and humidity and shows how effectively the human body can cool itself through sweating.

 

When wet-bulb temperatures rise too high, sweating becomes less effective and the risk of heat stress increases sharply as the human body struggles to cool down. Prolonged exposure to very high wet-bulb temperatures can become dangerous even for healthy individuals as heat exhaustion may quickly progress to heatstroke.

 

“Previous research suggested that the human body could withstand wet-bulb temperatures of up to 35°C, which is roughly equal to 95°F at 100 per cent humidity. However, recent medical research suggests the danger threshold for a healthy person carrying out routine daily activities may begin much earlier, at around 31°C,” said Dr Hitesh Punyani, director, internal medicine, BLK Max Hospital.

 

WBGT or Wet Bulb Globe Temperature, additionally factors in sunlight and wind along with heat and humidity. It is used to assess heat risks for outdoor workers, athletes and people exposed to extreme heat for long hours.