Why 40°C Feels Like 45°C: Understanding Heat Index and Heatwaves

If you’ve glanced at your phone’s weather app recently and seen a temperature of 40 degrees Celsius, followed by a smaller line saying “feels like 45,” you might have wondered what’s going on. How can it feel hotter than what the thermometer says? And more importantly, why does this matter?

The answer lies in the heat index, a measure that combines actual air temperature with relative humidity to tell us how hot it truly feels to the human body. While the air temperature is what the thermometer reads, the heat index is what your body experiences when it tries to cool itself through sweating. When humidity is high, sweat doesn’t evaporate as quickly from your skin, and evaporation is your body’s natural cooling mechanism. So on a humid day, your body retains more heat, and the result is that the temperature feels significantly hotter than what the air temperature indicates.

This difference between air temperature and what it feels like is not just a quirk of weather reporting. It has real implications for our health, comfort, and even survival, particularly during periods of intense heat known as heatwaves. In India, heatwaves are becoming more frequent and intense, stretching beyond their usual season of March to June and often pushing into July in some parts of the country. The India Meteorological Department (IMD) defines a heatwave based on both absolute temperature and how much the temperature departs from what is normal for that place and time of year. For the plains, a heatwave is declared if the temperature reaches at least 40 degrees Celsius, or if it is 4.5 to 6.4 degrees higher than average. It is classified as a severe heatwave if temperatures soar even higher, with some cities in India now regularly crossing the 45-degree mark.

But the numbers alone don’t tell the whole story. The combination of high temperatures and high humidity increases the risk during a heatwave. This is where the heat index comes in. Originally developed by the US National Weather Service, the heat index helps translate temperature and humidity into a single number that reflects how hot it feels. For instance, if the air temperature is 40 degrees Celsius and the relative humidity is 40 percent, it might feel like 45 or 46. And if the humidity goes up, so does the apparent temperature. This elevated “feels like” temperature can be dangerous, especially for people who work outdoors, the elderly, young children, and those with pre-existing health conditions.

In India, cities are beginning to factor in the concept of heat index into public health advisories and early warning systems, though it is still not as widely understood or communicated as the standard temperature. But with urban heat and humidity levels both on the rise due to climate change and rapid urbanisation, understanding how hot it feels, not just how hot it is, becomes essential.

The IMD and National Disaster Management Authority (NDMA) issue heatwave alerts using a color-coded system to help the public and officials prepare. A yellow alert suggests a warning that heat may be harmful for vulnerable groups. Orange means a severe heatwave that can cause health risks for most people, especially those exposed to the sun for extended periods or engaged in physical activity. A red alert indicates extreme conditions with high likelihood of heatstroke and heat-related deaths if proper precautions are not taken.

So why does humidity have such a powerful effect? The body relies on sweating to regulate its core temperature. As sweat evaporates from the skin, it cools the body down. However, when the air is already full of moisture, that process slows. The sweat stays on your skin and your internal temperature starts to rise. This can lead to conditions such as heat exhaustion, heat cramps or even life-threatening heatstroke. The body can no longer cool itself, and symptoms such as confusion, rapid heartbeat, dizziness or unconsciousness may follow. What begins as a hot afternoon can turn into a medical emergency within hours.

Interestingly, warm nights can also make heatwaves deadlier. When night-time temperatures do not fall enough, the body doesn’t get a break from the heat. This lack of recovery time increases stress on the heart and other organs, especially when high daytime and night-time temperatures continue for several days. The IMD notes that warm nights can worsen the impact of heatwaves by preventing the body from cooling down before the next day’s heat sets in.

The heat index, then, becomes an important measure not just for comfort but for safety. Internationally, health departments and meteorological services use it to guide warnings and responses. A heat index of 40 to 45 calls for caution, especially during outdoor work or physical activity. Above 45, people are advised to take serious precautions. In India, where both humidity and heat are common across large parts of the country, including coastal areas, the need to adopt the heat index in everyday weather communication is becoming urgent.

There is also a growing realisation that local variations matter. What qualifies as a heatwave in Delhi may not be the same in Chennai, where high humidity is the norm, or in Rajasthan, where dry heat prevails. The heat index can bridge this gap by translating both types of heat into a measure that is directly relevant to the human body.

The good news is that the information is now more accessible than ever. IMD provides daily heatwave bulletins and maps through its Mausam portal, and resources like the NOAA heat index chart make it easier to understand what “feels like” really means. But awareness must travel beyond official platforms and into everyday decision-making, especially as India braces for more extreme weather in the years to come.

If you find yourself outdoors on a day when the temperature says 40 but feels like 45, it’s not just in your head. It really does feel hotter, and your body is working harder than it seems. In such times, it’s important to stay hydrated, wear light and breathable clothing, avoid going out during peak afternoon hours, and seek shade or indoor cooling wherever possible. Pay attention to weather alerts, and most importantly, listen to your body. It knows when it is being pushed too far.

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