Delhi recorded its warmest July day and night in two years on Thursday, with temperatures soaring well above normal and the heat index touching 48 degrees Celsius, according to the India Meteorological Department (IMD).
The maximum temperature settled at 39.2 degrees Celsius, which was 4 degrees Celsius above the seasonal average, while the minimum temperature was 30.2 degrees Celsius, around 3 degrees Celsius above normal. At 2.30 pm, the city’s heat index, or ‘feels-like’ temperature, reached 48 degrees Celsius due to the combination of high temperatures and humidity.
According to IMD data, Delhi last recorded higher July temperatures in 2024, when the maximum reached 39.3 degrees Celsius, and the minimum stood at 30.4 degrees Celsius, as reported by The Times of India.
Let us take a look at why Delhi’s heat index peaked at 48 degrees Celsius.
What is a heat index?
The heat index, also known as the “apparent temperature” or “feels like” temperature, is a measure that combines actual air temperature and relative humidity to determine how hot the weather actually feels to the human body.
High humidity reduces the body’s ability to cool itself through sweating. As sweat evaporates slowly in humid air, the body retains more heat, making temperatures feel much higher. This is how the system works.
Why did Delhi feel like 48 degrees Celsius when the temperature was around 39 degrees Celsius?
Delhi is a massive, crowded city packed with concrete roads, tall buildings, and millions of vehicles and air conditioners. These materials trap heat during the day and slowly release it, making the city feel several degrees hotter than surrounding open, rural areas.
If the air temperature is 39 degrees Celsius but the air is filled with water vapour, your body cannot cool off, and your brain registers a much higher temperature. Therefore, the body experiences much greater heat stress than indicated by the thermometer. IMD considers Heat Index as an indicator of human discomfort rather than actual air temperature.
Why is July in Delhi so sultry?
July in Delhi often feels extremely uncomfortable as the arrival of the summer monsoon brings warm and moisture-filled winds to the city. While the rains provide some relief from the scorching temperatures, they also increase humidity levels, making the weather feel sticky and exhausting.
The combination of high temperatures and heavy moisture in the air creates a “feels like” temperature that is much higher than the actual reading. Even on days without rainfall, the cloud cover at night traps heat in the atmosphere, leading to more humidity. Thus, making it difficult for the body to cool down, leaving people struggling with a constant sense of heat and discomfort.
Why isn’t this called a heatwave?
According to the India Meteorological Department (IMD), a heatwave is declared based on the actual maximum temperature and not on the heat index.
A heatwave in the plains is declared when the maximum temperature generally must reach 40 degrees Celsius or above, along with prescribed departures from normal, or meet absolute threshold criteria.
Since the actual temperature is around 39 degrees Celsius along with high humidity, IMD will not consider it to be a heatwave.
What are the factors making Delhi hotter?
The increasing heat in Delhi is linked to a combination of global warming, fast-paced urbanisation and the city’s geographical conditions. As the population grows, natural landscapes are gradually being replaced by concrete buildings, highways and other infrastructure.
Unlike vegetation, concrete absorbs heat during the day and releases it slowly at night, preventing the city from cooling down. This creates the Urban Heat Island effect, where densely built-up areas experience much higher temperatures than nearby less-developed regions.
How humid southwesterly winds from the Arabian Sea are losing their moisture over Gujarat before reaching Delhi?
As the Arabian Sea branch of the Southwest Monsoon makes landfall, it strikes the coastal segments of South Gujarat, Saurashtra, and parts of Maharashtra. The initial impact causes heavy precipitation along the coast, discharging a significant percentage of the winds’ primary moisture content through immediate rain-out.
The monsoon winds moving across Gujarat and neighbouring regions gradually lose a large share of their moisture as they produce rainfall along the route. For Delhi to receive good rainfall, these winds need to remain strong and be supported by low-pressure systems that help transport moisture further inland. When these systems become weaker or shift position, the flow of moisture towards northwest India reduces, leaving Delhi with a combination of high humidity, rising temperatures and fewer rain showers.
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