Heat is a silent killer — some studies say it is the biggest cause of weather-related deaths — and the devastating heat wave that scorched India is one of the more tragic and perhaps under-appreciated stories of the spring.

The late May siege of blistering temperatures is being blamed for 2,500 deaths, the second-deadliest heat wave on record in India and fifth-deadliest in world history, according to Jeff Masters at Weather Underground, who examined weather mortality databases.

Temperatures averaged nearly 10 degrees (5.5 Celsius) above average for nearly two weeks, NASA noted.

In New Delhi, temperatures surged to 113 degrees, so hot they melted roads. At Titlagarh in Odisha, the mercury spiked to a searing 117 degrees, just 5 degrees below India’s hottest temperature ever recorded, reported Slate’s Eric Holthaus.

Coastal areas dealt with the double whammy of extreme heat and oppressive levels of humidity. At the height of the heat wave in Mumbai, the heat index — what the air feels like given the combination of heat and humidity — struggled to fall below 100 even at night, Holthaus reported.

“On May 23 at 14:30,  Bhubneshwar [near the northeast coast of India] recorded a temperature of 42.2°C (108°F) with a dew point of 29.3°C (84.7°F), giving an astonishing heat index of 62°C (143.6°F.),” wrote Weather Underground’s Masters.

And the heat was relentless. “The city of Ongole, in the state of Andhra Pradesh, high temperatures averaged 110°F (43.47°C) from May 24-30,” wrote Tom Di Liberto for NOAA’s Climate.gov.

Day after day and night after night of such punishing conditions caused heat stress on the human body to accumulate, hitting vulnerable groups, such as older adults, the homeless and outdoor workers, the hardest. In India, many people do not have access to air-conditioning.

Climate experts blamed a surge of hot, dry winds from Pakistan, a “heat bomb,” for the severity of the heat which overwhelmed a cooler, moister pre-monsoon flow of air from the Bay of Bengal. The push of cooler air from the Bay of Bengal might have resisted the heat bomb sooner had the Indian monsoon not been delayed this year.

The monsoon, which brings clouds, heat relief and beneficial rains to India, finally reached the south of India on June 5, several days behind schedule.

Because of El Nino, the Indian monsoon is forecast to be weaker than normal this summer, which may favor warmer and drier than normal conditions, especially in northern parts of the country.

The role of climate warming also should not be dismissed for playing a role in the magnitude of this heat wave. Elevated greenhouse gas levels likely boosted temperatures by some amount, making an already stifling situation even a bit more brutal. The climate literature predicts more frequent, intense and longer-duration heat waves in future decades.

“In a future with high carbon emissions, it is likely that a maximum temperature that occurs once every 20 years will at least double in frequency (to a 1-in-10-year event) by the end of the 21st century,” NOAA’s Di Liberto noted.