Europe is once again sweltering.
The heatwave sweeping across the continent in July 2026 has revived familiar warnings from scientists and public health experts. Extreme heat is no longer regarded as an unusual weather event but as one of the fastest-growing threats to human health. Hospitals prepare for surges in heat-related illness, governments issue emergency advisories, and climate researchers increasingly describe heat as a silent disaster.
History, however, reminds us that this is not the first time oppressive temperatures have profoundly shaped human events.
Nearly 169 years ago, during the First War of Indian Independence in 1857, another army found itself fighting not only an enemy but also the relentless Indian summer. On the rocky heights of Delhi Ridge, British troops confronted an adversary that could not be defeated by artillery or disciplined infantry. Alongside rebel fire came cholera, contaminated water, exhaustion—and heat so intense that officers struggled to describe it.
On June 7, 1857, British forces occupied the Delhi Ridge, a rocky escarpment overlooking the city from the northwest. It is also the northernmost terminus of the Aravallis, the oldest hill range in the subcontinent. For more than three months—until the assault of September 14—they held this exposed position while attempting to recapture Delhi from sepoys who had mutinied and were lying in wait on the plain below, perfectly acclamatised to the harsh Indian sun. Moreover, the Ridge in 1857 was not as green as it is now, according to records.
The Ridge thus offered an important strategic advantage, but little else.
Its rocky terrain provided scant natural shade. Soldiers spent weeks under an open sky while enduring almost continuous attacks from rebel positions. Yet, combat represented only part of their ordeal. Disease spread rapidly through overcrowded camps, drinking water was often unsafe, sanitation was poor, and the crushing summer heat magnified every hardship.
The official collection notes of Britain’s National Army Museum describe the conditions in stark terms. They record that numerous soldiers, including the British commander Major General Sir Henry Barnard, died from cholera while troops remained under constant rebel pressure throughout what the museum characterises as the “searing heat of the summer.” The description draws upon contemporary military correspondence and eyewitness accounts preserved from the siege.
Few descriptions capture those conditions more vividly than the memoir of Charles John Griffiths, an officer of the 61st Regiment of Foot, who participated in the siege and later published A Narrative of the Siege of Delhi: With an Account of the Mutiny at Ferozepore in 1857.
His words remain among the most frequently cited eyewitness accounts of the Delhi Ridge:
“The heat was insupportable, the thermometer under the shade of my tent marking 112°F.”
That translates to approximately 44.4°C.
The observation is remarkable because it comes directly from a participant rather than a later historian. Yet, it also deserves careful interpretation. Griffiths was recording the reading of his own thermometer beneath the shade of his tent—not a measurement taken at an official meteorological observatory using standardised equipment.
Even so, his memoir paints a remarkably consistent picture. Heat, cholera, flies and exhaustion dominated daily existence. Camp life became an exercise in endurance rather than routine military discipline.
Military operations themselves adapted to the climate. Whenever circumstances allowed, troops marched during the night and rested through the hottest daylight hours beneath canvas shelters. Extreme heat had begun to dictate military logistics.
The greatest killer on the Delhi Ridge was not necessarily gunfire.
Cholera devastated the British camp. On July 5, 1857, the army’s commander, Major General Sir Henry Barnard, died from the disease.
Military reports from the campaign repeatedly describe how prolonged exposure to extreme heat, physical fatigue and inadequate sanitation left soldiers increasingly vulnerable. Modern historians generally agree that disease inflicted casualties comparable to—or at times exceeding—those caused by direct combat during the Delhi campaign.
The experience offers an early historical example of how environmental conditions can transform military operations into public health emergencies.
One obvious question follows.
Was Delhi experiencing an unusually hot summer in 1857?
The historical evidence urges caution.
No continuous, standardised temperature record exists for Delhi during that year. Consequently, historians cannot scientifically claim that 1857 was the city’s hottest—or even an exceptionally hot—summer in instrumental terms.
Instead, researchers reconstruct environmental conditions through soldiers’ diaries, medical reports, administrative correspondence and later official investigations.
This distinction matters because eyewitness testimony reveals how people experienced the weather, while standardised meteorological observations allow scientists to compare temperatures across decades.
Important context comes from two research papers published in 2025 by R R Kelkar, former Director General of the India Meteorological Department (IMD).
In Journey of India Meteorological Department during last 150 years, Kelkar explains that although the IMD was formally established in 1875, systematic weather observations in India began much earlier.
The British East India Company established an astronomical observatory in Madras (now Chennai) in 1792. Meteorological observations started there the following year, with continuous records beginning in 1796. Similar observatories later appeared in Bombay (1823), Calcutta (1829), Shimla (1841) and Travancore (1841).
However, these observations were not conducted within a uniform national system.
Kelkar's second paper—A Need to Revisit Air Temperature Measurements in India in the 19th and Early 20th Centuries—explains that only after the IMD’s establishment did India begin standardising weather instruments and observational methods. Standardised thermometer sheds appeared during the late 1870s and were gradually replaced by Stevenson Screens during the 1920s.
Consequently, Griffiths’ famous temperature reading provides invaluable historical evidence, but it cannot be interpreted as equivalent to a modern official temperature record.
The experiences of 1857 prompted the British government to ask a broader question: why were European soldiers dying at such alarming rates in India?
Its answer appeared in the Report of the Commissioners Appointed to Inquire into the Sanitary State of the Army in India (1863).
The Commission concluded that climate alone could not explain the extraordinarily high mortality.
Instead, it identified a web of interacting factors: inadequate drainage, contaminated water supplies, overcrowded barracks, poor sanitation, exhausting military duties and infectious disease.
Among the Commission members was military physician Sir Ranald Martin, who challenged the long-held assumption that European soldiers gradually became acclimatised to India’s climate.
The available statistical evidence, he argued, did not support that belief. If anything, longer service in India appeared to coincide with increasing illness and mortality rather than physiological adaptation.
The Commission therefore reached an important conclusion that remains strikingly modern: climate mattered, but its effects were amplified by failures in sanitation, water supply and public health infrastructure.
Read together, Griffiths’ memoir and the 1863 sanitary inquiry tell a remarkably coherent story.
One records unbearable lived experience.
The other explains why that experience proved so deadly.
Today, public health researchers would likely describe extreme heat as a risk multiplier—a hazard that magnifies the effects of existing vulnerabilities. Heat alone is dangerous. Combined with contaminated water, infectious disease, heavy physical labour and inadequate shelter, its consequences become exponentially more severe.
Delhi Ridge in 1857 demonstrates this principle with unusual clarity.
Yet, an important distinction separates 1857 from today’s climate conversation.
The historical sources describe oppressive weather, not climate change. They cannot establish whether that particular summer was exceptional in the long-term climatic record.
Modern European heatwaves, by contrast, are analysed using more than 150 years of standardised meteorological observations, satellite measurements and advanced climate models, allowing scientists to assess trends and attribute changing risks with far greater confidence.
Moreover, heatwaves today affect European, Indians, everyone across the globe.
History cannot answer today’s climate questions.
But it can illuminate how societies respond when extreme heat collides with fragile public health systems.
That may be the enduring lesson of Delhi Ridge.
The British Army’s greatest adversary was never solely the rebel sepoys beyond its defensive lines. Scorching temperatures, cholera, contaminated water, inadequate sanitation and the exhausting realities of camp life combined to turn a military campaign into a humanitarian crisis.
More than a century and a half later, as extreme heat becomes an increasingly defining feature of the 21st century, the story of Delhi Ridge reminds us that weather is rarely just the backdrop to history.
Sometimes, it becomes one of its principal protagonists.