As heatwave conditions persist in West, North and Central India, it is a double whammy for farmers. Not only do they have to work under the scorching sun, they also have to brace for lower crop yields. High temperatures can cause heat stress in plants, affecting grain formation, thus lowering yield quantity & quality, writes S. Abhilash
l What exactly is a heatwave?
A HEATWAVE IS a period of unusually high temperatures that persists for at least two consecutive days. The India Meteorological Department (IMD) declares a heatwave in the plains when the maximum temperature is ≥ 40°C with a departure of at least 4.5°C above normal, while in hilly regions the threshold is ≥ 30°C.
For coastal areas such as Kerala, where absolute temperatures are relatively lower, a heatwave may be identified when the maximum temperature exceeds ~37°C along with a departure of ≥ 4.5°C from the climatological normal of that day persisting for two consecutive days.
Heatwaves are increasing mainly due to global warming caused by human activities such as burning fossil fuels and rapid urbanisation. In addition, deforestation and shrinking green spaces reduce natural cooling, making heatwaves more intense.
l Why does it feel much hotter than what it shows on the thermometer?
THE “FEELS-LIKE” temperature is often higher due to high humidity. Sweat then does not evaporate efficiently, reducing the body’s ability to cool itself. This is why a temperature of 37°C with humidity values of 50% can feel like 45°C or more. This combined effect of temperature and humidity is measured using the Heat Index and is recognised as the measure of apparent temperature.
l Is there a way to measure heat stress?
YES. WET-BULB temperature represents the lowest temperature to which air can be cooled through evaporation. It reflects the efficiency of the human body’s primary cooling mechanism—sweating. A wet-bulb temperature of 35°C is a critical threshold, beyond which the human body cannot effectively cool itself. However, real-world conditions are influenced by more than just temperature and humidity. This is why Wet-Bulb Globe Temperature (WBGT) is critical as it incorporates multiple environmental factors, including temperature, humidity, solar radiation and wind speed. It is a comprehensive measure of heat stress, especially in outdoor settings.
l What is the heat dome effect?
A HEAT DOME is a high-pressure system in the atmosphere that traps hot air beneath it, like a lid. Under this system, air sinks and compresses, causing temperatures at the surface to rise further. The trapped heat cannot escape easily, and clear skies allow continuous solar heating during the day.
In North India, heat domes are particularly important because they can persist for several days or even weeks, leading to prolonged heatwave conditions. The absence of cloud cover and rainfall, combined with dry winds and land heating, intensifies the heat. This results in continuously rising daytime temperatures and very warm nights, increasing heat stress and making conditions more dangerous for people, agriculture, and water resources.
l How does El Nino contribute to heatwaves in India?
EL NINO IS a climate phenomenon characterised by the warming of sea surface temperatures in the central and eastern Pacific Ocean, which alters global atmospheric circulation. In India, El Nino weakens the monsoon circulation and reduces cloud cover and rainfall. This leads to clear skies, dry conditions, and reduced soil moisture, all of which allow the land surface to heat up more rapidly. As a result, El Nino years are often associated with more frequent, intense, and prolonged heatwaves across many parts of India.
l How prolonged heatwaves can lead to flash droughts
PROLONGED HEATWAVES raise surface temperatures and accelerate evapotranspiration, rapidly depleting soil moisture even when initial conditions are adequate. When this is followed by a substantial deficit in rainfall, the lack of replenishment intensifies moisture stress, leading to flash drought — a rapid onset drought that develops over days to weeks rather than months. The impact on agriculture is severe and immediate. Crops experience water stress during critical growth stages, leading to reduced photosynthesis, impaired nutrient uptake, and increased susceptibility to pests and diseases.
l How can heat stress affect farm productivity?
HIGH TEMPERATURES CAN also cause heat stress in plants, affecting flowering and grain formation, thereby lowering yield quantity and quality. Rainfed agricultural systems are particularly vulnerable, as they depend directly on timely rainfall. Additionally, soil degradation, reduced groundwater recharge, and increased irrigation demand further strain agricultural sustainability. Overall, the combined effect of heatwaves and rainfall deficits poses a significant threat to food security by increasing crop failure risks and reducing agricultural productivity.
The writer is director, Advanced Centre for Atmospheric Radar Research, Kochi
Disclaimer: The views expressed are the author’s own and do not reflect the official policy or position of Financial Express.