An expert analysis | Understanding the September 2024 heatwave in Assam

Assam recorded its hottest ever September day last Monday (September 23, 2024) which was preceded by heat-wave like conditions in the region over the weekend. The record-breaking maximum temperatures in Guwahati stood at 39.3 degrees Celsius, Dibrugarh at 39.5, North Lakhimpur at 39.6 and Jorhat at 37.9 on September 23, the highest September temperatures recorded ever. Same was the condition beyond the state, across the northeastern region as Shillong recorded a maximum temperature of 29.9 degrees on the same date which was again the highest ever recorded in the month of September. Temperatures were 9.3 degrees higher than normal in Sohra of Meghalaya and 8.6 degrees above normal in Arunachal Pradesh’s Pasighat.

Seven people have reportedly lost their lives in the heat. Large quantities of fishes also died in the heat. The situation became such that the Kamrup Metropolitan District authorities took a decision to shut schools in the area for four days, September 24 to 27, citing excessive heat and rising temperatures. This was, however, revoked later when the weather improved.

How does a heat wave form?

CFC India reached out to Rahul Mahanta, an associate professor specialising in climate science at Cotton University, Guwahati for an expert consultation on the topic. Mahanta explained that heat waves are a normal part of the weather system and one factor at the heart of many heat waves is high pressure.

He explained, “The atmosphere is like a sea of air, and if there’s a particularly large amount of air piled up over a certain area, the air pressure increases, creating a high-pressure system. Areas of high pressure, known as anticyclones, are large, slowly rotating masses of air that blow in a clockwise direction in the Northern Hemisphere. As air moves away from the high-pressure center at ground level, air from above sinks down to take its place. This sinking air heats up as it is compressed, similar to how a bicycle pump gets hot when pumped. This not only makes the area hotter but also inhibits the natural cooling processes, such as convective air rising and cloud formation. This is what seems to be one of the factors responsible for the intense heat wave-like conditions over the region.”

What caused the unusual heat wave-like conditions in the region?

With the normal discourse after a heat wave-like condition being around climate change and its impact, we requested Mahanta for an insight around the topic. Mahanta agreed that climate change is a factor in the recent heat wave like conditions in the region but also opined that it is too early to blame it wholly on it as more studies are needed to attribute it to climate change.

‘’Climate change has altered monsoon patterns in Assam, leading to delayed or weakened rains. Longer dry spells can intensify heat waves, as there is less moisture in the air to moderate temperature increases. Assam has been facing more frequent extreme weather events like heavy rainfall or droughts. These weather shifts can create a feedback loop where dry spells contribute to heat waves, and subsequent rainfall may not be sufficient to cool the land,” Mahanta said.

He further pointed at the increasing frequency of anticyclones, which bring clear skies and little wind, which can lead to stagnant air. This prevents the dispersal of heat, allowing temperatures to rise continuously over several days.

“Last two weeks witnessed changes in atmospheric circulation, such as weakening or shifting of the Jet Stream and anticyclonic conditions over the region, which can trap heat and lead to prolonged heat waves. The lower level winds were mostly weak northeasterly, i.e. the southwest monsoon is weak over the region, and there is no moisture incursion from the Bay of Bengal because of the lack of any favourable synoptic condition,” Mahanta said.

Mahanta added that another factor at work is the lack of soil moisture in the region, as dry soil conditions can contribute to heat waves.

He explained, “Lack of moisture in the ground reduces evaporative cooling, causing surface temperatures to rise. there is higher insolation (the amount of solar radiation reaching the Earth’s surface). When the sun comes down on a summer’s day, much of its energy is spent evaporating water from soil moisture or plants. As the heat persists day after day, the soil moisture diminishes, and the energy that once went into evaporating water instead accumulates, increasing temperatures. This process is amplified when the sinking air of an anticyclone traps the heat, like the lid of a pot, creating what is sometimes known as a heat dome. It becomes a weight of oppression that can’t shift or be undermined.”

Local factors contributed as well

Mahanta named various local factors that might have contributed to the heat wave-like conditions. Some of them are:

Rapid urbanization in cities like Guwahati has led to a rise in built-up areas and a decrease in green spaces, intensifying the urban heat island effect where concrete retains heat.
Deforestation in nearby regions reduces the cooling impact of forests, while shifting cultivation practices contribute to higher surface temperatures.
Increased reliance on air conditioning strains Assam’s fossil-fuel-dependent power grid, worsening localized emissions.
Assam’s geographical location in the Brahmaputra Valley creates a heat dome effect, trapping hot air due to the surrounding hills.
Rising air pollution from vehicles, industries, and crop burning traps heat, while black carbon reduces surface reflectivity, increasing heat absorption.
Changes in local hydrology and shrinkage of water bodies, due to unregulated construction, pollution, and sand mining, have reduced their natural cooling effect in Assam.
Sand mining along the Brahmaputra and its tributaries destabilizes water flow, lowering groundwater recharge and evaporative cooling.
Decline in Traditional cooling practices, like homes made from bamboo and thatch, provided better ventilation and insulation, but modern concrete homes trap heat.
The decline of traditional water management systems, such as ponds and canals, has reduced their effectiveness in cooling the environment.

Solutions

Mahanta stressed that addressing heat waves locally requires a combination of strategies tailored to the specific climate, geography, and resources available. For regions like Assam or other parts of Northeast India, he offered the following solutions:

Urban Green Spaces: Increase the number of parks, trees, and green roofs in urban areas. Vegetation reduces heat by providing shade and cooling through evapotranspiration.
Restoration of Wetlands: Preserve and restore wetlands and water bodies, which help moderate local temperatures by cooling the air.
Cool Roofs and Walls: Use reflective or light-colored materials for roofs and walls to reduce heat absorption.
Traditional Materials: Encourage the use of traditional construction materials like bamboo, thatch, and mud, which provide natural insulation against heat.
Rainwater Harvesting: Implement rainwater harvesting systems to retain water for cooling purposes, such as for community use or watering urban green spaces.
Soil Moisture Retention: Encourage the use of organic mulching and cover crops in agricultural fields to retain soil moisture and reduce heat stress on crops.
Sustainable Agriculture Practices: Promote heat-resistant crop varieties that can better tolerate high temperatures and ensure that farmers have access to adequate irrigation methods like drip irrigation, which conserves water while keeping crops cool.
Energy Efficiency Measures: Encourage energy-efficient cooling systems such as ceiling fans, passive cooling methods (like cross-ventilation), and solar-powered air conditioners to reduce reliance on energy-intensive cooling systems.