EXPLAINED | NASA Detects Heat Islands Around Delhi Amid Extreme Heatwave. Know What It Is All About

Due to human activities and the materials used in the built environment, cities are usually markedly warmer than the surrounding countryside. The NASA image clearly delineates these urban heat islands

NASA's ECOSTRESS took an image shortly before local midnight on May 5, which shows urban areas and agricultural lands northwest of Delhi. Nighttime temperatures in Delhi and several smaller villages were above 35 degrees Celsius, peaking at about 39 degrees Celsius

Source : NASA

New Delhi: Since mid-March, a relentless heat wave has blanketed India, and its neighbouring country, Pakistan. This has resulted in dozens of deaths, fires, increased air pollution, and reduced crop yields. NASA's Ecosystem Spaceborne Thermal Radiometer Experiment on Space Station instrument (ECOSTRESS) has been measuring these temperatures from space, at the highest spatial resolution of any satellite instrument, the US space agency said on its website. 

The instrument took an image shortly before local midnight on May 5, which covers about 4,800 square miles, and shows urban areas and agricultural lands northwest of Delhi that are home to about 28 million people. 

Due to human activities and the materials used in the built environment, cities are usually markedly warmer than the surrounding countryside. The image released by NASA clearly delineates these urban "heat islands." 

According to NASA, nighttime temperatures in Delhi and several smaller villages were above 95 degrees Fahrenheit or 35 degrees Celsius, peaking at about 102 degrees Fahrenheit or 39 degrees Celsius. Meanwhile, the rural fields nearby had cooled to around 60 degrees Fahrenheit or 15 degrees Celsius. This implies that people living in the city are experiencing considerably higher temperatures than the average temperatures reported for their regions. 

NASA's instrument, ECOSTRESS, measures the temperature of the ground itself, which is very similar to air temperature at night. During the day, the ground may be warmer than the air. Though the primary mission of ECOSTRESS, which was launched to the space station in 2018, is to identify plants' thresholds for water use and water stress, the instrument also records other heat-related phenomena, like the heatwave. 

The instrument's high-resolution images have a pixel size of about 225 feet by 125 feet. These images serve as a powerful tool for understanding aspects of the weather event that might be overlooked by traditional observation networks, NASA said on its website.

What Are Heat Islands?

Heat islands are urbanised areas that experience higher temperatures than outlying areas, according to the US Environmental Protection Agency (EPA). Structures such as buildings, roads, bridges, and other infrastructure absorb and re-emit the Sun's heat more than natural landscapes such as forests and water bodies. As a result, the urban areas where these structures are highly concentrated and greenery is limited become "islands" of higher temperatures relative to outlying areas. These pockets of heat are referred to as "heat islands", according to the EPA. 

In urban areas, daytime temperatures are about one to seven degrees Fahrenheit higher than temperatures in outlying areas and nighttime temperatures are about two to five degrees higher.

Heat islands can form under different conditions, including during the day or night, in small or large cities, in suburban areas, in northern or southern climates, or in any season. 

The greatest temperature differences are experienced in humid regions and cities with larger and denser populations. According to several studies, the heat island effect will strengthen in the future as the structure, spatial extent, and the population density of urban areas change and grow.

According to NASA, the difference in temperature between urban and less-developed rural areas depends on how well the surfaces in each environment absorb and hold heat.

What Are The Causes Of Heat Island Effect?

Heat islands result from several factors including reduced natural landscapes in urban areas, urban material properties, urban geometry, heat generated human activity, and weather and geometry.

Reduced Natural Landscapes in Urban Areas

Trees, vegetation, and water bodies tend to cool the air by providing shade, transpiring water from plant leaves, and evaporating surface water, respectively, whereas hot, dry surfaces in urban areas, which include roofs, sidewalks, roads, buildings, and parking lots, provide less shade and moisture than natural landscapes.

Therefore, these structures contribute to higher temperatures.

Urban Material Properties

Conventional human-made materials used in urban environments such as pavements or roofing tend to reflect less solar energy, and absorb and emit more of the Sun's heat compared to trees, vegetation, and other natural surfaces, according to the EPA. Due to the slow release of heat from urban materials, heat islands, which build throughout the day, become more pronounced after sunset.

Urban Geometry

The dimensions and spacing of buildings within a city influence wind flow and urban materials' ability to absorb and release solar energy, especially in heavily developed areas, where surfaces and structures obstructed by neighbouring buildings become large thermal masses that cannot release their heat steadily. Cities with narrow streets and tall buildings become urban canyons, which is a place where the street is flanked by buildings on both sides. The urban canyon effect is the result of building and street architecture on airflow in the street. Urban canyons can block natural wind flow that would bring cooling effects. 

Heat Generated From Human Activities

Vehicles, air-conditioning units, buildings, and industrial facilities all emit heat into the urban environment, and these sources of anthropogenic waste heat can contribute to heat island effects. 

Weather and Geography

Calm and clear weather conditions result in more severe heat islands because they maximise the amount of solar energy reaching urban surfaces, and minimise the amount of heat that can be carried away. On the other hand, strong winds and cloud cover suppress heat island formation. 

Geographic features such as mountains can also impact the heat island effect. For instance, nearby mountains can block wind from reaching a city, or create wind patterns that pass through the city.

How Do Building Materials Result In Urban Island Effect?

Building materials such as asphalt, steel, and brick often have very dark colours such as black, brown, and grey. A dark object absorbs all wavelengths of light energy and converts them into heat, so that the object gets warm, while a white object reflects all wavelengths of light. Since light is not converted into heat, the temperature of the white object does not increase noticeably. However, dark objects such as building materials absorb heat from the Sun.

Why Are Rural Areas Cooler?

Rural areas are abundant in grass, trees, and farmlands covered with crops. These plants take up water from the ground through their roots, and store it in their stems and leaves. After this, the water travels to small holes on the underside of leaves, where the liquid water turns into water vapour and is released into the air. This process called transpiration acts as nature's air conditioner.

How Can Urban Islands Be Cooled Down?

Some cities are 'lightening' streets, in order to cool down urban heat islands, by covering black asphalt streets, parking lots, and dark roofs with a more reflective grey coating. With these changes, the urban air temperatures can drop dramatically, especially during the heat of summer.

The city can also be cooled down by planting gardens on urban rooftops. According to a study in Los Angeles, California, changes like these would be enough to save around $100 million per year in energy costs. 

Since many modern building materials such as brick or cement are impervious surfaces, water can't flow through them, like it would through a plant. These surfaces do not have a cycle of flowing and evaporating water, as a result of which they cannot be cooled down.

Therefore, builders can use materials that will allow water to flow through, in order to help cool the heat island. Building materials called permeable materials can be used, which promote the capture and flow of water, and help cool urban regions.