Explained: What are urban heat islands? Causes, and Why does this happen?

NASA Jet Propulsion Laboratory or NASA JPL, shared an image showing the temperature in Delhi's urban 'heat islands,' amid the heatwave.

New Delhi, May 18: Recently, NASA's Jet Propulsion Laboratory released images that show "heat islands" in and around the national capital Delhi.

In a press note, NASA said, this image, taken shortly before local midnight on May 5, shows urban areas and agricultural lands northwest of Delhi (the large red area in the lower right) that are home to about 28 million people. The image covers about 4,800 square miles (12,350 square kilometers).

Cities are usually markedly warmer than the surrounding countryside due to human activities and the materials used in the built environment. The image clearly delineates these urban "heat islands." Nighttime temperatures in Delhi and several smaller villages were above 95 degrees Fahrenheit (35 degrees Celsius), peaking at about 102 degrees F (39 degrees C), while the rural fields nearby had cooled to around 60 degrees F (15 degrees C), it added.

What are urban heat islands?

An urban heat island occurs when a city experiences much warmer temperatures than nearby rural areas. Research predicts that the heat island effect will strengthen in the future as the structure, spatial extent, and population density of urban areas change and grow.

Causes of Heat Islands

Heat islands form as a result of several factors:

• 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. Hard, dry surfaces in urban areas - such as roofs, sidewalks, roads, buildings, and parking lots - provide less shade and moisture than natural landscapes and therefore 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. Often, heat islands build throughout the day and become more pronounced after sunset due to the slow release of heat from urban materials.

• Urban Geometry. The dimensions and spacing of buildings within a city influence wind flow and urban materials' ability to absorb and release solar energy. In heavily developed areas, surfaces and structures obstructed by neighboring buildings become large thermal masses that cannot release their heat readily. Cities with many narrow streets and tall buildings become urban canyons, which 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. These sources of human-generated, or anthropogenic, waste heat can contribute to heat island effects.

• Weather and Geography. Calm and clear weather conditions result in more severe heat islands by maximizing the amount of solar energy reaching urban surfaces and minimizing the amount of heat that can be carried away. Conversely, strong winds and cloud cover suppress heat island formation. Geographic features can also impact the heat island effect. For example, nearby mountains can block wind from reaching a city, or create wind patterns that pass through a city.

Why does this happen?

An urban area is a city. A rural area is out in the country. The sun's heat and light reach the city and the country in the same way. The difference in temperature between urban and less-developed rural areas has to do with how well the surfaces in each environment absorb and hold heat.

If you travel to a rural area, you'll probably find that most of the region is covered with plants. Grass, trees and farmland covered with crops, as far as the eye can see.

Plants take up water from the ground through their roots. Then, they store the water in their stems and leaves. The water eventually travels to small holes on the underside of leaves. There, the liquid water turns into water vapor and is released into the air. This process is called transpiration. It acts as nature's air conditioner.