What is Urban Heat Island?

Introduction
Walking around a city full of tall buildings and bustling streets feels warmer, even if the weather is not hot. It is like the city is a heat magnet, holding onto heat, making everything feel warmer. This is what we call the urban heat island (UHI) effect. In the early 19th century, scholar Lake Howard first measured and discussed the UHI effect when studying urban climate in London, England. Since then, many scholars around the world have deeply researched its characteristics. When buildings are closely packed, heat becomes trapped and unable to escape, increasing temperatures in urban areas. The effect increases the temperature of cities, contributes to global warming, initiates storms/precipitation events, increases the energy demand of cities, and contributes to heat-related mortality. Its consequences are one of the easiest ways to see how human impact can change our planet. After all, sidewalks, parking lots, and skyscrapers would not exist if humans were not there to build them. Although these structures are essential to city living, the heat islands they create can be dangerous for humans. 

Why it’s hotter in the cities? 

Earth's energy is called a "budget" or "balance". The Earth constantly releases the same amount of energy it gains. In the diagram, the Earth's surface absorbs 50% of the sun's radiation (represented by yellow arrows). Eventually, the surface releases it through infrared radiation, conduction, convection, and evaporation (represented by red arrows). 

At the Earth's surface, sunlight absorbed - infrared radiation - conduction/convention – evaporation = 0.

The sun’s heat and light reach the city and the countryside in the same way. The difference in temperature between urban and less-developed rural areas has to do with land use, i.e., how well the surfaces in each environment absorb and hold heat. Several key factors that drive extreme heat in cities are: (picture)

• The low albedo of urban building material: This generally has the most significant influence on the effect. Materials such as asphalt, steel, and brick are often very dark colours—like black, brown, and grey which convert all the absorbed light energy into heat, and the object gets warm. Without a flowing and evaporating water cycle, these surfaces have nothing to cool them down.

•  Population Density: Heat emissions are generated from urban life activities such as transportation, industrial facilities, and heating and cooling of buildings. During a summer heat wave, air conditioning from urban buildings can add 20% more heat to the outside air.

• The power of trees: Plants act as nature’s air conditioner through transpiration. Rural areas are covered with grass, trees, farmlands, etc. Vegetation can help reduce peak summer temperature by 2-9 degrees Fahrenheit in urban areas.

Impact on airflow by the urban infrastructure: The size and dimensions of the buildings influence how air moves through a city during the day, playing a prominent role in trapping and dissipation heat. 

What is Albedo?

Albedo indicates what percentage of the incoming solar radiation (sunlight) is reflected by a surface. The less albedo a surface has, the more energy contained in solar radiation (sunlight) is getting absorbed. For example, if the moon has an albedo of 6%, that means 6% of the incoming solar energy that reaches the moon's surface is reflected and the other 94% of the energy is absorbed by the surface.

Effects of Urban Heat Island 

• Increased Energy Consumption: Increased temperature demands for air conditioning imply that the community will require 5 to 10 per cent more electricity demand to cater to the requirement. The resulting demand can also overload systems, leading to power outages and blackouts.

• Elevated Greenhouse gas emissions and ozone pollution: The increased electricity demand will rely on fossil fuels for production, resulting in greenhouse gas emissions. Sometimes, the UHI can also lead to the formation of ground-level ozone and acid rain. Research shows that high UHI correlates with increased levels and accumulation of air pollutants at night, affecting the next day’s air quality.

• Danger to aquatic life: Water at high temperatures is discharged into ponds, streams, rivers, lakes, and oceans, resulting in thermal pollution. As a result, the increased water temperature affects the aquatic system, especially the reproduction and metabolism of marine species, and can even be fatal to aquatic life.

• Threat to human health: Human health is negatively impacted by increased general discomfort, exhaustion, heat-related mortality, respiratory problems, headaches, heat stroke, and heat cramps. Research by the Centre for Disease Control and Prevention indicates that between 1997 and 2003, more than 8,000 premature deaths were registered in the United States due to excessive heat exposure. According to WHO, each year 1200 people die as a result of heat-stress-related diseases in South India.

Solutions to Urban Heat

Several short-term and long-term solutions exist to adapt to an increasingly warmer future and mitigate urban heat stress and related illness. Short-term solutions are about getting people out of extreme or dangerous heat levels and ensuring their health and safety. There are also ways to reduce urban heat island effects over the long term, such as

• Planting trees, particularly along paved streets. Vertical gardens are also a part of the solution. They help reduce city noise for residents, and plants act as a natural filter for rainwater.

• A green roof, or rooftop garden, is a vegetative layer grown on a rooftop that can provide shade and reduce rooftop temperatures.

• Cool roofs are made of highly reflective (i.e., high albedo) and emissive materials that remain cooler than traditional materials and help to reduce energy use.

• Agency report suggests that anywhere from 35-40% of an average city’s urban area is covered in pavement, making it a significant contributor to UHIs. Cool pavements, an alternative to conventional concrete or asphalt sidewalks and roads, are reflective grey coating and/or permeable materials that help reduce surface temperatures.

• Converting disused or abandoned urban properties into miniature parks, called pocket parks by urban planners and architects, has proven effective in combating the heat island problem.

• More efficient air conditioning is one of the several weapons deployed by future generations to combat the dangers of UHIs.