Urban Heat Island (UHI)

An urban heat island (UHI) is an urban area or metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The temperature difference usually is larger at night than during the day, and is most apparent when winds are weak.UHI is most noticeable during the summer and winter.

 

The main cause of the urban heat island effect is from the modification of land surfaces. Waste heat generated by energy usage is a secondary contributor. As a population center grows, it tends to expand its area and increase its average temperature.

 

UHI in Nepal

The topic is new for Nepal and most of the developing countries. The study carried out during the research in Kathmandu showed the presence of urban heat island affect and its growing trend.

How Do Heat Islands Form?

When it comes to coping with heat waves, our own cities are conspiring against us. Road surfaces, pavements and buildings all contribute to keeping urbanized environments three to four degrees hotter than surrounding non-urbanized areas.

The urban heat island effect occurs because the dense dark surfaces such as bitumen on roads and building materials used in cities accumulate and store heat during the day and then release it at night.

Heat islands form as vegetation is replaced by asphalt and concrete for roads, buildings, and other structures necessary to accommodate growing populations. These surfaces absorb—rather than reflect—the sun's heat, causing surface temperatures and overall ambient temperatures to rise. Displacing trees and vegetation minimizes the natural cooling effects of shading and evaporation of water from soil and leaves (evapotranspiration).

 

Tall buildings and narrow streets can heat air trapped between them and reduce air flow. Waste heat from vehicles, factories, and air conditioners may add warmth to their surroundings, further exacerbating the heat island effect. Other contributing factors to the heat island effect include:

• Size and shape of cities—Tall buildings act as obstacles and reduce wind speeds.

 

• Urban deserts—cities can be thought of as virtual deserts with almost no vegetation and materials that are almost completely impermeable to rain. This combination leads to a lack of evapotranspiration which increases sensible heat.

 

• Urban canyons—the tall canyons formed by city buildings trap radiant energy in their walls.

 

• Humidity effects—although there is little difference in the amount of water that cities and countrysides retain in their atmospheres (absolute humidity), the higher urban temperatures effectively lowers the relative humidity (since warm air can hold more water than cold air)

 

• Anthropogenic heat—The release of heat from the burning of fossil fuels can also raise urban temperatures

 

UHI control measures

Not all cities have a distinct urban heat island. Mitigation of the urban heat island effect can be accomplished through the use of green roofs and the use of lighter-colored surfaces in urban areas, which reflect more sunlight and absorb less heat.

One of the simplest solutions to reducing the urban heat island effect is to provide more shade, with trees.

But tree planting has its limitations: trees can't be planted in the middle of roads, they can?t necessarily be planted on private property, and there are also potential issues with having too many trees

Another approach that can cut down on heat absorption is to consider different surface materials for roads and pavements.

One alternative is green roads with a more porous surface that allows water to seep in and even grass to grow through, which in turn cuts down the amount of heat absorbed by the road surface.

A similar principle to green roads applies to green roofs and green walls, where the building is partly or fully covered by vegetation. This approach indirectly reduces urban heat by cooling the building itself and reducing its air-conditioning requirements, which in turn reduces the amount of waste heat released into the environment. But green roofs can also have unwanted side effects.

Contributors to the urban heat island effect and the potential solutions to it vary enormously from city to city, which is why modeling of individual cities is vital. What works in one city – like planting trees along the wide streets – is not going to be as effective or even as practical in the narrow street.

 

Are Heat Islands and Global Warming Related?

Heat islands describe local-scale temperature differences, generally between urban and rural areas. In contrast, global warming refers to a gradual rise of the earth's surface temperature. The local warming effect due to changes in urban fabric can be equally problematic as global warming because it is a major contributor for rising up the atmospheric temperature. This research focuses on urban heat island effect, a form of urban micro-climate, and suggests some mitigation measures for this phenomenon.

 

There are no simple solutions, but ignoring the problem is definitely not an option.

 

References:

https://www.theguardian.com/sustainable-business/2017/feb/21/urban-heat-islands-cooling-things-down-with-trees-green-roads-and-fewer-cars

 

https://en.wikipedia.org/wiki/Urban_heat_island