Author: Shankar Dev
An urban heat island (UHI) is a metropolitan area that is significantly warmer than its surrounding rural areas due to human activities.
We would have studied, for every 1 km there is temp drop of ~6° C. So when you go to the hill top of 1000m altitude, you will experience ~6° c lesser temperature than what you experience in sea level. But take it like this, mountain top comparatively nearer to sun & its likely to receive more sun rays than ground, so logically mountain top should experience more heat. But it is not so because Earth is not heated by incoming sunrays, instead by re-radiated rays.
While the sun is very hot it doesn’t emit this energy to the Earth as heat but rather as solar radiation. The majority of this solar radiation passes through the upper and lower atmosphere later being absorbed by land and water. Now the land and water releases (re-radiates) the absorbed heat to the atmosphere. Thus the diurnal temperature fluctuates. That is why though we receive maximum insolation at 12, we experience peek temperature at 3. while hot air rise (as it is less dense than cooler air) it expands, so the some energy is lost, thereby becoming cool. That is why mountain tops are colder.
The above explanation is actually irrelevant to this topic but we have to note 2 points from the above paragraphs.
- The temperature that we feel, measure is not of the land, but of the atmosphere (air).
- Air is heated by terrestrial radiations but not by incoming radiation.
(Back to topic)
In these days of rapid urbanization, vegetations are cleared for building multistoreys, roads, and other infrastructures. These structures are made of RCC, asbestos, steel roofing,tar etc..which are non-permeable and dry. Before such rapid urbanization the surfaces were permeable and moist. The rural areas adjoining the city still have permeable and moist surfaces. On a hot, sunny summer day, the sun can heat the dry and exposed urban surfaces, such as roofs and pavement, to temperatures 27–50°C hotter than the air, meanwhile shaded or moist surfaces in rural surroundings remain close to air temperatures. These variations cause urban regions to become warmer than their rural surroundings by 2-10°C, forming an “island” of higher temperatures in the landscape. Surface urban heat islands are pronounced both day and night, but tend to be strongest during the day when the sun is shining.
Increased Energy Consumption
Higher the temperatures higher the energy demand i.e for cooling purposes. According to different studies electricity demand for cooling increases 1.5–2.0% for every 0.6°C increase in air temperatures. During peak summer, the resulting demand for cooling can overload systems and leading blackouts to avoid power outages.
Elevated Emissions of Air Pollutants and Greenhouse Gases
Urban heat islands raise demand for electrical energy in summer. In most of the developing countries energy needs are met by fossil fuel power plants, which in turn leads to an increase in air pollutant and greenhouse gas emissions. The primary pollutants from power plants include sulfur dioxide (SO2), nitrogen oxides (NOx), particulate matter (PM), carbon monoxide (CO), and mercury (Hg).
These pollutants contribute to complex air quality problems such as the formation of ground-level ozone (smog), fine particulate matter and acid rain. Increased use of fossil-fuel-powered plants also increases emissions of greenhouse gases, such as carbon dioxide (CO2), which contribute to global warming and Global dimming (already seen). Increased use of air-conditioners releases CFC’s which affects ozone layer.
Compromised Human Health and Comfort
Increased daytime temperatures, reduced nighttime cooling and higher air pollution levels associated with urban heat islands can affect human health by contributing to general discomfort, respiratory difficulties, heat cramps and exhaustion, non-fatal heat stroke, new microbes, new diseases and heat-related mortality.
Impaired Water Quality
High pavement and rooftop surface temperatures can heat rainwater. This heated rainwater generally becomes runoff, which drains into urban sewage canal and raises water temperatures when it is released into streams, rivers, ponds, and lakes. Water temperature affects all aspects of aquatic life, especially the metabolism and reproduction of many aquatic species. Rapid temperature changes in aquatic ecosystems can be particularly stressful, even fatal to sensitive aquatic life.
Cities in cold climates may actually benefit from the wintertime warming effect of heat islands. Warmer temperatures can reduce heating energy requirements and may help melt ice and snow on roads. Anyways this is not a matter of rejoice as it will have its own consequences.
- Installing cool roofs or vegetated green roofs
- Switching to cool paving materials.
- Non-evasive land-use patterns
- Eco friendly materials used in road and building construction
To sum up, we should formulate an Eco-friendly urbanization.