Blazing Dhaka: An urban heat island
Whoever has experienced the sweltering summer days of Dhaka will agree that average temperature of Dhaka City has increased over the decades. The scorching heat during daytime and hot, see thing nights coupled with load shedding are the bane of the city dweller's life. But why the temperature is so high in Dhaka? The answer can be attributed to a unique feature of the urban climate known as Urban Heat Island (UHI) Effect. The urban heat island phenomenon was first discovered in the early 1800s in London. But it is not clear yet whether it is related to global warming.
A UHI is a metropolitan area which is significantly warmer than its surrounding rural areas. The temperature difference usually is larger at night than during the daytime and larger in winter than in summer, and is most apparent when winds are weak. The main cause of the urban heat island is modification of the land surface by urban development; waste heat generated by energy usage is a secondary contributor. Since we have covered most of the land surface of Dhaka with concrete and asphalt pavements, the city has become an oven with millions of people in it.
Though the UHI air temperature is generally most apparent at night, urban heat islands exhibit significant and somewhat paradoxical diurnal behaviour. The UHI air temperature is large at night and small during the day, while the opposite is true for the UHI surface temperature. Throughout the daytime, particularly when the skies are free of clouds, urban surfaces are warmed by the absorption of solar radiation. As described above, the surfaces in the urban areas tend to warm faster than those of the surrounding rural areas. By virtue of their high heat capacities, these urban surfaces act as a giant reservoir of heat energy. (For example, concrete can hold roughly 2000 times as much heat as an equivalent volume of air). This daytime heating creates convective winds that minimize the surface temperature to a great extent. At night, however, the situation reverses. The absence of solar heating causes the atmospheric convection to decrease. This traps the urban air near the surface, and allows it to heat from the still-warm urban surfaces, forming the nighttime UHI air temperature.
Significance
The effects of UHI are manifold. UHIs have the potential to directly influence the health and welfare of the urbanites. Compared to rural areas, cities experience higher rates of heat-related illness and death. The heat island effect is one factor among several that can raise summertime temperatures to levels that pose a threat to public health. The nighttime effect of UHIs can be particularly harmful during a heat wave, as it deprives urban residents of the cool relief found in rural areas during the night. Furthermore, the poor air quality that results from this increased energy usage can affect our health, aggravating asthma and other respiratory illnesses. People in Dhaka, especially the poor, suffer from various acute respiratory diseases during summer. In 1995, a heat wave in Chicago illustrated why excessive temperature and heat islands are of concern. This episode of unusually hot weather resulted in the deaths of over 700 people.
Another consequence of urban heat islands is the increased energy required for air conditioning and refrigeration in cities that are in comparatively hot climates. The Heat Island Group, a research and advocacy organization that works to educate the public and policymakers about the heat island effect, estimates that the city of Los Angeles spends about $100 million per year in extra energy costs to offset its heat island effect. Urban heat islands also can impact local weather, altering local wind patterns, spurring the development of clouds and fog, increasing the number of lightning strikes, and influencing the rates of precipitation as we are experiencing now in Dhaka. Sometimes it also affects growth of the trees. Using satellite images, researchers discovered that plants take more time to grow in UHIs than the rural areas.
Causes
Poor urban design is the biggest cause of heat island in our city. Heat islands are created when city growth alters the urban fabric by manmade asphalt roads and tar roofs and other features substituting forest growth. These surfaces absorb - rather than reflect - the sun's heat, causing surface temperatures and overall ambient temperatures to rise.
The principal reason for the night-time warming is (comparatively warm) buildings blocking the view to the (relatively cold) night sky. Two other reasons are changes in the thermal properties of surface materials and lack of evapotranspiration in urban areas. Evapotranspiration (ET) is a term used to describe the sum of evaporation and plant transpiration from the earth's land surface to atmosphere. Materials commonly used in urban areas, such as concrete and asphalt, have significantly different thermal bulk properties (including heat capacity and thermal conductivity) and surface radiative properties than the surrounding rural areas. This causes a change in the energy balance of the urban area, often leading to higher temperatures than surrounding rural areas. But that's not the only thing that causes the urban heat island effect. Scientists believe that vegetation plays a large part in keeping an area cool through a process called evaporative cooling. Plants take in water through their roots and depend on it to live.
But after the plant is done with it, dry air absorbs that water by turning it into gaseous water vapour. The air provides the heat that drives this process, so during the process, the air loses heat and becomes cooler. Because building a city means replacing vegetation with structures, the city loses the evaporative cooling advantages of vegetation.
Other causes of a UHI are due to geometric effects. The tall buildings within many urban areas provide multiple surfaces for the reflection and absorption of sunlight, increasing the efficiency with which urban areas are heated. This is called the "canyon effect". Meanwhile, tall buildings and narrow streets can heat the air trapped between them and reduce airflow. Waste heat from automobiles, air conditioning, industry, and other sources also contributes to the UHI. High levels of pollution in urban areas can also increase the UHI, as many forms of pollution change the radiative properties of the atmosphere.
Options
Luckily, since we know what causes the urban heat island effect, we can control it to a significant extent. The fact is green trees and only trees can help us. Yes, to escape the heat island effects, cities need a lot more vegetation and a lot fewer dark and hard surfaces. But the inconvenient truth is that some of our wise guys are plotting to cut down hundreds of trees here and there in UHIs. Celucas, what a strange country this is!
However, some people don't like the idea of a glaring, all-white city. Low-reflectivity coating offers an alternative and comes in non-white colors. These kinds of coatings reflect invisible radiation without reflecting all light. So, they keep an object relatively cool without sacrificing its dark color. Certain high-reflectivity coatings can also be applied to asphalt. Asphalt chip seals and emulsion sealcoats are two such examples that treat asphalt to make its surface more reflective.
One fad that's gaining popularity is the installation of green roofs atop city buildings. This solution doesn't have anything to do with colour. A "green roof" is simply a roof that includes plants and vegetation. Green roofs harness the same evaporative cooling effect that cities lose when they hack away vegetation. So a green roof not only prevents the building's roof from absorbing heat, but cools the air around it, offsetting the urban heat island effect to an extent. Many sustainable buildings use green roofs to reduce their reliance on energy consumption.
Several other methods help reduce the urban heat island effect as well. For instance, roof sprinkling is another evaporative cooling solution. Sprinklers on the roof wet the surface so that the air around it cools through evaporation. Urban planners also set up traditional parking lots along lots where trees and vegetation grow. Tall trees not only contribute to evaporative cooling but also provide much-needed shade.
Unfortunately, most of the above mentioned measures are costly, especially for a country like us. But there must be something that we can do to ameliorate the existing condition. We cannot be apathetic any more regarding the apocalyptic effects of climate change. Political governments should also ponder this matter seriously and more urban planners are needed to tackle the situation. Many problems already have beset our adored Dhaka. Let's not belie the situation and make it worse.
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