Urban Heat Islands. What are they?

Globalization facilitates many social and economic process, one of the major being migration. Rural population migrates to urban areas in search for employment. About half the world’s population lives in these urban areas. With the ever increasing cluster of the masses, it is no surprise of how it is affecting the environment, making it more harmful day by day.

The phenomenon of Urban Heat Islands is one such problem of the Urban area.  What is it really? It refers to the increase in temperature in contrast to the surrounding rural area.  The temperature difference is more larger at night than at day and more prominent during when the winds are weak. As the population center grows it tends to increase its area and with it the temperature. Increasing heat in urban areas heightens the length of growing seasons and decreases the occurrences of weak tornados. It is detrimental in the cases of preventing pollutants such as ozone and decreases water quality as warmer waters flow area streams and put pressure on their ecosystems.

What causes UHI?
 The main cause of UHI effect is the modifications brought about in the land that use materials that store short wave radiation. The reason why UHI causes warmer nights are that the short-wave radiation that are absorbed during the day hinder the cooling processes in the night, thus making it warmer. The other causes include changes in the thermal properties of surface substances and the lack of evapotranspiration (like the lack of vegetation) in urban area. A decrease in the amount of trees means loss of shade and cooling effect, less albedo of their leaves and reduced carbon dioxide. What doesn’t help here, is the fact that many materials used in urban areas for building pavements and roofs include concrete and asphalt which have different heating and thermal properties that create an energy imbalance in the urban areas, causing it to have higher temperature than the rural areas. Yet another cause is the geometric effect – the urban area contains a vast number of tall building and reflective surfaces which increases absorption of sunlight and the rate at which the area gets heated. This is also known as the ‘Urban Canyon Effect.’ The other cause of it is also the waste heat generated by energy usage. As the population center grows it tends to increase its area and with it the temperature.

How does it impact the weather?
Not only does it affect the temperature, but also produces secondary effects on meteorology, which includes the altering of local wind patterns, formation of clouds and fog, humility and the rates of precipitation. The heat provided by UHI leads to greater upward motion that can induce rainfall and thunderstorm activities. This can explain why when it gets really hot it suddenly rains – since during the day UHI creates a local low pressure area from where the relatively moist air from the surrounding rural area converges and possibility leads to favorable conditions for cloud formation.

But wait… How does it affect us?
UHI have proven to directly have potential to affect the health and welfare of urban population. In the United States alone, an average of a thousand die due to the extreme heat. Since the characteristic of UHI are increasing temperatures they can change the magnitude and duration of heat waves in the cities, which potentially causes very types of ailments in the citizens, most common being a sunstroke. It is seen that those cities prone to high temperatures throughout the year suffer a wee bit lesser than those cities which are not. It is also noted that the effects of UHI differ according to the location of the urban areas and their altitudes. The effect of UHI are particularly dangerous if they are accompanied by a heat wave – as the deprives the urban residents of the cooling process in the night, that would have been a relief to them in the normal heat.
Increased temperatures and sunny days lead to the formation of low level ozone from the volatile organic compounds and nitrate oxides which already exist in the air – this leads to worsened air qualities.

Is there a way to stop this?
As one knows, the color black absorbs the most heat – leading to conclusion that dark colors are significant heat absorbers. The color code in cities comprise of the prevalence of such dark colored materials throughout the urban areas. The simplest way of combating UHI is of changing this very color code into more light shades of white or pastel. The cooling roof, is one made from reflective material such as vinyl reflects almost 70% of the sun’s rays. (In comparison to asphalt roof which reflect only 6-26% of the sun’s rays.)
The other measure is using light colored concrete for pavements which reflect 50% more light than asphalt. However simple pavement changes don’t bring about a significant effect unless it is paired with changing the tiles of the surrounding building with materials that reflect the sun’s rays.
The second best option we have is to increase well-watered vegetation in urban areas. It can implemented by the use of green roofs (also known as a Living Roof that is a roof that is partially or completely covered by vegetation.) They are great insulators because during the warmer months, the plants cool the surrounding environment. Air quality is also improved since the plants take in and convert carbon dioxide into oxygen.

So in conclusion, there’s a simple take on combating UHI – plant a sapling, cool the city.      


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