Author: Siti yaumilia salsa (13.036)

Tropical architecture

Architecture and climate

Effect of buildiing on local climate

BUILDING DOES AFFECT MICRO CLIMATE SURROUNDINGS. IT

RAISESAMBIENT TEMPERATURE, CHANGE ITS RELATIVE HUMIDITY

AND AIRMOVEMENT SURROUNDING THE BUILDING. IN A LARGE

SCALE IT COULDCHANGE PRECIPITATION VARIATIONS

Buildings have a major impact on the environment over their

entire life cycle. Responsible for 40% of worldwide energy flow and

material use, conventional buildings have been identified as the largest source of green-house gas emissions, even more than

that of the transport and industry sector. They affect urban air quality and contribute to climate change. They are also

hazardous to health at times. During their lifecycle, conventional buildings harm the environment in many ways.

http://hubpages.com/forum/topic/4367

Urban Heat Island (UHI)

What is an urban heat island?

"Urban Heat Island" (UHI) refers to the tendency for a city or

town to remain warmer than its surroundings, by as much as

10 deg. F or more. This effect is caused mostly by the lack of

vegetation and soil moisture, which would normally use the

absorbed sunlight to evaporate water as part of photosynthesis

(a process called "evapotranspiration"). This is also the reason

why forests are cooler than deserts.

Instead, the sunlight is absorbed by manmade structures -- roads, parking lots, and buildings. With little or no water to

evaporate, the sunlight's energy goes into raising the temperature of those surfaces and the air in contact with them. As

the day progresses, a dome of warm air forms over the city asconvection transports heat from the surface to higher in the

atmosphere.

(http://www.weatherquestions.com/What_is_the_urban_heat_island.htm)

Over the last decade, heat waves have claimed an increasing number of casualties among the elderly - particularly in

southern Europe. Prolonged periods of high temperatures also put a strain on medical resources and place an additional

financial cost to society as a whole. High densely built-up areas trap the heat, especially at night, causing what is called

Urban Heat Islands (UHI) in which city centers can be up to 10° C warmer than surrounding rural areas. Another

consequence of UHIs is that energy consumption rises with the increased use of air conditioners and refrigeration appliance.

http://www.urbanheatisland.info/

As urban areas develop, changes occur in their landscape. Buildings, roads,

and other infrastructure replace open

land and vegetation. Surfaces that were once permeable and moist become

impermeable and dry.1 These changes cause urban regions to become

warmer than their rural surroundings, forming an "island" of higher

temperatures in the landscape.

http://www.epa.gov/hiri/about/index.htm

CAUSES

There are several causes of an urban heat island (UHI). The principal reason for the nighttime warming is that the short-wave radiation is still within the concrete, asphalt, and buildings that was absorbed during the day, unlike suburban and rural areas. This energy is then slowly released during the night as long-wave radiation, making cooling a slow process.[2] Two other reasons are changes in the thermal properties of surface materials and lack ofevapotranspiration (for example through lack of vegetation) in urban areas. With a decreased amount of vegetation, cities also lose the shade and cooling effect of trees, the low albedo of their leaves, and the removal of carbon dioxide.[7][8] Materials commonly used in urban areas for pavement and roofs, such as concrete and asphalt, have significantly different thermal bulk properties (including heat capacity and thermal conductivity) and surface radiative properties (albedo and emissivity) 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.[9]

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 "urban canyon effect". Another effect of buildings is the blocking of wind, which also inhibits cooling by convection and pollution from dissipating. Waste heat from automobiles, air conditioning, industry, and other sources also contributes to the UHI.[4][10][11] High levels of pollution in urban areas can also increase the UHI, as many forms of pollution change the

radiative properties of the atmosphere.[9] As UHI raises the temperature of cities, it will also increase the concentration of ozone in the air, which is a greenhouse gas. Ozone concentrations will increase because it is a secondary gas, aided by an increase in temperature and sunlight.[12]

.

Vancouver's downtown (source :

http://www.aboutvancouver.co.uk). Vertical surfaces

absorb the energy reflected from most of the three-

dimensional built-up surface.

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

IMPACT OF HEAT ISLAND

It has been largely demonstrated that cities with variable landscapes and

climates can exhibit temperatures several degrees higher than their rural

surroundings (i.e. UHI effect), a phenomenon which if increases in the future,

may result in a doubling of the urban to rural thermal ratio in the following

decades. Hence, assessment of the UHI and strategies to implement its mitigation

are becoming increasingly important for government agencies and researchers of

many affected countries.

As it would be expected, the characteristic inclination towards warming of urban

surfaces is exacerbated during hot days and heat waves, which reinforces the

air temperature increase, particularly in ill-ventilated outdoor spaces or inner

spaces of Source: PD.PHOTO.ORG

residential and commercial buildings with poor thermal isolation. This increases

the overall energy consumption for cooling (i.e. refrigeration and air-

conditioning), hence increasing the energy production by power plants, which leads to higher emissions of heat-trapping

greenhouse gases such as carbon dioxide, as well as other pollutants such as sulfur dioxide, carbon monoxide and particulate

matter. Furthermore, the increased energy demand means more costs to citizens and goverments, which in large metropolitan

areas may induce significant economic impacts. On the

other hand, UHIs promote high air temperatures that

contribute to formation of ozone precursors, which

combined photochemically produce ground level ozone.

A direct relationship has been found between UHI

intensity peaks and heat-related illness and fatalities,

due to the incidence of thermal discomfort on the

human cardiovascular and respiratory systems.

Heatstroke, heat exhaustion, heat syncope,and heat

cramps, are some of the main stress events, while a

wide number of diseases may become worse,

particularly in the elderly and children. In a similar way, respiratory and lung diseases have shown to be related to high

ozone levels induced by heat events. Several of the above mentioned impacts have been addressed by the US EPA (for

further information see links in the “External link” section of this website).

The anomalous warm of the city creates relatively low air pressures that cause cooler, rural air to converge on the urban

center, thus forcing warm air to ascend (i.e. convection), which at higher altitudes condensates and precipitates. Studies

carried out in several cities of the United States such as Atlanta, New York, Chicago and Washington, have shown that

urban-induced precipitation and thunderstorm events are mainly initiated by the UHI. Other meteorological impacts of the

UHI are associated with reductions in snowfall frecuencies and intensities, as well as reductions in the diurnal and seasonal

range of freezing temperatures. Lastly, high temperatures may produce physiological and phenological disturbances on

ornamental plants and urban forests.

Although in wintertime the UHI can result in energy savings (i.e. winter penalty), there is a great consent among

researchers that this benefit is outweighed by the detrimental effects that ocurr in summertime.

Mitigation of heat island effects

Figure 5. Workers apply a cool roof coating on a rowhouse in

Baltimore, Maryland. (Source: EPA)

There are a number of steps that communities can take to lessen the

impacts of heat islands. These heat island reduction strategies

include:

1. Installing cool roofs or vegetated green roofs

2. Installing green roofs

3. Switching to cool paving materials.

4. Planting trees and vegetation

The extent to which urban areas can benefit

from heat island reduction strategies depends on

several factors. Some of these factors, like prevailing weather patterns, geography, and pollution

transported from upwind regions, are largely beyond the influence of local policy.

However, factors such as land-use patterns, materials used in road and building construction, and the

coverage of urban trees and vegetation can be directly affected by decision makers. This is where

policies and programs to reduce the impacts of heat islands – and achieve related environmental and

energy-savings goals – can be most effective. http://www.eoearth.org/view/article/153461/