1.4.1 Urban microclimates Flashcards
Why are urban areas warmer than rural areas?
The urban heat island effect
What are urban heat islands?
Urban areas with higher air temperatures than the surrounding rural areas.
Changes in temperature within urban heat islands
- The highest temperatures are found in industrial areas and in the most densely built-up areas, e.g. the CBD.
There are pockets of cool air above park and bodies of water (e.g. rivers or ponds). These are called temperature ‘sinks’. - Areas within the city with the same land use (e.g. industry) generally have the same temperature. These are called temperatures ‘plateaus’.
- Temperatures can change rapidly when land use changes (e.g. from inner city housing to CBD high rise buildings). Rapid changes are referred to as temperature ‘cliffs’.
What are the main causes of the Urban Heat Island effect?
- Absorption of heat by urban surfaces
- Air pollution
- Heat from human activities
- Less evaporation
How does the absorption of heat by surfaces cause the urban heat island?
Concrete, brick and tarmac surfaces absorb and store heat from the sun during the day. They slowly release the heat as long wave radiation – this is most noticeable at night, when it warms the air.
How does air pollution cause the urban heat island?
Air pollution from cars and factories increases cloud cover over the city. It also creates a ‘pollution dome’ – a layer of pollution over the city. Both these things trap outgoing heat radiation and reflect it back to the surface.
How does heat from human activites cause the urban heat island?
Cars, factories, offices, central heating, air conditioning units and people all release heat.
How does decreased evaporation cause the urban heat island?
When it rains, the water is quickly removed by drainage systems, so there’s little surface water to evaporate. Also, there isn’t much vegetation, so there’s little transpiration. Evapotranspiration uses heat energy, so less evapotranspiration means higher temperatures.
Variation in the urban heat island effect
The UHI effect is:
- Stronger at night
- Stronger in summer
- Stronger when there is an anti-cyclone
Case studies for wind and the UHI
- Toronto
- Masdar City, UAE
Case Study: Wind in Toronto
In the 1980s, Toronto introduced climbing ropes to stop people being blown off their feet in the streets. Toronto are now encouraging the use of podiums in high rise buildings as they redirect wind. Hedges also act as barriers to stop wind.
Case study: Masdar City, UAE
A wind tower has been built, which creates a strong wind around the city and cools it down.
What is the Canyon Effect?
You get powerful gusts of wind when wind is channelled down streets – this is known as the canyon effect.
The canyon effect has implications for building design and town planning, e.g. positioning of buildings and the location of doorways on larger buildings.
Wind in urban areas
Average wind speeds is usually lower in cities than in rural areas. This is because tall buildings create friction that slows down the moving air.
There are areas where wind speed is zero, because some areas are totally sheltered from wind by buildings.
You get turbulence around buildings. This happens when wind hits the face of a building – some of its deflected down, some around the sides and some over the top. When these winds hit other buildings or the ground they cause vortices (bodies of swirling air).
Snow in urban areas
It doesn’t snow as often in urban areas, and when it does, the snow melts faster. This is because it’s warmer due to its UHI effect.
Rain and thunderstorms in urban areas
It rains more often in urban areas than in the surrounding countryside.
The rain is also more intense and there are more thunderstorms.
Why does it rain more in urban areas?
- The UHI effect means the air in urban areas is warm, and warm air can hold more water. The warm, moist air rises – this is called convectional uplift. As it rises it cools, the water vapour condenses and it rains. This type of rain is called convectional rainfall.
- Urban areas generate huge amounts of dust and pollution. Particles of dust and pollution floating about in the air act as condensation nuclei (they trigger water to condense around them). This encourages clouds to form, rather than allowing the warm, moist air to disperse.
Negative effects of the UHI effect
- Ponds in urban areas are drying up due to the higher temperatures.
- Damage to property.
- The use of air conditioning will increase to combat the higher temperatures, increasing the consumption of energy and making the problem even worse.
- Extreme heat island events could lead to heat stroke, asthma, organ damage and even death. In London there were 600 heat related deaths in August 2003. – Up to 9oC warmer than surrounding rural areas.
- Increased mortality associated with temperatures over 22oC.
- Increased water consumption.
- Can cause the heat and pollution to remain still and moving.
- Longer flowing time for hay-fever sufferers.
- Higher increase in Algal Bloom.
- Deterioration of historic monuments and buildings due to temperature related chemical weathering.
What are some strategies used to manage the UHI effect?
- Cool surfaces
- Permeable pavements
- Green roofs
- Urban trees or greening
How do cool surfaces help to reduce the UHI effect?
This is where surfaces, often houses, are painted white.
- Reflect short-wave radiation.
- Could reduce air conditioning by 30%+.
- Could cool cities by 2’C.
Issues: Colder in the winter and houses need to be kept clean for this to work.
How do permeable pavements help to reduce the UHI effect?
- Reduces heat island effect.
- Reduced storm-water run off.
- Lower tyre noise.
- Better night time visibility.
- Local comfort
Example: LA zoo
How do green roofs help to reduce the UHI effect?
- 70% reduction in air conditioning costs for a 1 storey-building.
- Reduce chance of flooding – vegetation holds water.
- Insulates in winter, cools in summer.
- Habitat for wildlife.
- Nice place to life.
Example: Bus stops in Sheffield
How do urban trees/greening help to reduce the UHI effect?
- Reduced temperature – Proven in urban parks
- Provide shade
- Absorption of UV
- Reduced noise
- Improved human comfort and well-being
- Increased property value.
- Aesthetics.
Case study: UHI effect in Chicago
Urban Heat Islands are more of an issue in North America.
The City of Chicago conducted research on the impacts of the urban heat island effect.
In 1995, temperatures in Chicago got over 40oC (120oF) for 4 days, which caused over 700 deaths (mainly elderly and ill people).
Research conducted has mapped Chicago’s hottest spots and its local government is targeting its cooling and energy efficiency efforts, such as green roof programs, to those areas.
Over the past 15 years, Chicago planted more than 500,000 trees and achieved a City-wide tree count of 4.1 million trees. The city plans to plant 1 million new trees by 2020.
Urban microclimate
The small scale variations in temperature, precipitation, humidity, wind speed and evaporation that occur in urban areas.
Climatic dome
The space above an area where different distinctive processes are occurring.
Urban Heat Island
The zone around and above an urban area, which has higher temperatures than the surrounding rural areas.
Thermal gradient
The temperature decline from the urban centre to the rural-urban fringe.
Albedo
The reflectivity of a surface, light surfaces reflect more than darker surfaces so will have a greater albedo and lower temperatures.
