Urban area, impacts of human activity on climate Flashcards
What is meant by the term Urban Heat Islands?
Urban Heat Islands, a situation where the cities’ or metropolitan area’s ambient temperature is dramatically altered and become warmer than the surrounding rural areas.
What happened to the urbanised area of Chicago between 1989 and 2010?
What was the impact of this?
The urbanised area of Chicago increase by 82.2% which is associated with a loss of 25.8% of agricultural lands and a 32.4% decline in urban open and green spaces.
This land transformation has caused noticeable climate changes, including increased energy demands thereby negatively impacting the quality of life. This was a contributing factor to the Chicago heat wave of 1995 that led to approximately 750 heat-related deaths in the city and 3300 excess emergency department visits.
Using the example of Chicago, describe the difference of temperature/location of areas that experience the phenomena of UHIs.
In general, the was a difference of 18oC between low UHI intensity areas around the Lake Michigan in the east and the high intensity areas in the densely populated northwestern neighbourhoods.
The distribution of the UHI phenomena were found to have a direct relationship to the city’s land use. By and large, while waterbodies, forests and woodlands have lowered the phenomena of UHIs; densely populated buildings, congested settlements, and industrial zones have intensified UHI effects.
In Seoul, what has been the effect following the removal of a large, downtown elevated motorway, and the restoration of a river and floodplain that had been built over.
Since the restoration of the stream, air temperatures decreased by up to 10-13%; that is, by 3-4oC during the hottest days. Before the restoration, the area was showing a temperature of about 5oC higher than the average temperature of the city. This is because rivers have a cooling effect:
As the river temperature increases due to evaporation, the humidity of the surrounding area will increase and the higher temperature difference means the potential for sensible cooling of air will also increase.
What is the effect of restoration of and floodplain that had been built over on wind speed?
Following the completion of restoration, the wind speed has become faster (by 2.2-7.1%). The average winds speed measured at Cheong Gye Cheon is up to 7.8% faster than before, apparently under the influence of the cool air forming along the stream.
Using the example of Melbourne, explain how urbanisation can effect the climate.
In Melbourne, for every 10km from the city centre, the tree cover drops by more than 2%. That means Melbourne’s inner suburbs might have more that 15% cover, but an outer suburb less than 10%.
A 5% fall in tree cover can lead to a 1-2oC rise in air temperature. This is because trees and vegetation absorb water through their roots and cool surrounding by releasing water vapour into the air through their leaves. This process is know as evapotranspiration.
Urbanisation involves more brick, concrete and dark tarmac surfaces.
What effect does this have on climate?
Brick, concrete and dark tarmac surfaces have a large specific heat capacity and low albedos. They rapidly absorb solar radiation by day, especially in summer. The heat is conducted away from the surface, stored and re-radiated steadily into the air at night as long-wave radiation.
Describe the difference in heating between densely populated areas and rural areas.
The greater the building density, the greater the heating, so city centres have the highest minimum temperatures at night and temperatures gradually cool through the more spacious suburbs to the countryside.
According to the University of Notre Dame, what is the effect of using green roof surfaces in Chicago.
According to Notre Dame research, the use of roofs with vegetation or reflective surfaces on top of Chicago’s current infrastructure could reduce the effect of the Urban Heat Island by lowering roof temperatures by a range of 3 to 4 oC.
This is because green roofs (roof that are partially covered with vegetation) reflect more sunlight and absorb less when compared to conventional roofs.
What is the problem of using green roofs in Chicago?
By using green roofs to reduce the temperature of the city, there will be less pressure difference caused by cool air from the lake mixing with the relatively less hot air of Chicago, thus decreasing lake-breeze which cools the city.
Explain how tall buildings effect wind speed?
Tall buildings provide frictional drag on the airs movement.
The frictional drag creates turbulence, which gives a rapid changes in the direction and speeds of the wind.
Pressure gradients are set up between the windward side and the leeward side of buildings which causes severe eddying of winds.
The windward side (the side facing the direction of the wind) of a building tends to have high pressure due to the air pushing against it, the leeward side which blocks the straight path has an area of low pressure causing a steep localised pressure gradient.
The steep pressure causes theses winds to be strong.
Why is Chicago named the windy city?
Chicago is names the windy city due to its grid system of buildings
Explain how is cloud cover influenced by human activity
Urban areas receive thicker and up to 10% more frequent cloud cover compared to rural areas.
This is because convection currents are generated by the higher temperatures of urban microclimates and so there are a large number of CONDENSATION NUCLEI which will therefore form more clouds.
The hydroscopic nuclei are made large due the greater amount of pollution in the urban area, with more dust from traffic congestion and industry.
Using an example, explain impacts on climate may be a combination on humans and physical influences.
On 4 December 1952, an anticyclone (an areas of high pressure) settled over a windless London, causing a temperature inversion with relatively cool, stagnant air trapped under a layer of warmer air. The resultant fog, mixed with smoke from home and industrial chimneys, particulates such as those from motor vehicle exhausts, and other pollutants such as sulphur dioxide, formed a persistent smog, which blanketed the capital the following day.
Explain why urban areas receive more precipitation.
The mean annual precipitation total in urban areas and the number of days with less than 5mm of rainfall can be both 5-15% greater than in rural areas. This means that urban areas get a larger amount of dry days, but more rainfall when they do experience rain. This happens due to convection currents which are generated by higher temperatures, and due to an increased amount of microscopic condensation nuclei which hold more moisture.
