Dublin’s Urban Heat Island

Dublin’s Urban Heat Island

04. Describe and comment on the extent of the urban heat island shown

The highest temperature is found just North of the River Liffey in B3 at 3.5 degrees. This is probably due to the fact that this area is the CBD of Dublin and the most densely populated area of the city. This also means that it is the area with the densest buildings, which emit and store large quantities of heat. This part of Dublin is 5 degrees warmer than the countryside surrounding the city. The rate at which this temperature drops as you move out from the centre varies on which direction you travel. The most dramatic and rapid temperature drop is to the North and West of the city, this is due to the fact that the urban area fades away, and the environment becomes more suburban and rural. To the east of the city there is the Irish Sea which will cause the area to have a less variable temperature due to the specific heat capacity of water. To the South of Dublin the temperatures drop off much more slowly which is probably due to the extent of the urban area that expands in this direction. As these figures were measured on a winter’s night, the heat island shows the heat absorbed during the day and is still head by the urban buildings.

05. Explain how urban structures and layout affect winds.

On the whole, urban areas have an annual mean of wind speeds of 10-20% lower than in rural areas. However gusts in cities are often much faster than the more constant wind of rural areas. In general, buildings exert a powerful friction drag on the wind, causing both its direction and speed to change in urban areas. For a single, isolated building wind will split when it hits the face of the building and will divert over and around the obstacle. On the windward side, some wind will push downwards creating a small eddy. On the leeward side, there will be some more defined eddies of wind which will spiral around the back of the building. This effect is exaggerated by the strong pressure gradient between the, low pressure, leeward side and the, high pressure, windward side. The orientation of buildings also plays a part in the winds in the urban areas, rounded edges and diagonal faces facing the wind will cause minimal disruption and less eddying.

In most urban areas however there are rarely isolated buildings it is more common that there will be groups of buildings and streets all interfering with the wind. Taller buildings will channel the wind into the canyons between them; this can give rise to the Venturi effect. This is due to the common fact that streets get narrower and buildings get taller as you get closer to the city centre. Therefore the winds are channelled and squeezed through the streets, causing wind speeds to increase. Due to the varying heights and surfaces of the buildings the exact effect that an urban area will have