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Climate on a Local Scale-Urban Climates

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Climate on a Local Scale-Urban Climates CLIMATE ON A LOCAL SCALE-URBAN CLIMATES Cities create their own climate. Geographers call it the climate dome, where weather is different from the surrounding rural areas. MODIFICATIONS TO URBAN CLIMATES • TEMPERATURES • WIND • SUNLIGHT • PRECIPITATION • CLOUD • ATMOSPHERIC COMPOSITION • RELATIVE HUMIDITY In comparison to rural areas cities have: • Higher max and min temperatures-greater temp range, • Lower wind speeds, with exception of tunnelling, • Greater uplift of air, • Greater cloud cover and precipitation, • Poor visibility, with haze and smog more likely. COMMENT UPON THE TEMPERATURE DISTRIBUTION SHOWN IN THIS MAP Map of temp dist over Montreal in Canada,7 March, 7am, with winds of 0.5ms • Lower temps on side of prevailing wind • The plateau-like temp zone running from the city edge towards the city centre, which has the higher temp • The correlation between the highest density of building and the highest temps • The fall in temp over the park, which has an open vegetated space. The effects of urban areas on local climate. The URBAN CLIMATE DOME THE DOME HAS THE FOLLOWING FEATURES: • Extends upwards 250-300m and its influence may continue for tens of kms downwind. • There are 2 levels within the urban dome-below roof level there is an urban canopy where processes act in the space between buildings (canyons) and above this is the urban boundary layer. This extends downwind and at a height as a plume into the surrounding rural areas. WIND • Wind speed is reduced by 12-14mph by the presence of buildings, which create friction drag and act as wind breaks.Create turbulence,causing changes in wind speed and direction • Wind-tunnelling can occur due to high-rise buildings channelling air into canyons between them. Called the VENTURI-EFFECT and can be avoided by building on stilts. • As wind hits buildings it can cause eddies and turbulence at street level. • On calm ,clear nights there is a surface inflow from cooler areas outside the city to the warmer areas in the city centre. AIRFLOW MODIFIED BY A SINGLE BUILDING • Air is displaced upwards and around the sides of the building and is also pushed downwards in the lee of the structure. AIRFLOW MODIFIED BY MORE THAN 1 BUILDING • If buildings are close each will interfere with the airflow around the next structure. • Important that chimneys are built high enough to release pollutants into undisturbed air flow. The VENTURI EFFECT • In the lee of the building there is a zone of lower pressure, causing vortices behind it. • If 2 buildings allow flow between them then the venturi effect is set up, in which pressure within the gap causes wind to pick up speed. • However, a reasonable flow of air is needed to remove air pollution. Vortex flows • As air flows around buildings suction is set up.On windward side overpressure causes a descending flow.Forms a vortex when reaches ground and sweeps around the windward corners. THE URBAN HEAT ISLAND EFFECT • Over the year London is 1.3c warmer than the surrounding rural area. • Temp differences can vary from 0.6cin the day to 3-4c at night- WHY? • Most noticeable when wind speeds are low as they tend to reduce temp diffs by dispersing the build-up of warm air or cloud cover. DEFINITION: A REGION OF WARMER AIR TEMP RELATIVE TO THE SURROUNDING RURAL AREA IN A METROPOLITAN AREA. Temperatures • Raised because heat from energy sources escape into atmosphere: • Industries, vehicles and people all generate heat • Air conditioning units release hot air into the atmosphere • Building materials such as concrete, bricks and tarmac act like bare rock surfaces, absorbing heat in the day and slowly releasing it at night. Some buildings with lots of glass have a high reflective capacity, and multi storey buildings tend to concentrate the heating effect in the surrounding street by reflecting energy downwards. • Air pollution increases cloud cover and creates a pollution dome, which allows in short-wave radiation but absorbs a large amount of outgoing radiation as well as reflecting it back to the surface. • Water falling to the surface is disposed of quickly and so this changes the urban moisture and heat budget – reduced evapotranspiration means more energy available to heat the atmosphere. Urban heat islands vary: • Over time –WHEN CONDITIONS ARE HIGH- PRESSURE WHY? ALSO WHEN THERE IS A TEMP INVERSION. • - IN WINTER ? • Over space – TEMPS RISE ABRUPTLY AT THE RURAL-URBAN BOUNDARY= A CLIFF! FROM THIS POINT TEMPS RISE STEADILY TO A PEAK IN THE CITY CENTRE (2-4C PER KM) THIS IS CALLED THE PLATEAU. WITHIN THIS PLATEAU THERE ARE VARIATIONS. PLATEAU VARIATIONS • Distribution of power stations, water areas and open spaces, • Due to ALBEDO. (amount of insolation that is reflected by the Earths surface and the atmosphere). Dark surfaces absorb insolation and then re-radiate it as long- wave energy that heats up the urban area.Reflective surfaces absorb little and reflect back into atmosphere keeping cool, of reflect it into small areas, which heat up. • Surfaces in cities are less reflective than in rural areas- tarmac! Snow reflects and stays much longer in the countryside. HOW CAN THE URBAN HEAT ISLAND BE REDUCED? • Create green spaces within the city. • Use lighter coloured materials for roads and roofing to reflect the heat. PRECIPITATION • Higher urban temps cause low pressure and convection rainfall, which is heavier and more frequent. • Also causes thunder and lightning. • WHY? • URBAN HEAT ISLAND GENERATES CONVECTION, • HIGH RISE BUILDINGS GENERATE VERTICAL MOTION, • WATER VAPOUR FROM INDUSTRIAL SOURCES AN DPOWER STATIONS, • POLLUTANTS ACT AS HYGROSCOPIC NUCLEI AND ASSIST CONDENSATION. THUNDERSTORMS • Develop in hot humid air. In urban areas the chance of them is increased.WHY? • Because of the convectional uplift created under conditions of extreme instability. • During uplift cooling, condensation and latent heat production occurs,further fueling the convectional uplift. • Raindrops are split and positive electrical charge builds up, when the charge is high enough to overcome resistance in the cloud a discharge occurs to areas of negative charge in the cloud or to earth.This is lightning.Thunder is heard as a result of rapid expansion of the air. FOG • Occurrence of fog increased with industrialisation.1700s 20 days of fog, by 1950s risen to over 50 days. • Particulate pollution creates condensation nuclei,encouraging fog formation at night, usually under high pressure conditions. • The clean air act of the 1950s reduced smoke production and particulate emissions, reducing the number of foggy days. • As LEDCs industrialise more fog is experienced.Eg New Delhi and Beiijing suffer winter fogs, crippling transport networks and affecting human health. CLOUD COVER • Clouds are thicker and 10% more frequent • Due to more rising air due to higher temps and a larger number of condensation nuclei. • All the same reasons that precip is higher. • Humidity is lower in cities because there is a lack of water surfaces or veg to emit moisture through transpiration. SUNLIGHT • Despite higher temps a city receives less sunshine. • Pollutants may reflect up to 50% of the insolation. • High rise buildings also block out sunlight..
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