A Level Geography - Urban Climate and Pollution

flood hydrograph

a graph showing the response of a river (change in discharge) to a particular period of heavy rainfall

flashy/peaky river

a river that responds quickly to a particular storm event with rapidly rising water levels

smog

air pollution by a mixture of smoke and fog which in the past was a major problem for european cities and is a growing problem in lic cities as they do not have changing fuel-use patterns which hic cities are focusing on

vortex

as air flows around the sides of buildings it becomes seperated form walls and roofs creating suction, on the windward side the overpressure causes a descending flow forming a vortex, in the lee side of the building there is a zone of lower pressure causing vortices behind it

factors affecting weather and climate

atmospheric composition, latitude, altitude, distance from the sea, prevailing winds, aspect, cloud cover and increasingly human activities

turbulence

caused by the uneven nature of the urban skyline as strong pressure gradients develop between the leeward and windward side of buildings which can lead to severe eddying of winds

rochdale

climatologists found that this area had significantly less rainfall on sundays when the mills and factories were not producing smoke which acts as condensation nuclei which shows the effect of air pollution on urban precipitation

carbon monoxide

colour, taste, odourless, poisonous gas produced by the incomplete combustion of fuel, estimated that road transport is responsible for 90% of all its emissions in the uk so concentrations tend to be closest next to busy roads, effects the transport of oxygen around the body by the blood, breathing in low levels can cause headaches and nausea

traffic emissions

current major threat to urban pollution levels and clean air, petrol and diesel emit a variety of pollutants and photochemical reactions due to the action of sunlight on pollutants can result in the formation of ground level ozone and acid rain, most acute in lics due to increasing numbers of vehicles, large numbers of old and inefficient vehicles, poor infrastructure that inhibits flow and poor vehicle regulations

winds

deflected by the shape of buildings in urban areas as well as by their size

urban heat island

describes the typical pattern of temperature in an urban area and its surrounding countryside, the typical heat profile of an urban heat island shows the maximum temperature at the city centre where building densities are highest, a plateau across the suburbs and a temperature cliff between the suburbs and rural areas

urban sunlight

despite higher mean temperatures cities recieve less sunshine and more cloud than rural areas, high rise buildings block out light, dust and other particles may absorb and reflect as much as 50% of insolation in winter when the sun is low in the sky and has to pass through more atmosphere compared to only 5% in summer

microclimate

distinctive climate of a small area e.g coastal areas have land and sea breezes making them mild in winter and cool in summer, mountain climates have valley ascending winds and mountain descending winds and forest climates are less windy, more humid and have a more even seasonal and diurnal temperature range

los angeles "eye sting"

famous because of high levels of pan being trapped, numerous attempts made to improve public transport but its large and sprawling nature, high population and basin topography means it is easier to use a car, recent strategies have included stricter city vehicle vehicle emission standards and the clean air action plan for the port

urban fog

frequency, length and intensity of fog is much greater in urban areas with up to 100% more in winter and 25% more in summer

issues associated with the urban heat island

greater demand for energy such as air conditioning and water by residents and businesses which could lead to restrictions e.g hosepipe bans and increased consumption, increase in allergies due to prolonged growing seasons, increased pressure on health services for the young and elderly, quicker deterioration of buildings due to more chemical weathering, increased survival and reproduction rates of pests, algal blooms, increased smog, higher evapotranspiration rates as plants may extract water from the soil and greater rates than normal, climate change expected to increase intensity

urban relative humidity

higher in urban areas where warmer air can hold more moisture and where lack of vegetation and water surface limits evapotranspiration

sulphur dioxide

in the uk the major contributors are coal and oil burning by industry such as power stations and refineries where short term exposure may cause coughing and narrowing of the airways

disadvantages of urban microclimates

increased air con bills, increased wind turbulence, increased thunderstorms and cloud cover, less sunshine, pollution and poor air quality, high temperatures linked to aggressive behaviour, particulate and photochemical smogs are a health hazard

ultra low emission zone

introduced in london to help improve air quality, operates 24 hours, 7 days a week except for christmas day within central London, most vehicles need to meet the ule7 emission standards or their drivers must pay a daily charge to drive within the zone (£12.50 for up to 3.5 tonnes and £100 for over)

urban atmospheric composition

large quantities of solid and gaseous impurities emitted by industrial processes and car exhausts causing pollution, compared to rural areas there is 3-7% more dust particles, 200x more sulphur dioxide and 10x more nitrogen oxide which cause acid rain, 10x more hydrocarbons, 2x as much carbon dioxide and reduced sunlight, higher temperatures, increased cloud cover, precipitation and smog

advantages of urban microclimates

lower heating bills, reduced wind speeds, less frost and snow, faster melting, reduced atmospheric humidity

hygroscopic

many of the particles produced by urban and industrial areas are water attracting hence water vapour condenses around them and as a result hours of sunshine are reduced in urban areas and fogs and smogs are more common

urban precipitation

mean annual precipitation is 10% greater in urban areas, strong thermals increase the likelihood of thunder by 25%, occurrence of hail by 400% and snow into sleet by 15%

rural hydrograph

more infiltration due to fewer impermeable surfaces therefore longer lag times and lower peak discharge as more interception slows the transfer of water

impacts of increased levels of condensation nuclei

more rainfall, especially in summer and heavier more frequent convective rain storms and thunder but in urban areas there is a relative lack of moisture due to lack of vegetation and high drainage density

urban temperature

normal building materials are non-reflective so absorb daytime heat, dark coloured surfaces have high thermal capacity so store heat during the day and release at night, heat obtained from car fumes, factories, power stations, central heating and people, under calm conditions city centres are the warmest, large cities have less snow, fewer frosts, earlier flowering and a greater need for air conditioning, day temperatures are 0.6 degrees warmer on average and night temperatures up to 4 degrees warmer due to dust and cloud reducing radiation loss, mean winter temperature 1 to 2 degrees warmer and up to 5 degrees warmer in summer

photochemical oxidants

ozone and pan associated with damage to plants and health issues such as headaches and chest pains, particular hazard during anticyclonic conditions as static descending air means an absence of wind and such weather systems can persist for weeks during the summer months

nitrogen dioxide

reacts with hydrocarbons in the presence of sunlight to create ozone and contributes to the formation of particles, road transport estimated to be responsible for 50% of total emissions of nitrogen oxides, it can inflame the lining of the lung and impacts are more pronounced in people with asthma and can cause accelerated weathering of buildings

changes in the nature of heat islands

recent research on london's heat island suggests that high levels of air pollution are actually decreasing the temperature by day, by blocking out the radiation from the sun however by night the same air pollution is trapping heat within the urban area thus the differences between urban and rural temperatures may be decreasing

urban wind

reduced by buildings which create friction and act as windbreaks, high rise buildings form canyons through which wind may be channeled, winds may be strong enough to cause tall buildings to sway and pedestrians to be blown over, in the 19th century south west city homes were the most sought after to be upwind of industrial smoke and pollution, urban mean annual velocities may be up to 30% lower than rural areas

albedo

reflectivity of a surface, the ratio between the amount of incoming insolation and the amount of energy reflected back into the atmosphere, light surfaces reflect more than dark surfaces so have a greater albedo

urban transport solutions

road schemes e.g urban bypasses, mass transit systems e.g metrolink in manchester, greater provision for cyclists e.g snake bridge in copenhagen, bus lanes, free public transport e.g luxembourg, new fuels, park and ride, greater use of waterways for transport e.g park and glide, congestion changes e.g london 2003 and strategies e.g banning cars from driving on certain days and alternating between cars with license plates that end in even or odd numbers

flood and storm hydrographs

show a river's discharge after a period of heavy rain, shows the link between rainfall and the level of the river and helps forecast river levels to predict possible floods, a river with a short lag time and a high discharge is more likely to flood than a river with a lengthy lag time and a low discharge

flat hydrograph

shows a slow response to a storm with only a slight change in river discharge after a storm, this kind of river is not likely to cause flooding problems

clear air act

smoke free zones introduced to urban areas in 1956 and tough regulations were imposed on levels of pm10 air borne pollution

venturi effect

some buildings have gaps in them or are built on stilts to avoid this but a reasonable flow of air at street level is essential to remove pollution

weather

state of the atmosphere at any particular moment and location in time, often looked at over a period of a few days to a week

climate

state of the atmosphere over a period of not less than thirty years, includes variables such as temperature, rainfall, winds, humidity, cloud cover and pressure and refers not just to the averages but also the extremes of these variables

photochemical smog

the action of sunlight on nitrogen oxides and hydrocarbons in vehicle exhaust gases causing a a chemical reaction which results in the production of ozone, peroxyacetyl nitrate (pan) has been deemed particularly hazardous

discharge (in cumecs)

the amount of water passing through the river in cubic metres per second

falling limb

the discharge at this point on a hydrograph is falling

rising limb

the discharge at this point on a hydrograph is rising

base flow

the discharge which is added via the ground and through the soil, this is a much slower reaction than that of surface run-off and shows small changes in reaction to a rainfall event

peak rainfall

the highest amount of rainfall in a flood episode

peak discharge

the very highest point to which the river discharge reaches

buildings

these exert a powerful frictional drag on air moving around them causing changes in wind speed and direction, average wind speeds are lower in cities and their centres

urban cloud cover

thicker and up to 10% more frequent than in rural areas, may result from convection currents generated by higher temperatures and presence of a larger number of condensation nuclei

urban thunder

thunderstorms are more common due to warmer temperatures over urban areas and stronger conventional uplift

lag time

time between peak rainfall and peak discharge, this is the time it takes for precipitation to run-off from where it has fallen to the river channel, many factors can affect this including the permeability of rocks and interception from vegetation

particulate matter

tiny bits of solid or liquid suspended in the air, originate mainly from power stations and vehicle exhausts and includes small bits of metal and rubber from engine wear, dust and ash, sea salt, pollens and soil particles, smaller than 10 micrometers are referred to as pm10 and can settle in the airway and deep in the lungs causing health problems

converging winds

under calm, high pressure conditions country breezes blow from colder rural areas to the warmer urban areas as warm air rises creating low pressure beneath it causing air to blow from the high pressure areas to the low pressure thus winds converge on central areas and may bring pollution into the city

urban hydrograph

unnaturally high rates of surface run-off due to impermeable surfaces, urban areas designed to drain water quickly through sloping roofs and high density drainage systems, less infiltration so lower soil and groundwater levels meaning a higher risk of flooding

strategies for managing the urban heat island

urban greening to create natural cooling, carbon stores, reduce flooding and filter pollutants, green roofs to reduce run-off and create biodiversity with habitats and reduce temperatures, cool surfaces with a high albedo used to store less solar energy during the day, cool coloured cars to reduce the need for air conditioning and more heat reflection (60% compared to 5%)

small scale variations

within the urban heat island these occur with the distribution of industries, open space, rivers and canals etc and significant minor peaks may be located close to areas of manufacturing industry and energy production