Urban climate

Fog history

-1700s 20 days of fog a year -1800s 50 days -1950s clean air act = reduction in smoke production and particulate emission and decrease in fog

Characteristics of urban climates

-5-10% more cloud -lower relative humidity -canyon effect -more chance of thunderstorms/hail -2-3 weeks fewer frosts -surface roughness: reduces wind speed, generates turbulence -5-10% more precipitation

Concerns with urban heat island

-Excessive heat - heat stroke, asthma, organ damage, death -Temperature rises in summer conditions become uncomfortable in buildings and on public transport = strain on air conditioning supply -Hot and still anticyclonic weathering conditions produce higher air pollution levels, chemical reactions that produce ozone and smog accelerated by high temps, lower wind speeds keep heat and pollution tapped in city. -Longer hay fever season, earlier flowering times of plants

Higher precipitation levels

-greater density of condensation nuclei -convection from urban heating

Fog

-increases in cities due to industrialisation -1950s average number of particles in city air was greater than rural areas -particles acted as condensation nuclei and encouraged fog formation at night, usually under high pressure weather conditions -effect transport networks, hazard to health when they trap pollutants

Three main effects of wind

1. Surface area in cities is uneven because of the varying height of the buildings, exert a powerful frictional drag on air moving over and around them. Average wind speeds are lower in cities than in surrounding areas, lower in CBD to suburbs 2. High-rise buildings may slow air movement, but they also channel air into canyons between them 3. On calm clear nights urban heat island effect is at its greatest convectional processes can draw in strong localised winds from cooler surrounding areas

Why cities are warmer than urban areas

1. Surfaces in the city are much less reflective so they absorb more heat which is slowly released at night 2. Air pollution increases clouds creating a 'pollution dome' reflecting a lot of radiation back to the surface 3. Water falling is quickly disposed of so there is reduced evapotranspiration so more energy is available to heat the atmosphere 4. Heat comes from industries and vehicles, higher populations create more heat

Reasons for precipitation levels

1. Urban heat island generates convection. Rapid evapotranspiration takes place and can result in convectional weather patterns 2. Presence of high rise buildings induces air turbulence and increases vertical motion 3. Low pressure caused by rising air converges as it is forced to rise over the higher urban canopy to converge and rise upwards forming rain clouds 4. Pollutants act as hygroscoptic (water attracting) nuclei and assist in raindrop formation 5. Water vapour from industrial sources and power stations

Climate dome

2 levels to urban dome Urban canopy - where processes act in the space between buildings (roof level / below Urban boundary layer above

Photochemical pollution

Air pollution that occurs mainly in cities and can be dangerous to health. Exhaust fumes become trapped by temperature inversions and the presence of sunlight - low level ozone forms High pressure weather systems

Inner city

An area of old housing and light manufacturing industry

Temperature inversion

An atmospheric condition in which temperature, unusually, increases with height. Stable conditions so do not allow convection, trapping the pollution in the lower layer of the atmosphere

Resdiential

Areas consist of housing from a range of periods, which has traditionally increased in both size and price as it moves to the outskirts

Cool surfaces

Built from materials with high solar reflects de or albedo

Particulate air pollution

Caused by the release of particles and noxious gases into the atmosphere

CBD

Central business district, central area contains the major shops, offices and entertainment facilities

Green roofs

Consists of a growing medium planted over a waterproof membrane

Managing urban heat island effect

Cool surfaces Green roofs Urban greening Sky view factor Cool cars

Business park

Found on the edge of urban areas where there is access to a major road

Thunderstorms

Increased chance in urban areas, late afternoon and early evening in summer months Produced by conventional uplift under conditions of extreme instability Cumulonimbus clouds develop up to the height of the tropopause, where the inversion produces stability The up draught of air through the central area of towering cloud causes rapid cooling and condensation =formation of water droplets, hail, ice which coalesce during collisions in the air During condensation, latent heat is released that further fuels the conventional uplift. As raindrops are split in the up draught, positive electrical charge builds up in the cloud

Increased heat from the urban island leads to

Increased water consumption = strain on supplies, restrictions Evaporisation rates higher Climate change = increase intensity Increased rates of temperature - chemical weathering Prolonged survival and higher reproduction for some animals/insect pests = greater potential for agal blooms in water as temperature increases

Industry

Large areas of land and tend to locate at the edge of cities where land is cheaper

Cool cars

Lighter coloured car shell

Urban heat island management example

London 'walkie talkie' skyscraper was melting cars so a sun shade (block) was fitted to the side to stop the building reflecting heat to cars

Precipitation

higher urban temperatures encourage the development of lower pressure over cities in relation to the surrounding area

Urban canyons

relatively narrow streets bordered by high-rise buildings funnelling and so concentrating winds

Thunderstorms formation

the rising heat, can trigger more frequent late afternoon/evening storms. The more intense the heating the more violent the storm. Updraughts of hot, humid air can rise higher in the atmosphere, quicker. Air cools and condenses rapidly dforming water droplets, hail, ice which can be discharged as lightning.

Hygroscoptic

water attracting

Out of town retail

Originally developed by supermarket, expanded to non-food retail units and entertainment complexes

Green space

Parks dotted through an urban area

Urban greening

Planting trees and vegetation to provide shade

Albedo

Reflectivity of a surface. Ratio between the amount of incoming isolation and the amount of energy reflected back into the atmosphere. Light surfaces reflect more than dark surfaces and have a greater albedo

Factors causing temperature fluctuations in urban areas

Season Weather conditions Sun intensity Ground cover

Landsat satellite

Shows the land surface temperature using colour shades to show the hottest / coolest

Informal settlements

Slums or shanty towns in LICs

Venturi effect

Squeezing of wind into an increasingly narrow gap resulting in a pressure decrease and velocity increase

Urban areas effect local climates

Temperature Precipitation Relative humidity Visability Wind speed Radiation Pollution

Sky view factor

The relativeness openness between buildings in an urban area

Urban heat island effect

Urban and suburban areas experience higher temperatures than the surrounding rural areas and this difference is the urban heat island effect (UHI)

Microclimate

Urban areas create their own climate Small-scale variations in temperature, precipitation, humidity, wind speed and evaporation that occur in a particular environment such as an urban area

Wind

Urban structures effect wind speed, direction and frequency Buildings can exert a powerful frictional drag on the air moving around them and this causes changes in speed an direction

How does the time of day affect temperatures

Urban temps are highest mid-afternoon Difference is greatest at night due to heat stored in building materials compared to vegetation

Urban heat island

Zone around an urban area, which has higher temps than the surrounding rural ones

Pollution levels higher

-smoke -diesel particulates -CO2, SO2, nitrous ocides and hydrocarbons