Urban Climates (AQA A Level Geography)

• The air in urban areas can be 2 - 5°C warmer than nearby rural areas 
• This is known as the urban heat island (UHI) effect 
• Most noticeable when there is little wind
• The highest temperatures are found in densely built-up areas and industrial areas, where activities generate more heat
• Temperature sinks are found above green spaces and water - e.g. parks and lakes
• Temperature plateaus occur in areas with the same land use - e.g. industrial areas
• Temperature cliffs occur when temperatures changes rapidly from one land use to another - e.g. suburban housing to high rise inner city buildings

Image showing characteristics of urban heat island effect

London's urban heat island - note how the isotherms curve around open areas such as Epping Forest, indicating large areas of reduced temperature. Central London shows a greater concentration of heat due to high density land use. 

Causes

• Main causes are:
  • Air pollution - pollution from cars, industry etc. increases cloud cover and produces a pollution dome. Both of these trap outgoing heat and reflect it back to the surface
  • Heat from human activities - air conditioning units, heating homes and offices etc. all release heat into the surround area
  • Absorption of heat by urban surfaces - urban surfaces have a low albedo. Tall buildings reflect and absorb sunlight
  • Less evapotranspiration - removal of green spaces and trees, reduces the cooling effect of evapotranspiration

Diurnal and seasonal temperatures

• There is a larger range between daytime and night time temperatures (diurnal range), compared to rural differences
• In urban areas daytime temperatures are approximately 0.6°C warmer and night time can be up to 4°C warmer 
• Rural areas do not store as much energy and release the heat quicker than urban areas
• Average urban summertime temperatures can be as much as 5°C warmer in mid-latitude cities with average winter temperatures of 2°C warmer 
• Temperatures can increase during periods of anticyclonic weather (high pressures), as these produce clear skies and low winds, therefore, allowing greater insolation to reach urban surfaces. Low winds prevent warm air from being dispersed

Cloud coverage

• There is greater cloud coverage in urban areas - approximately 5-10% more
• Due to a greater concentration of condensation on buildings - about 100 times more than rural areas

Humidity

• Relative humidity is affected by the temperature of the air and is reduced by as much as 6% as the air is warmer in urban areas, meaning it can hold more moisture, however, the lack of vegetation and surfaces limit evapotranspiration
• Humidity is seasonally affected
  
  • Summer - can be up to 10% lower as the air is much warmer, which is increased by the land surfaces within the urban area that absorb heat due to the dark colour of the buildings and road surfaces
  • In winter is approximately 2% lower
• Cities are designed to remove surface water, so humidity during the day is lower
• At night urban humidity is higher than rural surroundings

Precipitation

• Precipitation is more frequent - up to 15% more
• Air has to rise, cool and condense
• Urban pollution in the atmosphere increases aerosol particles that become cloud condensing nuclei
• This therefore, increases 'seeding' of cloud droplets, which in turn, increases precipitation rates
• Snow melts quite quickly due to the retained heat on darker surfaces and higher urban temperatures

• Fog is defined when visibility is less than 1000m
• Fog is effectively ground level cloud
• Urban areas generate large amounts of dust and pollution
• These particulates act as condensation nuclei and trigger cloud formation
• The higher the number of condensation nuclei in the urban air, the more fog that will form
• Advection fog is formed when warm moist air moves horizontally over a cold surface
  
  • San Francisco has a lot of advection fog as the surface water near the coast is much colder than the water further off shore
  • Warm moist air from the Pacific Ocean is advected across the cold coastal waters, chilling the advected air from below
• Radiation (ground) fog is formed by the radiative cooling at the surface
  
  • It takes place from the top of the fog layer
  • Mixing continues within the fog layer
  • Creating further cooling at the surface
  • Conditions necessary include:
    • Long, clear, cloud-free, cold night
    • Shallow layer of moist air near the ground - lake in a park
    • Light winds
• Steam fog forms when cold air moves over warm water (e.g. heated outdoor swimming pool)
  • Water evaporates from the pool into the air, increasing the dew point and if mixing is sufficient the air above becomes saturated
  • The colder air directly above the water is heated from below and rises forming what appears to be steam
  • Steam fog also forms above a wet surface on a sunny day - over a road after a summer rain shower
  • Can also be seen over unfrozen lakes in parks in autumn or winter

Thunderstorms

• Urban thunderstorm are increased by 25% and develop in hot, humid air and produce violent rain, lightning and thunder
• Urban convection is strong in the late summer afternoons, when heat energy has had the chance to build up in the atmosphere
• This creates an updraft of humid, hot air which rises, cools and condenses rapidly
• However, there are continued updrafts of warm, humid air that is carried upwards repeatedly
• Resulting in the formation of towering cumulonimbus clouds
• Air is moved by updrafts and downdrafts, so that water droplets increase in size (melt, freeze, melt, refreeze etc.)
• When large enough to overcome gravity, the fused ice crystal will fall
• As raindrops split in the updrafts, there is an electrical discharge into the air - this is lightening
• Thunder is a result of the sudden increase in pressure and temperature from the lightening causing a rapid expansion of are around it

• Rural wind speeds are higher than urban ones because the ground surface is smoother
• The rougher urban surfaces, tall buildings etc. act as barriers and reduce wind speeds
• Urban structures also redirect and redistribute airflow
• Urban structures extend their effect on wind by ten times the height of the structure downwind of the city
• However, there are local variations:
  • Wind flowing across a deep, narrow street, will create very little wind disturbance at ground level
  • Chicago is nicknamed the ‘windy city’ due to its grid system of buildings, creating wind tunnels where the wind can pick up speed
  • Some areas have zero wind speed as they are totally sheltered from wind by buildings
  • Powerful gusts of wind occur when wind is channelled down streets - known as the canyon effect
• Turbulence occurs around buildings
  • Wind is deflected downwards as it hits the face of a building
  • Wind can be deflected around the sides of the building
  • Or it can flow over the top of the building
  • This flow of air around buildings can create bodies of swirling air called vortices

• Particulates in urban areas are greater than in rural regions
• Sources include:
  • Vehicle exhausts
  • Burning - wood, coal, cigarettes, rubbish etc. release fine and coarse particulates
  • Construction, mining and quarrying
  • Plants and moulds generate coarse particulates such as pollen and mould spores
• Poor urban air quality impacts:
  • Respiratory problems such as asthma
  • Increases haze through increased emissions of sulphur dioxides and nitrous oxides
  • Carbon dioxide is increased adding to the enhanced greenhouse effect and global warming
  • Increased particulates in the atmosphere attack building facades
  • Photochemical oxidants cause eye irritations and headaches

Smog

• Smog happens when smoke particulates, sulphur oxides, hydrocarbons etc. mix with fog
  • The London smogs of 1940s were caused by the sinking of cold air trapping air pollutants in a pollution dome
  • Today smog is more likely due to photochemical reasons:
    • Sunlight reacts with the chemical pollutants in the atmosphere
    • UV light causes them to break down into secondary, harmful chemicals to form photochemical fog
  • Photochemical fog is a major problem in large cities - Los Angeles, Mexico City, Beijing etc.
  • More common in warm, sunnier cities as these places tend to suffer from temperature inversion fog (a layer of warm air is trapped below dense cooler air)
  • This keeps the pollutants at surface level