Urban Climates (AQA A Level Geography): Revision Note

Urban Heat Island (UHI)Effect

• The air in urban areas can be 2 - 5°C warmer than in nearby rural areas 

  • This is known as the urban heat island (UHI) effect 

• The UHI is most noticeable when there is little wind

• There is a variation of temperature within the UHI

  • The highest temperatures are found in densely built-up areas and industrial areas

    • In these areas, the 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 change rapidly from one land use to another - e.g. suburban housing to high-rise inner city buildings

Causes

• The main causes of the UHI effect 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 surrounding area

  • Absorption of heat by urban surfaces

    • Urban surfaces have a low albedo

    • Tall buildings reflect and absorb sunlight

  • Less evapotranspiration

    • The removal of green spaces and trees reduces the cooling effect of evapotranspiration

Diurnal and seasonal temperatures

Diurnal temperature fluctuations

• In urban areas, there is a larger range between daytime and nighttime temperatures (diurnal range), compared to rural differences

  • In urban areas

    • Daytime temperatures are approximately 0.6°C warmer

    • Nighttime temperatures can be up to 4°C warmer 

• Rural areas do not store as much energy and release the heat more quickly than urban areas

Seasonal temperature fluctuations

• Average urban summertime temperatures can be as much as 5°C warmer, with average winter temperatures of 2°C warmer 

• Temperatures can increase during periods of anticyclonic weather (high pressure)

  • These produce clear skies and low winds

  • This allows greater insolation to reach urban surfaces

  • Low winds prevent warm air from being dispersed

Precipitation Rates

Cloud coverage

• There is greater cloud coverage in urban areas

  • Approximately 5-10% more than in rural areas

• This is due to:

  • More convection, which is caused by the higher temperatures in the urban areas

    • This leads to increased condensation nuclei, which form clouds

  • Due to the levels of pollution, there are also higher levels of hygroscopic nuclei

Humidity

• Relative humidity is affected by the temperature of the air

• It is reduced by as much as 6% in urban areas because the air is warmer

  • This means it can hold more moisture

• Humidity in urban areas varies between the seasons

  • In summer, it can be up to 10% lower as the air is much warmer

    • This effect 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, it is approximately 2% lower

• Cities are designed to remove surface water via drains, so humidity during the day is lower

• At night, urban humidity is higher than in the rural surroundings

Precipitation

• Precipitation is more frequent in urban areas

  • This can be up to 15% more

• The warm air has to rise, cool and condense

• Urban pollution in the atmosphere increases hygroscopic particles that become cloud condensing nuclei

  • This increases 'seeding' of cloud droplets, which in turn increases precipitation rates

• If snow falls, it melts quite quickly due to the retained heat on darker surfaces and higher urban temperatures

Fog and thunderstorms

• Fog is when visibility is less than 1000m

  • Fog is effectively ground-level cloud

• Urban areas generate large amounts of dust and pollution

  • Some of the particulates are hygroscopic nuclei (a form of condensation nuclei)

  • All the particulates act as condensation nuclei and trigger cloud formation

• The higher the number of condensation nuclei in the urban air, the more fog forms

Advection fog

• 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 offshore

  • Warm, moist air from the Pacific Ocean is advected across the cold coastal waters, chilling the advected air from below

Radiation (ground) fog

• Radiation (ground) fog is formed when the ground rapidly loses heat at night through long-wave radiation

  • The cooled ground absorbs heat from the air

  • The air temperature falls, and condensation occurs, forming fog

  • 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

• Steam fog forms when cold air moves over warm water (e.g., a heated outdoor swimming pool)

  • Water evaporates from the pool into the air

  • This increases 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 areas have a higher frequency of thunderstorms

  • The rate is about 25% higher than in rural areas

• Urban convection is strong in the late summer afternoons

  • This creates updrafts of humid, hot air which rise, cool and condense rapidly

  • This results in the formation of towering cumulonimbus clouds

• Water droplets in the upper atmosphere are moved by updrafts and downdrafts

  • This means that water droplets increase in size (melt, freeze, melt, refreeze, etc.)

• When the droplets are 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 lightning

• Thunder is a result of the sudden increase in pressure and temperature from the lightning, causing a rapid expansion of air around it

Urban form and wind

• Rural wind speeds are higher than urban ones

  • This is because the ground surface is smoother and there are fewer barriers

• 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

Local variations in wind

• The form of urban areas causes local variations in the wind

  • 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

    • This creates wind tunnels where the air is funnelled between buildings, and 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

Urban Air Quality

• Particulates in urban areas are greater than in rural regions

• Sources of particulates include:

  • Vehicle exhausts

  • Burning - wood, coal, cigarettes, rubbish, etc., which releases fine and coarse particulates

  • Construction, mining and quarrying

  • Plants and moulds generate coarse particulates such as pollen and mould spores

Impacts of poor air quality

• There are a range of impacts of poor air quality, including:

  • Respiratory problems such as asthma

  • Increased cardiovascular health issues

  • Increases haze through increased emissions of sulphur dioxide and nitrous oxide

  • 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 irritation and headaches

Smog

• Smog happens when smoke particulates, sulphur oxides, hydrocarbons, etc., mix with fog

  • The London smogs of the 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.

  • It is more common in warm, sunnier cities

    • 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 the surface level