Temperatures: the Urban Heat Island Effect (AQA A-Level Geography): Revision Notes

Temperatures: the Urban Heat Island Effect

What is an urban heat island?

Cities and suburban areas tend to be warmer than the surrounding countryside. This temperature difference is called the urban heat island effect (UHI). The temperature variation can be significant - a city with one million or more inhabitants may experience annual mean temperatures that are 1 to 3°C higher than its rural surroundings. On calm, clear days, this difference can reach as much as 12°C.

The intensity of urban heat islands varies depending on several factors:

• Season and weather conditions
• Sun intensity
• Amount of ground cover
• Size of the urban area

Surface temperatures are typically highest during summer months. Smaller urban areas still produce heat islands, but the effect becomes less pronounced as city size decreases.

Temperature patterns across urban areas

Temperature varies significantly as you move from rural areas through to the city centre. The diagram above shows how both surface and air temperatures change across different zones:

• Rural areas: Baseline temperatures, coolest zones
• Suburban zones: Gradual temperature increase
• Warehouse/industrial areas: Significant temperature peaks
• Urban residential areas: Elevated temperatures
• CBD (Central Business District): Highest temperatures, particularly in the afternoon
• Parks and water bodies: Cooler zones within the urban area

Key patterns:

• Urban temperatures are typically highest in the mid-afternoon over the CBD
• Secondary temperature peaks occur over heavily built-up areas such as industrial zones
• The temperature range from rural fringe to city centre is often greatest at night
• Air temperatures over water bodies show very little variation
• Surface temperatures fluctuate much more dramatically than air temperatures

The urban boundary layer (UBL) forms a dome of warmer air that can extend as a plume downwind of the city. This dome develops above the urban canopy layer (the layer at street level between buildings).

Causes of urban heat islands

Cities are warmer than rural areas due to several interconnected factors:

1. Low albedo of urban surfaces

Albedo refers to how much solar radiation a surface reflects. Urban surfaces tend to have much lower albedo than natural rural surfaces, meaning they absorb more heat.

Common urban materials and their albedo values:

• Asphalt roads: 0.05–0.20 (very low reflectivity)
• Concrete: 0.10–0.35
• Brick/Stone: 0.20–0.40
• Coloured paint: 0.15–0.35
• White paint: 0.50–0.90 (high reflectivity)
• Grass/vegetation: 0.16–0.26 (rural areas)

Materials like concrete, brick, and tarmac absorb large quantities of heat during the day. Much of this heat is stored and slowly released at night, keeping urban areas warm long after sunset. Buildings with large windows have high reflective capacity and can concentrate heating effects by reflecting energy downwards to surrounding streets.

2. Air pollution and the "pollution dome"

Air pollution from industries and vehicles increases cloud cover and creates a "pollution dome" over cities. This dome:

• Allows short-wave solar radiation to enter
• Absorbs and reflects outgoing long-wave radiation back to the surface
• Traps heat within the urban area

3. Reduced vegetation and evapotranspiration

Urban areas are designed to remove surface water quickly through drainage systems. This significantly reduces cooling by evaporation. With less vegetation, there is also reduced evapotranspiration (water loss from plants), which would normally have a cooling effect.

4. Heat from buildings, vehicles, and industry

Heat sources from human activities include:

• Industries, buildings, and vehicles burning fossil fuels
• Air conditioning units releasing hot air into the atmosphere
• Body heat from large populations in confined spaces

Although air conditioning regulates indoor temperatures, the units pump hot air outside, contributing to higher outdoor temperatures.

5. Reduced wind speeds and trapped heat

High-rise buildings and varied building heights create turbulence and reduce overall wind speeds in urban areas (typically 20-30% lower than rural areas). When streets are positioned perpendicular to prevailing winds, this reduces ventilation and prevents heat and pollutants from being dispersed. Heat becomes trapped in "street canyons" between tall buildings.

Urban climate effects beyond temperature

Effects on local climate

Urban areas affect multiple climate elements compared to nearby rural areas:

Temperature changes:

• Annual mean: 0.5–3°C increase
• Winter minimum: 1.0–2°C increase
• Summer maximum: 1.0–3°C increase

Precipitation changes:

• Quantity: 5–10% increase
• Days with less than 5mm rainfall: 10% increase
• Snowfall (inner city): 5–10% decrease

Humidity changes:

• Annual mean: 6% decrease
• Winter: 2% decrease
• Summer: 8% decrease

Visibility changes:

• Fog in winter: 100% increase
• Fog in summer: 30% increase

Wind speed changes:

• Annual mean: 20–30% decrease
• Calm conditions: 5–20% increase
• Extreme gusts: 10–20% decrease

Radiation changes:

• Ultraviolet in winter: 30% lower
• Ultraviolet in summer: 5% lower
• Total on horizontal surface: 15–20% lower

Pollution:

• Dust particles: Up to 1,000% increase

Why is the urban heat island a matter of concern?

Research in London and other major cities has highlighted several serious concerns about urban heat islands:

Health impacts

As temperatures rise during summer months, conditions can become dangerously uncomfortable:

• Heat stroke cases increase
• Asthma rates worsen
• Organ damage becomes more common
• Death rates increase during extreme heat events

Air quality problems

Higher temperatures accelerate chemical reactions that produce ozone and smog. Lower wind speeds mean heat and pollution become trapped within the city, creating dangerous air quality conditions.

Cities like New Delhi and Beijing regularly experience severe winter fogs combined with high pollution levels - a phenomenon sometimes called "airpocalypse" due to its high death toll. The term describes the toxic smog created when fog traps pollutants.

Increased energy demands

Excessive heat increases the strain on energy supplies for cooling and air conditioning systems. This creates additional costs for residents and businesses.

Water supply pressures

In warmer periods, the added heat from urban heat islands leads to:

• Increased water consumption by residents and businesses
• Extra strain on water supply infrastructure
• Potential water-use restrictions
• Higher evapotranspiration rates (plants and trees extract water from soil at greater rates than normal)

Environmental changes

• Earlier flowering times for plants and trees
• Prolonged growing seasons causing discomfort and longer allergy seasons
• Higher reproduction rates of pests and insects
• Greater potential for algal blooms in water courses due to rising temperatures
• Increased risk of deterioration of historical monuments and buildings through temperature-related chemical weathering

Climate change amplification

Climate change is expected to intensify the urban heat island effect in most urban areas, making these problems more severe in the future.

Strategies for managing the urban heat island

Urban planning and design are increasingly focused on strategies to reduce the UHI effect:

Cool surfaces

Cool roofs are constructed from materials with high albedo. They absorb and store less solar energy during the day, meaning they do not become major emitters of heat at night. Similarly, cool roads and pavements use reflective coats and seals to reduce heat absorption.

Green roads

Green roads feature more porous surfaces that allow water to seep through and grass to grow. This approach reduces the amount of heat absorbed by the road surface whilst also improving drainage and biodiversity.

Green roofs

Green roofs consist of a growing medium (soil and plants) planted over a waterproof membrane. Benefits include:

• Reducing rooftop temperatures by 20–40°C on sunny days
• Acting as insulators to reduce heat loss from buildings
• Reducing rainwater run-off
• Increasing urban biodiversity by providing habitat for birds and small animals

Urban greening

Planting trees and vegetation in urban areas provides multiple benefits:

• Shade reduces surface peak temperatures (reductions of 5–20°C are possible)
• Natural cooling effect through evapotranspiration
• Trees lower overall air temperatures in urban parks
• Carbon storage reduces atmospheric CO₂
• Reduces urban flooding by intercepting rainfall
• Filters pollutants from the air

Many cities worldwide are expanding their urban parks and tree-planting programmes.

Sky view factor

Sky view factor describes the relative openness between buildings in an urban area. A restricted sky view (narrow streets with tall buildings) reduces heat escape from street and building surfaces. This contributes to heat accumulation within "street canyons" and increases air temperatures.

When streets are angled perpendicular to prevailing winds, ventilation is further reduced, trapping heat and pollutants between buildings during intense UHI events. Urban planning can address this by:

• Varying building heights
• Creating wider streets
• Ensuring street orientation allows for air flow

Cool cars

The interior of light-coloured cars does not heat up as much as darker vehicles due to higher reflectivity. This reduces the need for air conditioning and decreases the amount of heat emitted into surrounding air.

Other urban climate effects

Precipitation

Rainfall can be higher over urban areas than rural areas. This occurs partly because higher urban temperatures encourage the development of lower pressure over cities relative to the surrounding area. Several factors contribute to increased precipitation:

Convection processes:

• The urban heat island generates convection
• Rapid evapotranspiration from heated ground surfaces produces cumulus cloud formation
• Convectional weather patterns develop

Building effects:

• High-rise buildings and mixed building heights induce air turbulence
• This promotes increased vertical air motion

Air convergence:

• Low pressure caused by rising air draws surface winds from surrounding rural areas
• This air converges and is forced to rise over the higher urban canopy
• Similar to orographic (mountain) processes, air moving over cities may split and converge downwind, forming clouds

Pollution effects:

• City pollution increases cloud formation and rainfall
• Pollutants act as hygroscopic (water-attracting) nuclei, assisting raindrop formation
• Pollution may enhance lightning as cloud droplets take on different electrical charges
• Cities produce large amounts of water vapour from industrial sources and power stations

Convection rainfall tends to be heavier and more frequent in urban areas, as does the incidence of thunder and lightning.

Fog

The occurrence of fog increased dramatically with industrialisation. Records from London show approximately 20 days of fog per year in the early 1700s, rising to over 50 days by the end of the 1800s.

By the 1950s, particles in city air in developed countries were much more numerous than in rural areas. These particles acted as condensation nuclei and encouraged fog formation at night, usually under high-pressure weather conditions.

Summary and key takeaways