Urban Heat Island & Urban Climate

Urban Heat Island (UHI)

• Definition: A persistent area of higher temperature located over a city and surrounded by cooler rural conditions.
• Represents a “dome” of warmth produced by human-modified surfaces, anthropogenic heat and atmospheric changes.
• Typical central business district (CBD) temperature excess: up to 5^{\circ}C above the neighbouring countryside.

• Core physical origin
• Replacement of natural, moist, light-coloured, irregular surfaces (soil, grass, trees) with dark, dry, flat man-made materials (asphalt, concrete, glass, metal). These surfaces
• have lower albedos ⇒ absorb more solar short-wave radiation.
• store sensible heat which is reradiated at night as long-wave radiation.
• Anthropogenic heat release from traffic, industrial furnaces, HVAC systems, lighting, etc.
• Reduced vegetation ⇒ less shading, less evapotranspiration (latent heat uptake) ⇒ weaker evaporative cooling.
• Greater surface roughness (skyscrapers, streets) enhances absorption (larger 3-D surface area) and traps radiation multiple times between walls (“urban canyons”).
• High dust & pollution concentrations trap outgoing terrestrial radiation (greenhouse-like blanket) at night.

Daily & Seasonal Dynamics

• Idealised diurnal profile (figure in transcript)
• Sunrise → mid-morning: UHI weak; surface warms.
• Afternoon: depth & intensity at daily maximum; vigorous convection, pollutants dispersed into a tall, less-concentrated column.
• Sunset → midnight: rapid growth of temperature difference as rural areas radiate strongly; urban surfaces re-emit stored heat; inversion begins to build.
• Near dawn: strongest “dome” plus most intense pollution shading.

• Seasonal behaviour (Southern Hemisphere example; Johannesburg)
• Winter (June–July): strong surface inversions, subsidence under anti-cyclonic (Kalahari anticyclone) conditions, stagnant air.
• Night-time excess up to 5^{\circ}C.
• Day-time average increase much smaller (≈ 0.1^{\circ}C) because turbulence reduces stratification.
• Summer: higher absolute temperatures but smaller relative difference because intense convection ventilates the canopy layer.

Vertical Structure: Heat & Pollution Domes

• Heat island often described as a "vertical dome" of warmer air.
• Generated by nocturnal long-wave radiative loss from warm urban fabrics.

• Pollution dome
• At night an inversion traps aerosols and gases beneath a lid; pollutants accumulate, forming a visibly darker/ orange-tinted layer.
• Definition: "mass of polluted air in and above the city, prevented from rising by an inversion".
• Day-time heating “pushes” the dome upward; stronger winds partially disperse it.

• Smog formation
• Smog = smoke + fog; pollution particles act as hygroscopic nuclei attracting water vapour ⇒ orange, sulphurous haze.
• Health impacts: respiratory illness, reduced visibility, traffic accidents.

Mechanisms & Causes (full list)

• Artificial surface materials: lower albedo, higher heat capacity.
• Increased 3-D surface area (tall buildings) intercept more sunlight.
• Anthropogenic heat sources: furnaces, engines, air-conditioners, industrial machinery.
• Suppressed evaporation: paved surfaces are impervious; less soil-water; therefore lower latent heat flux.
• Dust & pollutants: radiatively active at night, absorb/emits long-wave ⇒ additional warming.
• Reduced vegetation:
• Less transpiration (latent heat absorption).
• Lost photosynthetic uptake of solar energy.
• Fewer leaves/ twigs ⇒ smaller absorbing area spreads insolation over a smaller effective surface.
• Lower relative humidity (warmer air has higher moisture capacity but less supply) ⇒ saturation less likely.

Urban vs Rural Climate Comparison

• Temperature
• Urban: warmer (UHI) due to reasons above.
• Rural: cooler, enhanced by radiative cooling, evapotranspiration.

• Pollution
• Urban: high (industrial, vehicular, energy use).
• Rural: relatively clean.

• Cloud cover
• Urban: more clouds; extra condensation nuclei.
• Rural: fewer clouds.

• Precipitation
• Urban: more rain & hail, snow less likely (melts); stronger up-draughts.
• Rural: smaller totals though may see more snow.

• Relative humidity
• Urban: lower (dry surfaces, warmer air).
• Rural: higher (soil moisture, vegetation).

• Wind
• Urban: mean speed lower (buildings are windbreaks) but turbulence higher (channeling, vortices).
• Rural: higher speeds, smoother flow.

• Fog & Visibility
• Urban: frequent fog/ smog in winter inversions.
• Rural: clearer visibility.

Weather Impacts — Precipitation, Thunderstorms, Fog

• Enhanced evaporation over hot surfaces ⇒ larger moisture flux into boundary layer.
• Greater number of hygroscopic/ condensation nuclei ⇒ easier droplet formation.
• Result: increased rainfall frequency and intensity downwind of major cities; hail more common, thunderstorms stronger (heavier convective up-drafts).
• Thunderstorm triggers
• High surface temperatures.
• Strong upward convection currents.
• Abundant nuclei.

Regional Context: South African Highveld & Kalahari Anticyclone

• Kalahari anticyclone
• A semi-permanent high-pressure cell dominating winter synoptics.
• In winter the anticyclone descends closer to the plateau ⇒ subsidence inversion enhanced.
• Urban areas on the Highveld (e.g.
Johannesburg) therefore experience pronounced winter pollution domes and UHIs.

Pollution Sources & Composition

• Urban energy use (heating, cooling, transport) consumes vast fossil fuel quantities.
• Major emission categories
• Vehicle exhausts ⇒ \text{CO}, \text{NO}x. • Stationary combustion (coal, oil) ⇒ \text{CO}2, \text{SO}_2.
• Industrial processes ⇒ particulates, nitrous oxides.
• Temperature inversions maintain high near-surface concentrations.
• Visual: Central Johannesburg vs suburbs — smoke trapped under warm inversion layer above the city core.

Mitigation & Urban Planning Strategies

• Albedo modification (“cool roofs” & pavements)
• Paint/coat roofs light colours, install white membranes; dark roofs can be up to 21^{\circ}C hotter than light surfaces.
• Pavements: use light-coloured concrete, embed white chips.

• Urban greening
• Plant street trees, parklands, large-canopy species on sunniest sides of buildings.
• Benefits: shading, evapotranspirative cooling, CO$_2$ uptake, reduced air-conditioning demand.
• Roof gardens lower internal building temperatures.

• Smoke restrictions & fuel-switching (e.g.
electrification of townships to replace coal fires).
• Energy efficiency to curb anthropogenic heat.
• Urban form: layout to encourage ventilation corridors.

Global Warming Linkage

• Global warming: rise in mean earth surface temperature.
• Greenhouse gas inventory (with approximate contribution to anthropogenic warming)
• \text{CO}2 – combustion, decay, fires – 50\%. • Methane (CH$4$) – livestock, rice – 18\%.
• CFCs – aerosols, refrigeration – 14\%.
• Ozone (tropospheric) – photochemical smog – 12\%.
• Nitrous oxide (N$_2$O) – fertilisers, bacteria – 6\%.
• Urban areas both contribute to and suffer from climate change; UHIs compound summer heat-stress and raise cooling energy demand, creating feedback loops.

Ethical, Practical & Health Implications

• Equity: Low-income residents often live in heat-stressed neighbourhoods with few trees, limited access to cooling, and higher pollution.
• Public health: Increased cardiovascular & respiratory illness, heat stroke, and accident risk in smog events.
• Urban planning must integrate climate resilience: green infrastructure, emissions reduction, improved building codes.