Study Notes: Urban Climate (9635 Geography)

Introduction: The City's Own Weather System

Hello future urban geographers! This chapter explores one of the most fascinating aspects of human-environment interaction: how our cities create their very own local weather and climate. When we build massive urban areas, we fundamentally change the Earth's surface and the atmosphere above it. This leads to distinct microclimates that affect everything from air quality to energy consumption and public health. Understanding these processes is crucial for developing sustainable and livable urban environments. Let's dive into the fascinating world of urban meteorology!

1. Urban Temperatures: The Urban Heat Island (UHI) Effect

The most significant change to local climate caused by urbanisation is the rise in temperature. The difference in temperature between urban areas and surrounding rural areas is called the Urban Heat Island (UHI) Effect. City centres can be several degrees Celsius warmer than the surrounding countryside, especially at night.

What causes the Urban Heat Island?

The UHI effect results from four primary factors related to how cities interact with solar energy and heat. Don't worry if this seems tricky; think of the city as a giant, dark stove that stays hot long after the sun sets.

Key Factors Contributing to UHI (The S.A.T.E. Model)

Here is a simple way to remember the main causes:

  • Surface Materials and Storage
  • Albedo Change
  • Thermal/Anthropogenic Heat Release
  • Evaporation Reduction

1. Surface Materials and Storage (High Thermal Mass):

  • Urban surfaces like concrete, asphalt (tarmac), and brick have a high thermal capacity. This means they absorb and store a large amount of heat during the day.
  • Rural areas (grass, soil) release heat quickly. Cities, however, act like a battery, releasing the stored heat slowly throughout the night, keeping the city warmer.

2. Albedo Change (Less Reflection):

  • Albedo is the measure of how reflective a surface is. Rural surfaces (e.g., fields) often have a high albedo (they reflect solar energy).
  • Dark urban surfaces (black roads, dark roofs) have a low albedo, meaning they absorb most of the incoming solar radiation, converting it directly into heat.

3. Thermal/Anthropogenic Heat Release:

  • This is the heat generated purely by human activities.
  • Examples: Cars burning fuel, air conditioning units dumping hot air onto the streets, industrial processes, and heating systems in buildings. This waste heat contributes directly to the UHI.

4. Evaporation Reduction (Lack of Latent Heat):

  • Rural areas have lots of vegetation and open soil, allowing evapotranspiration (cooling through water evaporation). This process uses energy (latent heat) that would otherwise warm the air.
  • In cities, drainage systems (pipes and gutters) quickly remove rainwater. There is less open water and far fewer plants, leading to a huge reduction in cooling through evaporation.

Did you know? The UHI effect is usually strongest during calm, clear nights when heat loss in the rural areas is rapid, while the city continues to radiate its stored heat back into the atmosphere.

Quick Review: The UHI Effect

Key Takeaway: Urban areas are thermal traps. They absorb solar energy efficiently (low albedo), store it well (high thermal mass), lose less heat through natural cooling (low evaporation), and add their own waste heat (anthropogenic heat).

2. Urban Precipitation, Fogs, and Thunderstorms

The UHI doesn't just make the air warmer; it also influences the moisture and stability of the atmosphere, leading to noticeable changes in rainfall and storm activity.

Precipitation: Frequency and Intensity

Cities often experience greater frequency and intensity of rainfall, particularly downwind of the city centre. This is due to two main factors:

1. Thermal Uplift (Convection):

  • The UHI creates a plume of warmer, lighter air rising over the city. This convectional uplift encourages the air to cool, condense, and form storm clouds.

2. Condensation Nuclei:

  • Pollution from traffic and industry releases countless tiny particles (soot, dust, sulphate aerosols) into the atmosphere.
  • These particles act as condensation nuclei—they give water vapour tiny surfaces to condense onto, making cloud droplet formation easier and leading to quicker and more intense rainfall.

Fogs and Thunderstorms

Fogs:

  • Urban areas have historically suffered from increased fog, mainly due to high concentrations of soot and smoke particles acting as condensation nuclei.
  • When this fog mixes with smoke or pollutants, it forms smog (smoke + fog). While classic London smog (coal smoke) is rare now due to environmental laws, modern urban areas still suffer from photo-chemical smog (see Section 4).

Thunderstorms:

  • Thunderstorms are more frequent over urban areas because the intense thermal uplift from the UHI provides the strong convection needed to develop towering cumulonimbus clouds.
Key Takeaway

The heat and pollution sources in a city combine to increase atmospheric instability, leading to more frequent and sometimes heavier rainfall and storms.

3. Urban Wind Patterns and Structures

Cities are not smooth surfaces. They are a jumble of tall, blocky structures that dramatically interfere with the flow of air.

Effect of Urban Structures on Wind

The impact of urban structures and layout affects wind speed, direction, and frequency.

1. General Wind Speed and Friction
  • Overall, the massive number of tall buildings creates huge surface friction.
  • This drag effect generally reduces the average wind speed over the entire city compared to the flat, open countryside.
2. Specific Local Wind Effects

However, wind speeds can be locally increased due to the way air is forced around obstacles:

The Canyon Effect (Urban Street Canyons):

  • When wind flows down a street lined with tall buildings (a "street canyon"), the air is funnelled, causing its speed to increase significantly, much like placing your thumb over a hosepipe outlet.

Eddies and Turbulence:

  • When wind hits the face of a tall building, it splits. Some air is forced up, and some is forced down to the ground.
  • This creates complex, swirling air movements (eddies or turbulence) at ground level, which can pick up and circulate dust and pollutants.
3. Wind Direction and Frequency
  • Building layouts can deflect the prevailing wind direction, causing local variations in wind flow that are channelled along street orientations.
  • The UHI itself can sometimes create a local wind circulation cell, where cooler air flows from the rural periphery towards the warmer urban centre—a process known as the urban breeze.
Key Takeaway

Cities generally slow down wind due to friction but create localised zones of very fast, turbulent air flow in street canyons.

4. Air Quality: Particulate and Photo-Chemical Pollution

The concentration of pollutants in urban air is perhaps the most serious consequence of urban climate modification, directly impacting human health.

Particulate Pollution (PM)

Particulate Matter (PM) refers to tiny solid or liquid particles suspended in the air.

  • Sources: Vehicle exhaust (especially diesel), industrial emissions, construction dust, and power generation.
  • Size Matters: Particles are measured in micrometers (\(\mu m\)). PM10 (particles less than 10 \(\mu m\)) and PM2.5 (less than 2.5 \(\mu m\)) are the most dangerous because they can penetrate deep into the lungs and even the bloodstream.
  • Concentration: Low wind speeds (due to friction) and sometimes temperature inversions (where a layer of warm air traps cooler, polluted air beneath it) prevent pollutants from dispersing, leading to high concentrations at ground level.

Photo-Chemical Pollution (Smog)

This modern form of pollution is caused by chemical reactions in the atmosphere and is strongly linked to sunlight and traffic.

  • The Process: Vehicle exhaust releases Nitrogen Oxides (NOx) and Volatile Organic Compounds (VOCs). When intense sunlight reacts with these primary pollutants, they form secondary pollutants, most notably Ozone (\(O_3\)) at ground level.
  • Characteristics: This highly toxic mix of ground-level ozone and other irritants is known as photo-chemical smog. It often has a brown, hazy appearance.
  • Real-world Example: Cities like Mexico City and Los Angeles, which are located in basins (encouraging inversions) and experience intense sunlight, are historically famous for this type of smog.
Key Takeaway

Urban areas trap tiny particles and chemicals, leading to respiratory diseases and environmental harm. Sunlight transforms primary pollutants into dangerous secondary pollutants like ground-level ozone (photo-chemical smog).

5. Pollution Reduction Policies

Managing urban climate challenges requires effective governmental and planning policies aimed at reducing the sources of pollution and mitigating the UHI effect.

Strategies for Air Quality Improvement

Policies generally target the sources of particulates and NOx/VOCs:

  • Legislation and Standards: Implementing strict emissions standards for vehicles (e.g., European Euro Standards) and industrial chimneys.
  • Traffic Management: Introducing congestion charges (like in London or Stockholm), creating Low Emission Zones (LEZ) that restrict polluting vehicles, and investing heavily in efficient public transport (buses, trams, rail).
  • Renewable Energy: Shifting away from coal and gas power generation within or near urban areas towards clean energy sources.

Strategies for UHI Mitigation

These policies focus on reversing the factors that cause heat retention and encouraging cooling:

1. Increasing Albedo:

  • Cool Roofs: Promoting the use of highly reflective, light-coloured materials for roofs and roads. These surfaces reflect solar energy instead of absorbing it.

2. Increasing Evaporation (Green Infrastructure):

  • Green Roofs/Walls: Planting vegetation on rooftops and walls. This increases shade and cooling through evapotranspiration.
  • Urban Parks and Green Spaces: Increasing the total area of vegetation provides cooling and acts as a minor sink for air pollutants.

3. Urban Planning and Layout:

  • “Cool Corridors”: Designing city layouts to allow wind to flow through, preventing heat stagnation and helping disperse pollutants (the opposite of the friction effect).

Analogy: Green roofs are like a city putting on a moist towel, using evaporation to cool itself down!

Key Takeaway

Pollution and UHI can be managed through regulatory strategies (controlling emissions) and design strategies (using green infrastructure and reflective materials) to make urban environments more sustainable.