9.5 - Urban Climate
The urban heat island effect and temperature patterns in cities
Urban areas often experience higher temperatures compared to surrounding rural regions, a phenomenon known as the urban heat island (UHI) effect. This results in distinct temperature variations across different parts of a city, influenced by land use and building density.
Key features of temperature distribution in urban areas
- UHIs - These are urban zones with significantly higher air temperatures than nearby rural areas, often most pronounced in industrial zones and densely built-up areas like the central business district (CBD).
- Temperature plateau - Regions within a city that share similar land use, such as residential zones, tend to have consistent temperatures, forming a 'plateau' of uniform heat levels.
- Temperature cliff - A sharp increase in temperature occurs when land use changes abruptly, for example, transitioning from suburban housing to high-rise commercial structures.
- Temperature peak - The hottest part of the urban heat island, typically found in the most built-up or industrial areas.
- Temperature sink - Cooler spots within the city, often associated with green spaces like parks or bodies of water, which counteract the surrounding heat.
Causes of the urban heat island effect
The urban heat island effect arises from several factors related to the built environment and human activity. These elements work together to elevate temperatures in urban settings compared to rural surroundings.
Main contributors to urban heat islands
- Heat absorption by urban surfaces - Materials like concrete, brick, and tarmac absorb solar heat during the day and store it, releasing it slowly as long-wave radiation, especially at night. These surfaces have a low albedo, meaning they absorb more energy than they reflect.
- Air pollution impact - Emissions from vehicles and factories create a 'pollution dome' over cities, increasing cloud cover. This layer traps outgoing heat and reflects it back to the ground, amplifying warming.
- Heat from human activities - Sources such as vehicles, industrial processes, offices, heating systems, air conditioning units, and even human bodies contribute additional warmth to the urban environment.
- Reduced evapotranspiration - Urban drainage systems quickly remove water, leaving little for evaporation. With less vegetation, transpiration is limited, and since evapotranspiration uses heat energy to convert water to vapour, its reduction leads to higher temperatures.
Temporal variations in the urban heat island effect
The intensity of the urban heat island effect varies over different time scales, influenced by daily cycles, seasonal changes, and specific weather conditions. These variations impact how much warmer cities are compared to rural areas at different times.
Patterns of UHI intensity over time
- Diurnal differences - The UHI effect is more significant at night. Daytime urban temperatures are typically around 0.6°C warmer than rural areas, while at night, the difference can be as much as 3-4°C. This happens because urban surfaces release stored heat after sunset, preventing significant cooling.
- Seasonal differences - In mid-latitude cities, the UHI effect is stronger in summer. Winter temperature differences may be around 2°C, while summer differences can reach up to 5°C due to increased solar radiation.
- Weather-related influences - The effect is most pronounced during anticyclonic conditions with clear skies and calm winds. Clear skies allow more solar energy to heat the ground, and low winds prevent the dispersion of warm air.
Wind patterns and effects in urban environments
Wind behaviour in cities differs from rural areas due to the presence of tall structures and complex urban layouts. These features alter wind speed and direction, creating unique microclimates.
Characteristics of urban wind patterns
- Reduced average wind speed - Tall buildings create friction, slowing down winds in urban areas compared to open rural landscapes.
- Sheltered zones - Some city areas experience almost no wind due to buildings blocking airflow.
- Turbulence around structures - Wind is deflected in multiple directions—downwards, around sides, and over tops of buildings—causing turbulent air movement.
- Canyon effect - When wind is funnelled through narrow streets between tall buildings, it can accelerate, resulting in powerful gusts.
Precipitation and atmospheric conditions in urban areas
Urban climates also affect rainfall patterns and other atmospheric conditions, often leading to more intense and frequent weather events compared to rural regions. These changes are driven by the unique characteristics of urban environments.
Urban influences on precipitation and atmosphere
- Increased rainfall frequency and intensity - Urban areas often see more frequent and heavier rainfall, including more thunderstorms, due to the UHI effect. Warm air holds more moisture, rises through convectional uplift, cools, and forms rain.
- Higher fog occurrence - Urban dust and pollution particles act as condensation nuclei, increasing the likelihood of fog formation.
- Reduced snowfall and frost - Cities experience less frequent snowfall, quicker snow melt, and fewer frost days because of higher temperatures.
- Role of pollution particles - The high concentration of particles in urban air enhances condensation processes, contributing to both fog and rainfall.