Urban temperatures: the urban heat island effect. Precipitation: frequency…

The idea

Urban microclimates differ vastly from rural areas. The Urban Heat Island (UHI) effect makes cities warmer due to low albedo surfaces absorbing insolation and anthropogenic heat. This thermal uplift increases the frequency and intensity of precipitation and thunderstorms via enhanced convection. Abundant condensation nuclei from particulate pollution also increase urban fog. Urban structures alter wind; high-rise buildings create canyon effects, channelling winds to increase speed and alter direction, though overall surface roughness reduces average wind speed. Finally, air quality suffers from particulates (PM10) and photo-chemical smog, which forms when sunlight reacts with vehicle exhaust emissions like nitrogen oxides.

What earns the marks

• Explain the UHI effect using surface albedo, the thermal capacity of building materials, and anthropogenic heat sources.
• Link increased urban temperatures to enhanced convection, leading to a higher frequency and intensity of thunderstorms and precipitation.
• Describe how urban structures create friction that lowers overall wind speed, while urban canyons channel wind, increasing localised speed and altering direction.
• Explain the formation of photo-chemical smog through the reaction of sunlight with nitrogen oxides and volatile organic compounds.
• Connect high levels of particulate matter to their role as hygroscopic condensation nuclei, increasing the incidence of urban fogs.

Common mistakes and how to put them right

• Stating that urban areas always have higher wind speeds.

  Put it right: Specify that while urban canyons increase localised wind speeds, overall urban surface roughness decreases average wind speed.

• Confusing the global greenhouse effect with the UHI effect.

  Put it right: Explain UHI through localised surface albedo and anthropogenic heat, not global atmospheric gas concentrations.

• Describing photo-chemical smog as just a mixture of smoke and fog.

  Put it right: Define it as a secondary pollutant formed by a chemical reaction requiring sunlight, nitrogen oxides, and hydrocarbons.

Exam tips

• Use specific examples of urban structures, such as the Venturi effect in urban canyons, to explain localised wind speed increases.
• When discussing precipitation, explicitly mention both frequency and intensity, linking them directly to thermal uplift and condensation nuclei.
• Clearly distinguish between particulate pollution (physical matter like soot or dust) and photo-chemical pollution (chemical reactions requiring sunlight).