Welcome to Urban Climates!
Ever noticed how it feels a few degrees warmer in the city centre than in the countryside? Or how it seems to get suddenly windy when you walk between tall skyscrapers? That’s not your imagination—cities actually create their own microclimates. In this chapter, we are going to explore how the "concrete jungle" changes the weather and the air we breathe. Don't worry if it seems like a lot of science at first; we'll break it down step-by-step!
1. Urban Temperatures: The Urban Heat Island (UHI) Effect
The Urban Heat Island (UHI) effect is the name given to the phenomenon where urban areas are significantly warmer than the surrounding rural areas. This temperature difference is usually most noticeable at night when the city stays warm while the countryside cools down quickly.
Why are cities hotter?
Think of a city like a giant storage heater. During the day, it soaks up heat, and at night, it slowly lets it out. Here is why this happens:
1. Low Albedo: Albedo is a measure of how much light a surface reflects. Dark surfaces like tarmac and bricks have low albedo, meaning they absorb more solar radiation (heat) rather than reflecting it.
2. Thermal Mass: Building materials like concrete and stone act as "heat sinks." They have a high capacity to store heat during the day.
3. Lack of Evapotranspiration: In the countryside, plants "sweat" (transpiration) and water evaporates from the soil, which uses up energy and cools the air. In cities, we replace grass with concrete and hide water in underground pipes, so there is no natural cooling.
4. Anthropogenic Heat: This is "human-made" heat. Think about the heat coming out of car exhausts, air conditioning units, and industrial chimneys.
5. The Urban Canopy: Tall buildings create a "canyon" effect that traps heat and prevents it from escaping into the atmosphere.
Analogy: Imagine wearing a black woolly jumper (the city) vs. a white t-shirt (the countryside) on a sunny day. The black jumper soaks up all the sun and keeps you hot long after the sun goes down!
Memory Aid: The 5 'A's of UHI
Albedo (Low)
Anthropogenic Heat (Cars/AC)
Absorption (By concrete)
Absence of plants
Airflow (Blocked by buildings)
Quick Review Box: The UHI is strongest at night, in the city centre, and during anticyclonic (calm, clear) weather conditions.
2. Urban Precipitation, Fogs, and Thunderstorms
Believe it or not, it actually rains more in cities than in the rural areas nearby! Cities can see 5–15% more precipitation on average.
Why does it rain more in the city?
1. Convection: Because the city is a "Heat Island," the warm air rises (convection). As it rises, it cools and condenses to form clouds and eventually rain.
2. Condensation Nuclei: For rain to form, water vapour needs something to "sit" on. Cities have lots of particulates (dust, soot, and smoke from cars and factories). These act as condensation nuclei, giving water droplets a surface to form on.
3. Increased Friction: Tall buildings slow down the wind, causing air to pile up and be forced upwards (this is called convergence), leading to more cloud formation.
Thunderstorms and Fog
Thunderstorms are more common in summer in cities because the intense UHI effect creates massive updrafts of warm air.
Fog is also more frequent because the high number of pollutants (nuclei) helps water droplets form at ground level, especially during winter nights.
Did you know? Cities can experience "flash flooding" not just because of more rain, but because the water can’t soak into the concrete ground!
Key Takeaway: More heat + more dust = more rain and more storms.
3. Wind: Structures and Layout
Buildings don’t just block the wind; they can also speed it up or change its direction entirely. On average, cities are less windy than the countryside because buildings act as windbreaks (friction), but there are two major exceptions you need to know:
The Venturi Effect (The Canyon Effect)
When wind is forced through a narrow gap between two tall buildings, it has to speed up to get through. This is the Venturi Effect.
Analogy: Think of a garden hose. If you put your thumb over the end to make the gap smaller, the water squirts out much faster!
Turbulence and Eddy Currents
When wind hits a tall building, it is deflected down the face of the building toward the ground (a down-draught). This creates "vortices" or mini-whirlwinds at the base, which can be quite dangerous for pedestrians!
Common Mistake to Avoid: Don't assume cities are always windier. Overall, wind speeds are lower in cities due to friction; it's only in specific "urban canyons" that the wind becomes localized and fierce.
4. Air Quality: Pollution in the City
Urban air quality is often lower than rural air quality because of two main types of pollution:
1. Particulate Matter (PM): These are tiny bits of solids or liquids suspended in the air (like soot from diesel engines or dust from building sites). They can settle deep in the lungs and cause health problems.
2. Photochemical Pollution: This is often called Smog. It happens when sunlight reacts with chemicals like Nitrogen Oxides (\(NO_x\)) from car exhausts. This creates Ozone at ground level, which is harmful to breathe.
Temperature Inversions
Normally, air gets colder as you go up. However, during a temperature inversion, a layer of warm air sits on top of cold air near the ground. This acts like a "lid" on a pot, trapping all the pollution down where people breathe it. This often happens in valley cities like Los Angeles or during cold, still winter nights in the UK.
Quick Review Box:
- Source: Cars, factories, heating.
- Problem: Smog and respiratory issues.
- Risk Factor: Temperature inversions trap the "muck" near the ground.
5. Pollution Reduction Policies
Since air pollution is a major health risk, governments use different policies to clean it up. You should be able to name and explain a few of these:
1. Clean Air Acts: In the UK, the 1956 Clean Air Act introduced "smoke-control areas" where only smokeless fuels could be burned.
2. Vehicle Restrictions: Many cities now have LEZs (Low Emission Zones) or ULEZs (Ultra Low Emission Zones) where drivers of polluting cars have to pay a daily fee to enter.
3. Public Transport Improvements: Building trams, cycle lanes, and electric bus networks to get people out of private cars.
4. Urban Greening: Planting more trees and creating "green roofs." Trees filter the air and help reduce the UHI effect through shade and transpiration.
5. Zoning: Keeping heavy industry and "smokestack" factories far away from residential areas or on the edge of the city where the wind can blow the smoke away.
Key Takeaway Summary:
Cities are hotter (UHI) because they absorb heat and lack plants. They are rainier because the heat makes air rise and pollution provides condensation nuclei. Wind is generally slower but can "tunnel" between buildings (Venturi Effect). Pollution is a major issue, but policies like LEZs and greening help mitigate the damage.