Thunderstorms, the Wind and Air Quality (AQA A-Level Geography): Revision Notes

Thunderstorms, the Wind and Air Quality

Thunderstorms in urban environments

Thunderstorms form in hot, humid air masses and bring violent weather conditions with heavy rainfall, thunder and lightning. Urban areas experience a higher frequency of thunderstorms, particularly during late afternoon and early evening in the summer months. This happens because cities create ideal conditions for storm development.

The formation process begins with convectional uplift under conditions of extreme instability. As the sun heats urban surfaces, warm air rises rapidly to form towering cumulonimbus clouds. These clouds can reach the height of the tropopause, where an inversion layer creates stability and prevents further upward movement.

Wind in urban environments

Buildings fundamentally alter how wind moves through cities. Whilst it might seem that buildings would simply slow wind down by creating obstacles, the reality is far more complex. Urban designers and architects must carefully consider these wind effects to create comfortable and safe environments for people.

How buildings modify wind patterns

Cities experience four main wind effects that create challenging conditions for pedestrians and residents:

• During calm, clear nights, the urban heat island effect is strongest. This temperature difference between the warm city and cooler surrounding areas causes convectional processes that draw in strong localised winds from the countryside.

• Many city districts have developed clusters of high-rise towers that dramatically reshape urban skylines. Cities like La Défense in Paris demonstrate how tall buildings can increase or reduce wind speeds at street level, depending on how the surrounding streets are aligned.

• Long, straight streets create channelling effects where wind is funnelled with less friction, significantly increasing wind speed. This can make walking uncomfortable and even dangerous in some locations.

• The Venturi effect intensifies wind as it is forced through narrow gaps between buildings. This can create severe gusts at street level and make walking difficult, particularly affecting people with mobility challenges.

Wind behaviour around individual buildings

When wind encounters a single building, it creates distinct patterns on different sides of the structure.

On the windward side (the side facing the wind), several effects occur:

• Overpressure builds up as wind hits the building face
• This pressure increases with building height
• Wind is deflected both upwards over the building and downwards towards the ground
• The descending flow on the windward side can be very strong
• When this downward-moving air reaches ground level, it creates a vortex that sweeps around the windward corners

On the lee side (the downwind side sheltered by the building):

• Low pressure develops, drawing air inward
• Complex vortices form as air circulates in the sheltered zone
• Multiple swirling patterns develop downwind of the structure

Effects of building spacing

The spacing between buildings determines how their individual wind effects interact with each other. This significantly affects air flow patterns and pollution dispersal.

Widely spaced buildings act as individual obstacles. Each building creates its own pattern of wind flow with distinct windward and lee side effects. The wake from one building doesn't significantly interfere with airflow around the next building.

Narrower spacing causes the wake from one building to interfere with airflow around the next structure. Wind patterns become more complex as the buildings interact.

Wind patterns around tall buildings

High-rise structures create particularly strong wind effects because wind speed naturally increases with height above ground level.

Urban design solutions

Careful planning and architectural features can reduce the negative impacts of urban winds. Different design approaches address specific wind problems.

Street alignment: When main streets are aligned with the direction of prevailing winds, pollutants are more effectively flushed out of the urban area. This also helps reduce the urban heat island effect by allowing cooler air to flow through the city.

Building on stilts: Elevating buildings on support columns allows wind to flow freely through the base of the structure. This improves air circulation at ground level and prevents stagnant air pockets from forming.

Building on a podium: Placing buildings on raised platforms prevents downdraughts from reaching ground level where pedestrians walk. This design separates the strong wind effects from the areas where people spend time.

Doorway porches: Building porches or covered entrances over doorways protects people from downdraughts when entering or leaving buildings. This is particularly important for buildings that experience strong wind effects.

Wind barriers: In areas where the Venturi effect creates particularly strong winds, physical barriers can be installed to reduce wind speed and provide shelter. La Grande Arche de la Défense in Paris serves as an example of how barriers can be incorporated into urban design to protect pedestrians in high-wind zones.

Air quality in urban environments

Urban areas typically experience poorer air quality than rural areas. Particulate air pollution results from the release of tiny particles and harmful gases into the atmosphere.

Types of urban air pollutants

Several major pollutants affect air quality in cities:

Carbon monoxide (CO) is a colourless, tasteless and odourless gas produced when fuels burn incompletely. Road transport generates an estimated 90% of all carbon monoxide emissions in the UK. Concentrations are highest close to busy roads. This gas is particularly dangerous because it affects oxygen transport around the body through the blood. Breathing in low levels of carbon monoxide can cause headaches, nausea and fatigue.

Nitrogen oxides (NOx) form when vehicles are exposed to sunlight. They react with hydrocarbons in the atmosphere to produce components of photochemical smog, including peroxyacetyl nitrates (PANs) and other secondary pollutants. Road transport generates approximately 50% of total nitrogen oxide emissions. Nitrogen oxides inflame the lining of the lungs, with impacts being more pronounced in people with asthma. They also accelerate weathering of buildings and contribute to acid rain. PANs damage plants as well as affecting human health.

Particulate matter (PM) consists of tiny solid or liquid particles suspended in the air. Particles mainly originate from power stations and vehicle exhausts, but other sources include metal and rubber from engine wear, dust, ash, sea salt, pollens and soil particles. Particles smaller than about 10 micrometres are referred to as PM$_{10}$. These can settle in the airway and deep in the lungs, causing serious health problems including respiratory disease.

Sulphur dioxide (SO$_2$) is a colourless gas with a sharp odour. It reacts readily with other substances to form harmful compounds such as sulphuric acid, sulphurous acid and sulphate particles. In the UK, the major contributors to coal and oil burning are power stations and refineries. Short-term exposure may cause coughing, tightening of the chest and narrowing of the airways. Sulphur dioxide can also produce haze, acid rain, damage to lichens and plants, and corrosion of buildings.

Temperature inversions and pollution trapping

Air pollution levels vary throughout the year depending on weather conditions. Concentration of pollutants may increase significantly during winter because temperature inversions trap them over the city.

Under normal atmospheric conditions, air temperature decreases with altitude, allowing pollutants to rise and disperse. However, during a temperature inversion, a layer of warm air sits above cooler air near the ground, acting like a lid that prevents vertical air movement.

Photochemical smog

The mixture of fog and smoke particulates produces smog. This was common in European cities throughout the nineteenth and first half of the twentieth centuries due to high rates of coal burning. Britain suffered particularly severely, with many smogs becoming so thick they were known as 'pea-soupers'. In December 1952, smog in London was responsible for more than 4,000 deaths.

Whilst traditional coal-based smog has largely been eliminated in developed countries, smog remains a feature of many cities worldwide. Headlines regularly highlight the negative impact smog has on human health. A particular concern has been the increase in photochemical smog.

Pollution reduction policies

Reducing air pollution in cities has become increasingly important for national and local government. Different strategies have been adopted to tackle the problem.

Clean Air Acts

After the catastrophic London smog of 1952, the British government decided legislation was needed to prevent excessive smoke from entering the atmosphere. The Act of 1956 introduced smoke-free zones into urban areas, and this policy slowly began to clean up the air. The 1956 Act was reinforced by later legislation.

In the 1990s, for example, tough regulations were imposed on levels of airborne pollution, particularly on PM$_{10}$s. Local councils in the UK are now required to monitor pollution levels in their areas and to establish Air Quality Management Areas where levels are likely to be exceeded.

In London, various measures to clean up construction sites have been introduced. These changes are said to account for around 12% of the city's nitrogen oxide emissions. The use of dust suppressants at industrial sites has also been increased to reduce particulate emissions.

Vehicle control and public transport

Providing better public transport and imposing general restrictions on polluting vehicles can be very effective in reducing urban air pollution.

In London, a variety of measures have been introduced: