Baba microclimate

What is an urban microclimate and what causes it?

Cities create their own microclimate domes and have unique:

· Temperature ranges

· Wind patterns

· Clouds and precipitation rates

· Pollution

Urban microclimates vary according to urban areas' size, shape and location. There are a number of causes, some of which are:

· Changes to land surface; concrete, brick and tarmac

· Cities have fewer trees than surrounding rural areas. Trees shade the ground, preventing heat from the Sun being absorbed

· Dark rooftops and dark pavement absorb more solar radiation

· Tall buildings reflect and absorb sunlight

· Cars engines and factory exhaust produce heat

· Fewer plants in urban settings mean that less evapo-transpiration occurs, a process that cools the air

· Poor building insulation means release of heat at night

Within these microclimate domes, there are two levels:

· Urban canopy - processes act in the spaces between buildings below roof level

· Urban boundary - processes acting above roof level and extends downwind as a plume into the surrounding rural areas

Patterns of precipitation and air quality are extended to immediate areas via the prevailing winds.

What are the characteristics of urban microclimates?

Urban climates show the following characteristics:

· 2-3 weeks fewer frosts

· Pollution levels are higher

· Fog and photochemical smog is likely

· 5-15% more precipitation

· Lower relative humidity

· 5-10% more cloud

· Increased thunderstorms

· Lower wind speeds, with exception of tunnelling effect

· Pressure gradients

· Greater diurnal temperature range - higher max and min temperatures as compared to rural areas

What is the urban heat island (UHI) effect?

The air in urban areas can be 2-5°C warmer than in nearby rural areas. This is known as the urban heat island (UHI) effect. The UHI is most noticeable when there is little wind.

There is a variation of temperature within the UHI:

· The highest temperatures are found in densely built-up areas and industrial areas. In these areas, the activities generate more heat

· Temperature sinks are found above green spaces and water, e.g. parks and lakes

· Temperature plateaus occur in areas with the same land use, e.g. industrial areas

· Temperature cliffs occur when temperatures change rapidly from one land use to another - e.g. suburban housing to high-rise inner city buildings

What are the main causes of the urban heat island effect?

Air pollution: Pollution from cars, industry, etc., increases cloud cover and produces a pollution dome. Both of these trap outgoing heat and reflect it back to the surface.

Heat from human activities: Air conditioning units, heating homes and offices, etc., all release heat into the surrounding area.

Absorption of heat by urban surfaces: Urban surfaces have a low albedo. Tall buildings reflect and absorb sunlight.

Less evapotranspiration: The removal of green spaces and trees reduces the cooling effect of evapotranspiration.

What are the diurnal and seasonal temperature fluctuations in urban areas?

Diurnal temperature fluctuations: In urban areas, there is a larger range between daytime and nighttime temperatures (diurnal range), compared to rural differences.

· In urban areas: Daytime temperatures are approximately 0.6°C warmer. Nighttime temperatures can be up to 4°C warmer

· Rural areas do not store as much energy and release the heat more quickly than urban areas

Seasonal temperature fluctuations: Average urban summertime temperatures can be as much as 5°C warmer, with average winter temperatures of 2°C warmer.

Temperatures can increase during periods of anticyclonic weather (high pressure). These produce clear skies and low winds. This allows greater insulation to reach urban surfaces. Low winds prevent warm air from being dispersed.

How does urban form affect cloud coverage and humidity?

Cloud coverage: There is greater cloud coverage in urban areas, approximately 5-10% more than in rural areas. This is due to:

· More convection, which is caused by the higher temperatures in the urban areas. This leads to increased condensation nuclei, which form clouds

· Due to the levels of pollution, there are also higher levels of hygroscopic nuclei

Humidity: Relative humidity is affected by the temperature of the air. It is reduced by as much as 6% in urban areas because the air is warmer. This means it can hold more moisture.

Humidity in urban areas varies between the seasons:

· In summer, it can be up to 10% lower as the air is much warmer. This effect is increased by the land surfaces within the urban area that absorb heat due to the dark colour of the buildings and road surfaces

· In winter, it is approximately 2% lower

Cities are designed to remove surface water via drains, so humidity during the day is lower. At night, urban humidity is higher than in the rural surroundings.

How does urban form affect precipitation, fog and thunderstorms?

Precipitation: Precipitation is more frequent in urban areas, up to 15% more. The warm air has to rise, cool and condense. Urban pollution in the atmosphere increases hygroscopic particles that become cloud condensing nuclei. This increases 'seeding' of cloud droplets, which in turn increases precipitation rates. If snow falls, it melts quite quickly due to the retained heat on darker surfaces and higher urban temperatures.

Fog and thunderstorms: Fog is when visibility is less than 1000m. Fog is effectively ground-level cloud. Urban areas generate large amounts of dust and pollution. Some of the particulates are hygroscopic nuclei (a form of condensation nuclei). All the particulates act as condensation nuclei and trigger cloud formation. The higher the number of condensation nuclei in the urban air, the more fog forms.

Thunderstorms: Urban areas have a higher frequency of thunderstorms, about 25% higher than in rural areas. Urban convection is strong in the late summer afternoons. This creates updrafts of humid, hot air which rise, cool and condense rapidly. This results in the formation of towering cumulonimbus clouds. Water droplets in the upper atmosphere are moved by updrafts and downdrafts. This means that water droplets increase in size (melt, freeze, melt, refreeze, etc.). When the droplets are large enough to overcome gravity, the fused ice crystal will fall. As raindrops split in the updrafts, there is an electrical discharge into the air - this is lightning. Thunder is a result of the sudden increase in pressure and temperature from the lightning, causing a rapid expansion of air around it.

What are the different types of fog?

Advection fog: Advection fog is formed when warm, moist air moves horizontally over a cold surface. San Francisco has a lot of advection fog, as the surface water near the coast is much colder than the water further offshore. Warm, moist air from the Pacific Ocean is advected across the cold coastal waters, chilling the advected air from below.

Radiation (ground) fog: Radiation (ground) fog is formed when the ground rapidly loses heat at night through long-wave radiation. The cooled ground absorbs heat from the air. The air temperature falls, and condensation occurs, forming fog. Conditions necessary include:

· Long, clear, cloud-free, cold night

· Shallow layer of moist air near the ground - lake in a park

· Light winds

Steam fog: Steam fog forms when cold air moves over warm water (e.g., a heated outdoor swimming pool). Water evaporates from the pool into the air. This increases the dew point, and if mixing is sufficient, the air above becomes saturated. The colder air directly above the water is heated from below and rises, forming what appears to be steam. Steam fog also forms above a wet surface on a sunny day, over a road after a summer rain shower. Can also be seen over unfrozen lakes in parks in autumn or winter.

How does urban form affect wind?

Rural wind speeds are higher than urban ones. This is because the ground surface is smoother and there are fewer barriers. The rougher urban surfaces, tall buildings, etc., act as barriers and reduce wind speeds. Urban structures also redirect and redistribute airflow. Urban structures extend their effect on wind by ten times the height of the structure downwind of the city.

Local variations in wind: The form of urban areas causes local variations in the wind.

· Wind flowing across a deep, narrow street will create very little wind disturbance at ground level

· Chicago is nicknamed the 'windy city' due to its grid system of buildings. This creates wind tunnels where the air is funnelled between buildings, and wind can pick up speed

· Some areas have zero wind speed as they are totally sheltered from wind by buildings

· Powerful gusts of wind occur when wind is channelled down streets - known as the canyon effect

· Turbulence occurs around buildings. Wind is deflected downwards as it hits the face of a building. Wind can be deflected around the sides of the building. Or it can flow over the top of the building. This flow of air around buildings can create bodies of swirling air called vortices

What is urban air quality like and what are the impacts of poor air quality?

Particulates in urban areas are greater than in rural regions. Sources of particulates include:

· Vehicle exhausts

· Burning - wood, coal, cigarettes, rubbish, etc., which releases fine and coarse particulates

· Construction, mining and quarrying

· Plants and moulds generate coarse particulates such as pollen and mould spores

Impacts of poor air quality: There are a range of impacts of poor air quality, including:

· Respiratory problems such as asthma

· Increased cardiovascular health issues

· Increases haze through increased emissions of sulphur dioxide and nitrous oxide

· Carbon dioxide is increased, adding to the enhanced greenhouse effect and global warming

· Increased particulates in the atmosphere attack building facades

· Photochemical oxidants cause eye irritation and headaches

What is smog?

Smog happens when smoke particulates, sulphur oxides, hydrocarbons, etc., mix with fog. The London smogs of the 1940s were caused by the sinking of cold air, trapping air pollutants in a pollution dome.

Today, smog is more likely due to photochemical reasons:

· Sunlight reacts with the chemical pollutants in the atmosphere

· UV light causes them to break down into secondary, harmful chemicals to form photochemical fog

Photochemical fog is a major problem in large cities - Los Angeles, Mexico City, Beijing, etc. It is more common in warm, sunnier cities. These places tend to suffer from temperature inversion fog (a layer of warm air is trapped below dense cooler air). This keeps the pollutants at the surface level.

What technical innovations reduce urban pollution? (Filters, photocatalytic materials and self-cleaning concrete)

Rapidly developing LDEs and EMEs have some of the highest rates of air pollution and reducing urban air pollution globally is a challenge. Strategies include: Technical innovations, Vehicle restrictions, Government legislation.

Filters: Fitted to industrial gas and particulate exhausts, filters carbon out of the gases released during industrial processes. Any emitted gases are chemical and pollutant free when released back into the environment. Catalytic converters fitted to vehicle exhausts remove harmful pollutants before being released.

Photo-catalytic materials (smog eating material): Façades are retrofitted to the front of old buildings or new buildings are constructed with photocatalytic concrete. Special tiles are coated with titanium dioxide, which is a pigment that acts as a catalyst and is also used in sunscreen. When UV rays hit the tiles, a reaction occurs, converting mono-nitrogen oxides (smog producing substances) into less harmful calcium nitrate and water. Titanium dioxide within the tiles isn't altered and continues to work indefinitely.

Self-cleaning concrete - Tiocem: This is photocatalytic concrete that has titanium dioxide mixed in. Buildings will be able to break down nitric and nitrogen oxides when hit by sunlight. Can also be used for constructing paving, roofing tiles, roads, and in sound buffering walls on the sides of roads.

Smog Eating Poem: The University of Sheffield, UK, has dedicated one of its walls to a smog-eating poem called In Praise of Air. The poem is printed on material infused with titanium dioxide that reduces air pollution. The banner removes the pollution of at least 20 cars every day. Following this, smog-eating advertising posters have begun appearing in other parts of the world, including a mural made of photocatalytic paint pigments in Warsaw,

Poland.

What other technical innovations reduce urban pollution?

Greening the urban area: Improve air quality through planting trees and vegetation. Vertical gardens - around concrete columns and on the sides of buildings. Roof gardens. Urban agriculture using open and derelict spaces.

Air purification towers: Dutch designed "Smog Free Tower" which is an air purifying tower that sucks in pollution and expels clean air. The first tower was installed in Rotterdam, and cleans 3.5 million cubic metres of air per day.

Self-driving cars: Studies have estimated that self-driving vehicles could improve fuel efficiency by 15-40%, which would reduce local emissions of pollutants as well as global greenhouse gases.

Hydrogen fuel additives: Additives improve fuel combustion and reduces emissions in existing vehicles. UK developed 'zero1' technology feeds small amounts of hydrogen into the vehicle air intake, creating a more efficient burn. By increasing fuel efficiency, there is an overall reduction in nitrogen oxide (NOx), particulate matter (PM), hydrocarbon and carbon monoxide emissions. The technology can be retrofitted to existing vehicles and is available commercially in small numbers.

Alternative fuels:

· Electric: Hybrid electric vehicles (HEVs), Plug-in hybrids (PHEVs), Fully electric (EVs)

· LPG: Dual fuel or bi-fuel vehicles that can switch between LPG and petrol. LPG is a by-product of the crude oil refining process. Contains a mix of hydrocarbon gases and considered having a relatively low environmental impact. LPG has many uses from cooking to heating, refrigeration to vehicle fuel

What is synthetic gas to liquid (GTL)?

Shell has developed a new GTL fuel as a replacement for diesel and the engine needs no modification. Testing of GTL in heavy vehicles (trucks, buses and ships etc.) and depending on the vehicle age, showed reduced nitrogen oxide (NOx) emissions of 5-37% and particulate matter (PM) emissions of 10-38%.

Natural gas can also be converted into dimethyl ether (DME) as another alternative to diesel. Although the engine needs modifying, using DME reduces NOx emissions by around 25% and PM emissions virtually eliminated. Car makers Ford and Volvo, are considering marketing vehicles already able to use DME as a fuel.

What vehicle restrictions reduce pollution? (Congestion charge and selective bans)

Congestion charge: Charges for using vehicles in certain places at certain times (e.g. London's congestion charge). This reduces pollution through the reduction in road traffic (London's emissions dropped 15% in its first year). However, it can increase fringe/outer zone traffic and emissions as people try and avoid the charge by using alternative routes.

Selective bans: Certain days and times are designated as no travel times for vehicles. Mexico City's air quality is among the worst in the world, according to the World Health Organization (WHO). Over 5 million cars within a valley with a population of more than 21 million people. The city instituted a late start for schools 10am instead of 8am. No car day, where commuters are banned from using their car depending on the last digit of their number plate (e.g. on Mondays, plates ending in 5 or 6 cannot be used).

What other vehicle restrictions reduce pollution?

Pedestrianisation: Vehicles are restricted from entering certain places at certain times. This reduces emission by reducing road traffic. Many urban areas have pedestrianised zones, however, it impacts retailers as restrictions on parking can discourage customers.

Park and ride: Buses are laid on by local authorities at the urban fringe and charge a flat fee for all day parking and travel from the parking area to the urban centre.

Improvements to public transport: Improved bus services makes accessing areas cheaper, faster and more efficient. Many cities have brought in bus lanes, which keeps the buses moving in peak traffic times. Trams and light railway services run on lines that avoids congestion. Less polluting than buses and can be accessed at various points on the journey. Not always successful and can shift congestion issues to the urban fringe. Improvements can be very expensive and run over time and budget (e.g. HS2).

Car sharing/pooling: Many urban centres have designated lanes for cars with two or more people in them. This keeps the flow of traffic moving and reduces journey times and emissions.

How does legislation reduce pollution?

Legislation can be local or global. However, according to the UN:

"One in three countries in the world lack any legally mandated standards for outdoor air quality" - UNEP 2021

Laws aim to reduce pollution by limiting emissions from industry, private and public facilities and vehicles.

Industries are regulated under Integrated Pollution Prevention and Control (IPPC) set up under the Pollution Prevention and Control Act of 1999.

· Factories are not allowed to emit 'dark' smoke under the Clean Air Act of 1993, except in unavoidable circumstances (e.g. starting up)

· The amount of dirt and dust emitted is also strictly monitored/controlled

· Chimneys must have up-to date modern filters/scrubbers fitted

In the UK, key air pollutants are regulated by the Air Quality Standards Regulations 2010.

Laws set air quality standards such as:

· UK Clean Air Act of 1956 and 1968 reduced domestic pollution through the introduction of smoke free zones

· Industrial pollution was reduced by introducing tall chimney thereby dispersing pollutants higher into the atmosphere

· Introduction of the MOT emissions test by the Road Vehicles Regulations means all vehicles have to pass an emissions test to be allowed on the UK roads

· In Scotland, roadside emissions tests are carried out and fines issued if the vehicle fails

· Local authorities in the UK can issue fines to people leaving their engines running unnecessarily