UHI's (Urban Heat Islands) Winds

Specification: Winds: the effects of urban structures and layout on variations in wind speed, direction and frequency.


The geometric features of urban structures and their layouts have major effects upon wind speed, direction and frequency.


There are three main types of effects of wind within urban areas:

• Due to the variance in building height within the urban canopy layer, buildings can exert major frictional drag upon wind speed which in turn creates turbulence and gives about abrupt changes in both direction and speed of the wind. Ultimately, this slows the average wind speeds within urban areas in comparison to rural areas which might see higher velocity winds.
• Where the geometric features of tall buildings can slow wind speeds, they can too channel airflow into urban canyons beneath them. This leads to an increase in wind speed as a result of the overpressure of airflow between buildings (effect is known as the Venturi effect) and can increase the frequency of turbulent winds.
• As a result of temperature increases within cities, convection can create localised low pressure cells within urban areas in contrast to rural areas which can influence surface inflow from colder rural areas into the urban cities. This can led to a greater frequency and increase to wind speed.

As mentioned in the above points, the size and shape (geometry) of buildings can affect wind speed, its direction and its frequency. As shown within the photo below, the obstruction of a building can cause a modification to airflow passing over it. Air is displaced upward, around the sides and then pushed downwards in the lee side (downwind) of the building.

Urban structures modify airflow around it causing eddying of vortices on the lee side and forcing airflow upward and around the buildings.

In reference to the windward side of the building, air pushes against the walls of the structure with high pressure. Air which then flows around these sides of a building forms vortex's as the airflow reaches the ground and sweeps into windward corners. Essentially, the presence of urban structures cause these abrupt changes in wind direction. Furthermore, the frequency of these changes is then decided by the density of buildings with the urban area (which will be discussed in the below section).

On the lee side of the building, a zone of lower pressure causes vortex formation behind it which are essentially turbulent winds that can cause building damage and strong winds that may knock pedestrians off their feet. If two buildings share airflow between them, they can become subject to the Venturi effect. This is where the overpressure in the gap between the buildings on the windward side causes increases to wind speed which can reach extremely high velocities. It is for these reasons, some urban layouts are planned with buildings spaced far apart with a large gap to prevent this overpressure of airflow. Furthermore, some buildings have used stilts to attempt to offset the Venturi effect which can cause major damage to buildings. However, the way in which urban layouts are planned is important to maintain a sustained airflow to remove surface level pollution from vehicle exhausts.

In most cases, buildings are narrowly spaced and so airflow between them is interfered and wind speed is increased. However, the impact of this is dependent upon the height of the buildings and the amount of space between them. Widely spaced buildings can act as single buildings/isolated blocks however in contrast, as the gap decreases between buildings, airflow becomes more and more likely to become subject to overpressure and thus the Venturi effect which can change the frequencies of airflow by producing a complex pattern of changes in wind direction and speed.


Often, the urban layout is structured in such a way that winds speeds are often abrupt and turbulent as a result of frictional drag. Furthermore, the modifications to airflow by urban structures and the potential Venturi effect caused by this means there are vast variations in wind speed direction, velocity and frequency  as a result of geometric features of urban areas. As a means to prevent pollution integrating within the urban canyons of airflow, pollution emitting chimneys are often situated high enough so pollutants released are released into an undisturbed flow above the buildings. This means pollutants are not emitted into the downward lee-side eddying of vortex, air flowing around the sides or downward flowing air which circulates on ground level in vortices and within general airflow and can act to cause respiratory problems, corrosion of buildings, damage to ecologies (especially if pollutants are present within the downwind urban plume), haze, acid rain and the soiling of buildings depending upon the pollutants involved.

Alas, the end of Urban Heat Islands.

UHI's (Urban Heat Islands) Air Quality

Specification: Air Quality: particulate pollution, photochemical smog and pollution reduction policies.

The air quality of urban areas is often more polluted than that of rural areas. This relationship can too develop in reference to scales of development. For example, within developed countries, governments are more likely to pass pollution-managing legislation that in developed countries.

In order to understand why air quality is an issue, one must first learn the main contributors toward urban pollution and why they cause issues in terms of economics, society and within the environment.

The main contributors behind urban pollution are the infamous...

• Suspended particulate matter - These are particulates exhausted from industrial units, power stations and vehicle exhausts. These guys are responsible for fog (acting as condensation nuclei), respiratory problems and the soiling of buildings. SPM's can sometimes be referred to as PM10's due to their relative diameter.

The soiling of buildings creates a thick dirt layer on buildings

• Sulphur dioxide - These pollutants produce haze, acid rain, respiratory problems, corrode buildings and damage the environment.
• Oxides of nitrogen - These pollutants product haze, acid rain, respiratory problems, corrode buildings and damage the environment. (Identical to Sulphur dioxide)
• Carbon monoxide - Will cause heart problems, headaches and fatigue.
• Photochemical oxidants - Pollutants which associate with smog and cause headaches, eye irritation and chest pain.

Air quality will vary seasonally and with changes to the weather: Pollution domes are most prevalent within static, high pressure temperature inversions in which a stable climate traps pollutants over an urban area. If conditions worsen, the mixture of fog and smoke can create smog which was known in the UK as the 'pea souper (killer fog)' which killed up to 4,000 people. For these reasons, legislation was introduced by the government to manage the levels of pollution and smog. 

Smog of the past, photochemical smog of the present: 

Where smog in developed countries is relatively a thing of the past, there has now been cited an increase to photochemical smog which is essentially low-level ozone. As with other pollutants, photochemical smog is problematic when it remains static under high pressure anticyclonic weather conditions as the pollutants remain stationary owing to the absence of wind. As a result of this, pollution domes containing photochemical smog can remain for a long duration (a few weeks) during summers (the time where pollutant trapping is most prevalent). Urban areas located within river basins where high relief creates a pollutant trapping temperature inversion are where the effects of photochemical smog are most founded. Athens is often quoted as the worst affected city due to its surrounding topography, dense populations (leading to vast exhaust emissions) and frequent sunshine which means pollutants can be trapped over the area for weeks.

In response... Pollution reduction policies: 

Pollutants cause major damage to both the human and physical environments of urban areas. For example, damage to infrastructure, human health (through respiratory problems), economies (through building damage and to an extent agricultural damage where pollution develops an urban plume downwind) and toward the environment through the loss of ecology have justified government legislation to reduce pollution in urban areas.

These policies...

• Clean Air Acts - In the 1950s and as a response to the incidence of the 'pea souper' smog, the Uk government decided to prevent smoke entering the atmosphere by introducing smoke-free zones which slowly began to clean up the air. This was enhanced in later years the Air Quality Management Areas. These are essentially areas ran by local councils in areas where pollution levels are likely to exceed their allowance.
• Vehicle Control in Inner Urban Areas - Many urban areas have controlled the output of pollution into the atmosphere by restricting or banning the movement of vehicles within the urban city centres. For example, Athens uses a driving restriction in which 2.5km² of the city centre is traffic free. The UK has too acted on pollutants by using park and ride schemes which essentially pedestrianise the CBDs. Within London, a congestion charge has been used to deter vehicle entry from the city centre by exercising a road toll to force cars entering to pay a high price to enter. In Mexico, a driving restriction scheme has been used which bans all vehicles from driving one day per week (license plate decides which day). This has helped to clear up the pollutants within the atmosphere however in times where the city is particularly endangered by pollutants such as photochemical smog, this driving restriction can be extended to two weekdays and one weekend day. 
• Increased public transport networks - Councils have launched mass propaganda campaigns to persuade the public to use public transport instead of cars. For example, London's extensive public transport network uses bus only lanes and is continuing to develop routes across the city where areas are restricted for car usage by congestion charges.
• Zoning of industry - Where possible, industry has been located downwind in cities so pollutants do not integrate into urban canopies (thus inducing a variety of socio-economical problems). In addition to this, legislation has now forced industrial chimneys to locate above an inversion layer sot hat pollutants are not trapped within the urban areas.
• Vehicle emissions technology and legislation - Motor vehicles have been fit with technologies such as the catalytic converter which removes dangerous gas such as carbon monoxide however increases the output of carbon dioxide. The promotion of lead-free petrol has too been used to reduce pollution in the UK.

Next time on the season finale of 'Urban Heat Islands'... Does the Wind have an important role in influencing the micro-climates of urban areas? Will the Venturi effect increase wind speeds along your street? 

Stay tuned, Leonard. 

UHI's (Urban Heat Islands) Precipitation

Specification point: Precipitation: frequency and intensity; fogs, thunderstorms, and their relationship to urban form and processes.

The generic pattern within climatic domes such as the UHI is the occurrence of higher precipitation over urban areas in contrast to the surrounding rural areas.

Key Words:

Convection - Vertical heat transfer whereby the expansion of a gas, liquid or solid via increased temperature causes the material to become less dense (Lower Pressure) and therefore rise.

So why is precipitation more frequent and more intense in UHI's?

• An important principle that one must apply to this course is the concept of convection. As temperatures rise,  as do the prevalence of convection currents meaning the vertical heat transfer of low pressure air increases (relate this to the ITCZ between the Hadley cells and how precipitation is formed).
• Rising air through convection is enhanced by the geometry of buildings. For instance, the presence of tall buildings induces both air turbulence and a vertical motion of this air.
• Finally, the presence of industry can promote precipitation formation via producing water vapour and pollutants which act as hygroscopic nuclei and encourage raindrop formation.

Essentially, the creation of low pressure as a result of higher urban temperatures means convectional rainfall is more likely, as is the presence of thunderstorms and lightning. 

And why is the frequency of fog more prevalent in urban areas?

Urban heat islands typically have a greater affiliation with fog formation in relation to their rural counterparts. Evidence of this was founded in the 1800s where the urban centres of London and Manchester saw the increasing prevalence of fog as a result of industrialisation. In the 1950s, it was discovered that these cities were so badly affected as a result of pollutants such as suspended particulate matter which acted as condensation nuclei and encouraged fog formation at night (which were able to stabilise if under anticyclonic temperature inversions).

In response, the 1950s saw legislation to counter fog which was epitomised in the Clean Airs Act. This statute reduced the amount of suspended particulate matter in the atmosphere over the urban areas which correlated to a decrease in the number of foggy days. This meant, in modern context, the UK as a developed country see's less fog as it did under industrialisation. In contrast however, the developing countries of Beijing are now seeing the same issues as a result of industrialisation which has resulted in fog creating major infrastructural damage and contributing to health conditions such as asthma and bronchitis.

Furthermore, why are thunderstorms more frequent within urban areas?

Thunderstorms develop in hot humid air and are associated with heavy precipitation, thunder and lighting. Their frequency is increased under summer months as a result of temperature increases which drive convection.

Thunderstorms are created by convectional uplift under conditions of extreme instability. Cloud formations typically associated with thunderstorms are the cumulonimbus clouds which can create towering columns reaching up to the height of the Tropopause. These clouds can stabilise particularly under temperature inversions.

The process: The updraught of low pressure air (as a product of convection) through the towering cloud causes rapid cooling and condensation of rising air into precipitation which then forms a down-draft toward the ground surface. Condensation produces latent heat (as we know from SALRs) which further fuels convectional uplift so the weather system can sustain. However, as raindrops split in the cumulonimbus cloud, positive electrical charge builds up until the charge is big enough to overcome resistance in the cloud. When this happens (charge overcomes resistance) discharge will strike a negatively charged area in the cloud on the ground surface which creates what is known as lightning. 

Next time on 'Urban Heat Islands'.... Will Air Quality be able to amend itself through pollution reduction policies? Will you be able to list the types of pollution impacting urban areas across the world? 

Stay tuned.

UHI's (Urban Heat Islands) Temperature

Specification point: Climate on a local scale: urban climates

This spec point is split into 4 sections, temperature, precipitation, air quality and winds. In this post I will talk about temperature.

An urban heat island is generally described as a climatic dome which is warmer in comparison to surrounding rural areas. This is a result of several reasons relating to the human environment of urban areas.

• Building materials such as brick, tarmac and concrete act to absorb large quantities of heat during the day due to their high thermal conductivity and heat capacity. This heat is slowly radiated out at night whereby the contracts in rural-urban diurnal temperature ranges will be less within urban centres as radiated heat will warm the cities during the night. This causes a change in the energy balance where urban areas will exhibit high temperatures than rural areas.
• The geometric features of tall buildings can provide surfaces in which radiating heat is trapped within the urban canopy layer creating what is known as the 'urban canyon effect' where temperatures are elevated above their mean latitude average.
• Heat from both industry and buildings which burn fuel too generate heat with contribute to increased temperatures within urban areas. Similarly, as people generate heat and there is a large density of populations within cities, this too attributes toward increasing temperatures.
• Air pollution from industry and cars increases cloud cover creating a 'pollution dome' allowing the entrance of short wave radiation but preventing its reflected back into the atmosphere. In the events of anticyclonic weather conditions in which the localised climate remains static, the creation of photochemical smog resulting from pollutants can trigger severe health problems such as asthma and bronchitis.
• The impermeable surfaces of urban areas mean water is disposed via complex drainage systems. Whilst useful, this change means the natural moisture budget of urban areas is disrupted as reduced evapotranspiration from vegetation  means more energy is available to heat the atmosphere of urban areas rather than evaporating surface water.

What is also important to note is that heat islands vary seasonally and diurnally. As mentioned, the effect of a UHI is greatest under high-pressure anticyclonic weather conditions especially where temperature inversions maintain a stable, static climate.

UHI's also vary over space. The UHI effect has been likened to a cliff in which upon entrance to an urban areas, temperatures generally increase 2-4 degrees for every km toward the city centre. However, there are variations within this pattern that reflect the distribution of green space and industry (Green space will see lower temperatures, industry higher). This could be a result of differing albedo effects of the physical environment compared with the human environment as man-made structure, consisting of dark materials, tends to absorb insolation and re-radiate this energy increasing the heat output within the urban area. In contrast, green areas can be highly reflective and reflect radiation back into the atmosphere.