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Urban Climates

Urban climate

The impact of urban forms and processes on local climate and weather.

Human activity has a big influence on the climate of an urban area. Climate is the long-term behaviour of the atmosphere in a specific area, with characteristics such as temperature, pressure, wind, precipitation, cloud cover and humidity etc. An urban area is an area with a high density of human created structures in comparison with the areas surrounding it. In this topic we will look at how the climatic characteristics of an urban area are affected by human factors such as pollution, the colour of buildings, people themselves and factories etc. Urban microclimates are perhaps the most complex of all microclimates. With over 82% of the UK population1 being classed as urban, it is no surprise that they are also the most heavily studied by students of geography and meteorology.

Key words:

Urban microclimate - Human activity chemically and physically alters air and weather characteristics over and around urban areas, making it different from air and weather over rural areas

Microclimate - Climate within a small area that differs significantly from the climate of the surrounding area

General climatic changes caused by urban areas in comparison to rural areas;

Urban microclimates characteristics

Source 2

URBAN HEAT ISLANDS

Due to human activity, the temperature in an urban microclimate is higher than that of the surrounding areas. Urban areas are said to be urban heat islands as under calm conditions, temperatures are highest in the built-up city centre and decrease towards the suburbs and countryside.

Newcastle upon Tyne heat island

There are several reasons why this pattern occurs;

  1. In urban areas, the building materials are non-reflective and therefore absorb heat.
  2. Also, road surfaces such as tarmac and concrete have a high thermal capacity and low albedo (the fraction of solar radiation that is reflected off the surface of an object, it tends to be lower in urban areas where surfaces are more likely to absorb radiation) therefore also absorb large amounts of heat due to their dark colour. This heat is absorbed during the day and then released slowly at night, increasing the temperature.
  3. Further heat is given off by the presence of factories and increased car use within the city, causing pollution which causes smog and a pollution dome to form. This pollution dome allows short-wave insolation to enter, but traps outgoing terrestrial radiation due to its longer wavelength, therefore increasing the amount of heat obtained.
  4. The reflection of solar radiation by glass buildings and windows. The central business districts of some urban areas can therefore have quite high albedo rates (proportion of light reflected).
  5. The emission of hygroscopic pollutants from cars and heavy industry act as condensation nuclei, leading to the formation of cloud and smog, which can trap radiation. In some cases, a pollution dome can also build up.
  6. The relative absence of water in urban areas means that less energy is used for evapotranspiration and more is available to heat the lower atmosphere.
  7. The absence of strong winds to both disperse the heat and bring in cooler air from rural and suburban areas. Indeed, urban heat islands are often most clearly defined on calm summer evenings, often under blocking anticyclones.

Due to these reasons, the mean winter temperatures are on average 1-2 degrees Celsius higher in urban areas, in comparison to rural areas. The mean summer temperature may be on average 5 degrees Celsius higher than surrounding rural areas. This can be seen in London’s Heat Island below3 and has noticeable impacts upon atmospheric pressure in urban areas. The Heat Island for London shows temperatures in excess of 6°C warmer than surrounding rural areas, and a decline in those excess temperature away from the high-density built-up area of central London. Local features, such as Richmond Park, have a dampening effect on temperatures.

London Heat Island

Temperature inversions

A temperature inversion is a reversal of the normal decrease of air temperature with altitude. It occurs where a layer of warm air lays on top of colder air underneath effectively trapping it and preventing it from rising. This can happen in cities trapping pollutants. Physical geography plays a role as temperature inversions can occur in cities that are closely surrounded by hills and mountains, or on plains which are surrounded by mountain chains, which makes an inversion trap the air in the city. During a severe inversion trapped air pollutants form a brownish haze that can cause respiratory problems. For example, the Great Smog of 1952 in London, England, is one of the most serious examples of such an inversion. It was blamed for an estimated 8,000 to 12,000 deaths. 4

AIR PRESSURE AND WINDS

Urban heat islands can also have an effect on air pressure. Locally as warm air rises over an urban area it draws in heat and air from the surrounding area and creates an area of localised low pressure. Strong pressure gradients develop between the windward and leeward side of buildings and can lead to severe eddying winds. Pressure gradient is the main driving force that causes the movement of wind from areas of high pressure to areas of low pressure. In effect the winds are equalising pressure differences across the Earth’s surface. This means winds converge on central areas and consequently may bring pollution from outer areas, into the city centre. There are often patterns and differences between rural and urban areas and these comparisons are greatest seen under calm high-pressure conditions. Indeed, urban winds are different from rural ones in terms of their speed and direction.

Winds - the effects of urban structures and layout on wind speed, direction and frequency

 

Winds in an urban microclimate are affected by its surroundings. There are many impacts of the urban environment on winds:

  1. The general pattern is that wind speeds tend to be lower in urban areas than in surrounding suburbs or rural areas because of the friction forces of the surface of the buildings etc.
  2. Although wind speeds generally increase with altitude away from the Earth's surface, the speed of winds can also be lower at higher altitudes directly above built up areas because winds are deflected over high buildings and are slowed by the friction of urban surfaces.
  3. Buildings affect the winds due to their size, shape and spacing. Tall buildings provide frictional drag on the movement of air. The frictional drag creates turbulence, which gives rapid changes in the direction and speeds of the wind.
  4. Pressure gradients are set up between the windward and leeward side of buildings which causes severe eddying of the winds. The windward side (the side facing the direction of the wind) of a building tends to have high pressure due to the air pushing against it, the leeward side which blocks the straight path of the wind has an area of low pressure and the wind moves around the building to the area of low pressure causing a steep localized pressure gradient. The steep pressure gradient causes these winds to be strong. When high pressure conditions occur in rural areas breezes move towards the low pressure created in the urban area by the rising convectional heat currents, the air therefore moves from the high pressure in the rural areas to low pressure in the city.

Buildings and wind

5. The spacing of buildings can also affect winds and can sometimes increase the speed. Single buildings which are widely spaced apart, act on the wind by themselves. Closely spaced buildings work together with each other creating more frictional drag causing winds to skim over the top and eddy in between them. Buildings also act as wind channels where the wind moves at faster speeds causing pedestrians to be blown over and causing a lot of litter. This is known as the Venturi effect, where acceleration of wind occurs as winds are forced to move through narrowing terrain, buildings in this case. These are known as Urban Canyons - a place where the street is flanked by buildings on both sides creating a canyon-like environment, can increase wind speeds as wind is funnelled down the canyon. 5

Venturi Effect

Cloud Cover

In urban areas cloud cover is changed and influenced by human activity. There tends to be more cloud cover over urban areas, and they receive thicker and up to ten per cent more frequent cloud cover than that compared to rural areas. The reason for this is because there is more convection caused by higher temperatures and a larger number of condensation nuclei which will therefore form more clouds. The amount of hygroscopic nuclei is greater, and this is all a result from the greater amounts of pollution in the urban area, with more dust from cars fuel, industry and quarrying which contribute to the hydroscopic nuclei making them larger. Cloud cover may also often be the result of smog, a mixture of fog and smoke, as this causes smoke which will appear as low-lying clouds. The increase or decrease in amount of cloud cover can directly impact the precipitation levels in urban areas. Precipitation The mean annual precipitation total in an urban area and the number of days with less than 5mm of rainfall can both be between 5-15% greater than in rural areas; what this means is that they get a larger amount of dry days, yet have more rainfall when they do have rain. This happens because of convection currents which are generated by the higher temperatures, and due to an increased amount of microscopic condensation nuclei. Precipitation levels are also increased by more thunderstorms. However, warmer city temperatures turn snow coming from rural areas into sleet, meaning the number of days with snow laying on the ground is decreased by 15%.

Thunderstorms

Furthermore, higher temperatures of urban areas mean that the likelihood of thunderstorms is increased by 25%. Thunderstorms develop in hot humid air and are accompanied by violent rain, lightning and thunder. They are particularly common in the late afternoon when heat energy has had the chance to build up in the atmosphere. Thunderstorms are created by;

  1. Heating of the air causes uplift of the air which rises through the troposphere
  2. The air cools rapidly as it rises, causing condensation and rapid cloud formation
  3. This results in the formation of towering cumulonimbus clouds
  4. Rapid cooling leads to the formation of water droplets, hail and ice
  5. Coalescence of water droplets or fusion of ice crystals makes them bigger when they will fall
  6. As raindrops are split in the uplift of air, positive electric charge is released into the air
  7. This electrical charge builds up until high enough to overcome resistance in the cloud, released to areas of negative charge on the ground or in the cloud.
  8. This is known as lightning.
  9. Thunder occurs as a result of the sudden increase in pressure and temperature from lightning which produces rapid expansion of the air surrounding it.

Intensity, frequency and length of fogs are much greater in urban areas particularly under anticyclone conditions. For example, Kew in the middle suburbs of London has 79 hours of very dense fog, with visibility being less than 40metres. Whereas, London Airport on the outer suburbs has only 46 hours, and south east England (the mean of 7 weather stations) has 20 hours. This shows that further away from the urban areas of a city, towards rural areas, fog density decreases. Obviously, the larger the city and the greater the quantity of urban structures and materials the greater the impacts of these microclimatic changes.

London Skyscraper melts car parts;

A London skyscraper dubbed the "Walkie-Talkie" was blamed for reflecting light which melted parts of a car parked on a nearby street. The 37-storey skyscraper is at 20 Fenchurch Street, and has been nicknamed the "Walkie-Talkie" because of its shape. Martin Lindsay parked his Jaguar on Eastcheap, in the City of London, on Thursday afternoon. When he returned about two hours later, he found parts of his car - including the wing mirror and badge - had melted. The building was fitted with a sun shade on its windows to rectify the problem.

Source 6

Urban microclimates management strategies

  1. Trees and Vegetation - Increasing tree and vegetation cover lowers surface and air temperatures by providing shade and cooling through evapotranspiration. Trees and vegetation can also reduce storm water runoff and protect against erosion.
  2. Green Roofs - Growing a vegetative layer (plants, shrubs, grasses, and/or trees) on a rooftop reduces temperatures of the roof surface and the surrounding air and improves storm water management. Also called “rooftop gardens” or “eco-roofs,” green roofs achieve these benefits by providing shade and removing heat from the air through evapotranspiration.
  3. Cool Roofs - Installing a cool roof – one made of materials or coatings that significantly reflect sunlight and heat away from a building – reduces roof temperatures, increases the comfort of occupants, and lowers energy demand.
  4. Cool Pavements - Using paving materials on pavements, car parks, and streets that remain cooler than conventional pavements (by reflecting more solar energy and enhancing water evaporation) not only cools the pavement surface and surrounding air, but can also reduce storm water runoff and improve night time visibility.
  5. Smart Growth - These practices cover a range of development and conservation strategies that help protect the natural environment and at the same time make our communities more attractive, economically stronger, and more liveable.
  6. Building design – needs to be considered to limit the impact of winds on buildings. The Burj Khalifa stands 828m tall and has curved sides to deflect wind around the building and prevent the formation of whirlpools or vortices. It is also orientated towards the prevailing wind direction. The building is known to sway 2m at the very top!

NEXT TOPIC - Urban Air quality

SOURCES

1 – World Bank (2016), accessed at https://tradingeconomics.com/united-kingdom/urban-population-percent-of-total-wb-data.html

2 Met Office UK, National Meteorological Library and Archive Fact sheet 14 — Microclimates, Accessed 30th December 2019 at https://www.metoffice.gov.uk/binaries/content/assets/metofficegovuk/pdf/research/library-and-archive/library/publications/factsheets/factsheet_14-microclimates.pdf

3 Mayor of London (2016), London’s Urban Heat Island: A Summary for Decision Makers, accessed 11th January 2020 from https://www.puc.state.pa.us/electric/pdf/dsr/dsrwg_sub_ECA-London.pdf

4 Christopher Klein (2012) - The Great Smog of 1952, History. Accessed 11th January 2020 from https://www.history.com/news/the-killer-fog-that-blanketed-london-60-years-ago

5 J.Calamia (2009) Urban Street Canyons – Wind, Accessed 11th January 2020 from http://web.mit.edu/nature/archive/student_projects/2009/jcalamia/Frame/05_canyonwind.html

6 BBC (2013) 'Walkie-Talkie' skyscraper melts Jaguar car parts. Accessed 11th January 2020 from https://www.bbc.co.uk/news/uk-england-london-23930675

Posted by Rob Gamesby April 2020

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