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Micro-Climateof an Urban Area

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IndexIntroductionKey TermsWhat is a Micro-Climate?Factors changing Climate in Cities?Results of Micro-Climates in Cities?Urban Heat IslandVideo: Urban Heat IslandSummary

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IntroductionClimates change according to a number of

factors.Longitudinal and Latitudinal LocationAir MassesWeather Condition

In Cities and Large Urban Areas other factors come in to place.

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Key TermsMicro-Climate

Pollution

Albedo

Urban Heat Island

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What is a Micro-Climate?A Micro-climate is a local atmospheric zone

where the climate differs from the surrounding area.

The term may refer to areas as small as a few square feet (e.g a garden bed) or as large as many square miles (e.g a valley). 

Microclimates can be found near bodies of water that can cool the local atmosphere, or in heavily urban area

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Climatic Zones of San Diego

1. Coastal Zone2. Inland Area3. Further Inland Area4. Mountainous Region

Each Area is influenced by different conditions, one of which is the impact of the local population on the weather.

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Major Micro-ClimatesUpland- Upland areas have a specific type of

climate that is notably different from the surrounding lower levels. Temperature usually falls with height at a rate of between 5 and 10 °C per 1000 m, depending on the humidity of the air.

Coastal-The coastal climate is influenced by both the land and sea between which the coast forms a boundary. The thermal properties of water are such that the sea maintains a relatively constant day to day temperature compared with the land. 

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Forests- Tropical rainforests cover only about 6% of the earth's land surface, but it is believed they have a significant effect on the transfer of water vapour to the atmosphere. This is due to a process known as evapo-transpiration from the leaves of the forest trees.

Urban- These are perhaps the most complex of all microclimates. With over 75% of the British population being classed as urban, it is no surprise that they are also the most heavily studied by students of geography and meteorology. Therefore, the rest of these notes focus on the various elements that constitute an urban microclimate.

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What Changes a Cities Climate?

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.  

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An urban area is an area with a high density of human created structures in comparison with the areas surrounding it.  

Climate in Urban Areas are affected by human factors such as pollution, the colour of buildings, people themselves and factories etc.

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Pollution:Companies that supply electricity typically rely on fossil fuel

power plants to meet much of this demand, which in turn leads to an increase in air pollutant and greenhouse gas emissions.

The primary pollutants from power plants include Sulphur Dioxide, Nitrogen Oxides, Particulate Matter, Carbon Monoxide and Mercury.

Increased use of fossil-fuel-powered plants also increases emissions of greenhouse gases, such as Carbon Dioxide, which contribute to Global Climate Control.

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Building Colour:Colour affects the amount of heat which is reflected by a

surface. This is referred to as the Albedo.Certain colours are able to absorb heat energy thus

warming the temperature of the surrounding area.All weather is a result of the uneven heating of the Earth

caused by different areas of the planet having different Albedos.

Generally the Lighter the colour the higher the Albedo and the Darker the colour the Lower the Albedo.

The higher the Albedo, the less heat there is in an area.Polar Regions have the highest Albedo

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PeopleThere are a number of ways in which people

can affect the climate.Pollution is the main contributor by man.

Traffic- In Large Urban Areas Transport is always an issue and the build up of Traffic leads to more emissions.

Industry- Industries are necessary for everyday life but the pollutants they ‘spew’ into the atmosphere lead to increase in temperatures.

Central Heating/Cooling- Electricity is needed for both heating and cooling of houses. The more an urban area continue to grow and warm the more energy will be needed.

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Results of an Urban Micro-ClimatePollutants are harmful to human health and

also contribute to a reduction in Air Quality- Smog or Acid Rain.

Warmer temperatures in cities mean that in the summer during heat waves, many cities and their residents experience even greater temperatures and heat stress. In 2003 this was a major problem in Europe, particularly France, and many people died.

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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.

 Locally as warm air rises over an urban area it draws in heat 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

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The table below summarises some of the differences in various weather elements in urban areas compared with rural locations:

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Urban Heat IslandAs a result of the Micro-Climate in Urban

areas the Term Urban Heat Island was coined.

By Definition: An urban heat island (UHI) is a metropolitan area that is significantly warmer than its surrounding rural areas due to human activities.

The temperature different is usually larger at night than in the day and recorded when wind is minimal.

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The formation of a heat island is the result of the interaction of the following factors: the release (and reflection) of heat from industrial and domestic buildings; the absorption by concrete, brick and tarmac of heat during the day, and its

release into the lower atmosphere at; 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);

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;

recent research on London's heat island has shown that the pollution domes can also filter incoming solar radiation, thereby reducing the build up of heat during the day. At night, the dome may trap some of the heat from the day, so these domes might be reducing the sharp differences between urban and rural areas;

the relative absence of water in urban areas means that less energy is used for evapo-transpiration and more is available to heat the lower atmosphere;

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.

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London: Urban Heat Island

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Video: Urban Heat Island

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Overall Changes in WeatherPrecipitation- As noted previously, the greater presence of

condensation nuclei over urban areas can lead to cities being wetter and having more rain days than surrounding rural areas. Indeed, it was often said that Rochdale, the famous mill town, had significantly smaller amounts of rain on Sundays when the town's factories were closed.However, other factors play a major role, especially the heat islands. These can enhance convectional uplift, and the strong thermals that are generated during the summer months may serve to generate or intensify thunderstorms over or downwind of urban areas. Storms cells passing over cities can be 'refuelled' by contact with the warm surfaces and the addition of hygroscopic particles. Both can lead to enhanced rainfall, but this usually occurs downwind of the urban area.

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Smog- Smogs were common in many British cities in the late 19th and early 20th centuries, when domestic fires, industrial furnaces and steam trains were all emitting smoke and other hygroscopic pollutants by burning fossil fuels. The smogs were particularly bad during the winter months and when temperature inversions built up under high pressure, causing the pollutants to become trapped in the lower atmosphere and for water vapour to condense around these particles.One of the worst of these 'pea-soup fogs' was the London Smog of the winter of 1952/53. Approximately 4,000 people died during the smog itself, but it is estimated that 12,000 people may have died due to its effects. As a result, the Clean Air Act of 1956 was introduced to reduce these emissions into the lower atmosphere. Taller chimney stacks and the banning of heavy industry from urban areas were just two of the measures introduced and, consequently, fewer smogs were recorded in the UK during the 1960s and 1970s.Research in the 1990s has shown, however, that another type of smog - photochemical - is now occurring in some urban areas as a result of fumes from car exhausts and the build up of other pollutants in the lower atmosphere which react with incoming solar radiation. The presence of a brown-coloured haze over urban areas is an indication of photochemical smog, and among its side effects are people experiencing breathing difficulties and asthma attacks.

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Wind- Tall buildings can significantly disturb airflows over urban areas, and even a building 100 metres or so high can deflect and slow down the faster upper-atmosphere winds. The net result is that urban areas, in general, are less windy than surrounding rural areas.However, the 'office quarter' of larger conurbations can be windier, with quite marked gusts. This is the result of the increased surface roughness that the urban skyline creates, leading to strong vortices and eddies. In some cases, these faster, turbulent winds are funnelled in between buildings, producing what is known as a venturi effect, swirling up litter and making walking along the pavements quite difficult.

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Class Work1. Define what you understand by a n‘Urban Heat

Island’.

2. What are the three major factors affecting the temperature of an Urban Area?

3. Draw a detailed diagram of the effects of the Urban Heat Island.

4. How is the precipitation and wind of an Urban Area affected by the affects of the Urban Micro-Climate?

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SummaryTemperatures tend to be higher in Urban Areas.This is a result of number of human-induced factors which

tend to increase the amount of heat in an urban areas.Urban Micro-climates do not only refer to hotter

temperatures in cities- Often due to the presence of low pressure systems and high rise buildings, cities are often very windy places.

Urban Heat Islands refer to urban areas which have a significantly higher average temperature than the surrounding suburbs.

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HomeworkUsing examples from your own experiences

in Urban areas, either in Malta or abroad, discuss why you think that the climate within

these areas is different to its’ suburbs and other rural areas in Malta or abroad.

(200 - 300words).