Cities create their own microclimates and their sites are almost always warmer, compared to a nearby rural location, than if the city were not there. The differences in urban climates are due to a number of different factors.
The Urban Heat Island effect is known as the zone of hot air that develops over the urban areas and consequently higher temperatures than the surrounding rural area. Temperatures progressively decline as you move further from the urban areas; an example of this can be seen between Central London and the surrounding area, as there is a temperature difference of 6 degrees. However, even small urban areas can have urban heat island effects. Heat islands form for a variety of reasons that aren’t present in the surrounding rural areas. For example, urban areas have extensive dark surfaces (roads and roofs) and absorb heat during the day and release it slowly at night when the temperatures cool. Urban heat island effect is particularly visible at night time due to this reason, as the urban areas still retain heat even when the rural areas are much cooler.
Temperature differences between urban and rural areas can vary from 0.6 degrees (day) to 3-4 degrees (night). People and human activities such as transport, industry and power stations also generate their own heat. The pollutants from cars act as a condensation nuclei leading to the formation of cloud and smog, which traps radiation within the area. On average urban areas contain 10 times the amount of condensation nuclei than rural areas. The lack of vegetation in urban areas reduces the amount of moisture in the air. This is intensified by drains and sewers, which remove surface water quickly.
On average, precipitation rates are 5-10% higher in urban areas than in rural areas. The main reason for this trend is that urban areas tend to make more condensation nuclei; the tiny particles are essential for water droplets to form. Despite the fact that humidity levels are lower in urban areas (due to less vegetation and more drains/sewers), the greater concentration of condensation nuclei means that there is more cloud and rain, as well as mist and fog. Condensation nuclei is produced from car/lorry exhausts, as well as this, dust from human activity adds to the general mix. In the past, a huge amount of coal dust was produced and pumped into the atmosphere again, adding to the condensation nuclei.
When pollutants become trapped within the lower atmosphere, water vapour can condense around these particles and cause smog to form. The occurrence of Fog is dramatically higher within urban areas, the great increase in the amount of fog within cities is in accordance with the industrialisation of the city, e.g. In London 1700 there was 29 days of fog but by the 1800 (industrial revolution had occurred) there was over 50 days of fog, this statistic shows that the amount of fog is influenced by the amount of industrial activity within a city. Winter city fogs are created by dense cold air associated with anticyclones trapping pollutants, making a pollution dome. This situation often reflects a temperature inversion. This is when instead of the usual situation of air becoming colder with altitude, the situation is reversed- the temperature is ‘inverted’- so a layer of warm air sits above the cooler air at the ground.
The heat island effect, especially in summer, can lead to convective activity (showers, thunderstorms, hail) over large cities. During unstable atmospheric conditions rapidly rising air may trigger periods of heavy rainfall and even thunderstorms. Cities with tall buildings can also have a slight orographic effect (similar to hills) causing air to rise.
Air quality within urban areas is also dramatically different than surrounding areas. Within cities there is a significant amount of pollution, there are different types of pollution and they have different effects. Suspended particulate matter is solid matter within the urban atmosphere such particles are usually less than 25 micrometers in size and cause problems such as fog/smog and respiratory problems. Sulphur dioxide is usually released from combustion of fossil fuels, causing a hazard known as acid rain (due to its acidic nature) and this can corrode away at buildings, it also contributes to respiratory diseases. Carbon monoxide is released from incomplete combustion of fossil fuels and is a poisonous gas that prohibits haemoglobin within the blood to uptake oxygen, therefore it is potentially life threatening.
Consequently due to all the pollutants the number of “blue sky days” were the sky is visibly blue can be significantly reduced consequently reducing the amount of sunlight upon the city. Clearly the air quality will be significantly better in rural areas as there is less transport and burning of fossil fuels etc. There have been policies introduced in cities worldwide to reduce pollution levels and increase the number of ‘blue sky days’. An example of a policy is in Mexico where only odd number plates are allowed to enter the city centre some days and only even plates other days. The serious effects of poor air quality can be perfectly illustrated with Beijing were the public wear masks to protect themselves from the poor air and serious efforts have been made to cut down on polluting sources.
The urban layout of buildings of various different sizes vastly affects the wind pattern in the upper atmosphere; there are effects on wind speed, direction and frequency. Tall buildings can greatly interfere with airflow – they disturb the lower layers of the atmosphere, exerting a frictional drag and slowing the overall speed of the wind over an urban area.
You can see from the diagram above that there is a fairly light and even wind speed through rural areas as there is much less friction from high rise buildings. On the ground the wind can become gusty. Building shape, spacing and orientation are important controls on wind speed and direction. Narrow streets with tall buildings either side can produce ‘an urban canyon’ with winds being funnelled along the street. Approaching the centre of a CBD (central business district) where the streets become progressively narrower, wind speeds may increase as airflow is effectively ‘squeezed’ this is known as the venturi effect.
In conclusion urban areas modify their climates to a large extent and cause it to be very different compared to surrounding rural areas. Urban areas cause numerous amounts of changes in the climate surrounding them and cause higher temperatures, higher precipitation rates, effect the air quality and completely disturb the upper atmosphere wind pattern.