To what extent do urban areas create their own climate (40 Marks)
An urban area is the region surrounding a city. Jobs are those in a non-agricultural field, making urban areas very developed causing a high density of human structures such as houses, buildings, and transport links. Urban areas contain microclimates which measures the average weather condition in a small area.
Due to human activity, temperature in an urban area is a fundamental characteristic, as temperature is relatively higher compared to surrounding rural areas, named as an urban heat island. This is due to housing and buildings being constructed close together. The material used for building is non-reflective and therefore absorbs heat, also road surfaces such as tarmac and concrete have a high thermal capacity and result in large insulation due to the albedo effect, where only light surfaces reflect more heat. Therefore urban heat islands have a low albedo effect due to the dark coloured nature of the area, only 5-10% of radiation is reflected due to the dark colours. There are large amounts of buildings with glass windows within urban areas due to high building capacity, the glass contributes a high reflective capacity and therefore reflects heat towards the surfaces where it is absorbed. Average night temperature in an urban area is 4 degrees Celsius warmer than surrounding areas due to the dust and clouds acting as a blanket to reduce radiation. During the night, temperatures remain high as the buildings block heat escaping from the surface into the cold night sky. This results in heat being trapped at lower levels thus causing temperature to be warmer. The dust acts as a blanket as it reduces insolation, this is increasing as more dust is occurring due to pollution from factories and car fumes. More clouds are also forming as a result of an increase in condensation nuclei which leads to water droplets collecting around the condensation nuclei to form more clouds. Temperature within an urban area makes a different climate compared to surrounding areas as it is anthropogenically influenced, as human activity has led to more harmful exposures, such as an increase in car ownership as well as more commuting due to globalisation and movement and an increase in tourism, leading to an increased demand in central heating, air conditioning and producing goods within manufacturing.
Winds within urban areas are affected by the surroundings. Tall buildings will disturb the airflow in urban areas and they will deflect and slow down the faster upper-atmosphere winds. Tall buildings will channel the air into canyons between the buildings, this causes winds to increase in speed, known as the Venturi effect where high pressure is formed causing high velocity winds. The tall buildings in an urban area exert a powerful frictional drag on the surrounding air, forming turbulence. On the other hand, with single buildings, air is displaced upwards and around the sides, which forces air downwards on the lee side of the structure, causing a vortex. On the windward side of the structure, the air is pushed against the wall with high pressure increasing with height. Wind in an urban area influences its own climate, as it is different compared to surrounding areas. When high pressure conditions occur in rural areas, wind will 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.

Precipitation rates are also significantly different within rural areas, research shows that rainfall is generally higher within urban areas than surrounding rural areas. A reason for this may be that an urban heat island carries the meaning that temperatures are higher therefore lower pressures will be encouraged to develop above the city in the lower layers of the troposphere. This also means that evapotranspiration rates will be higher within the city causing an increase in cloud formation. Convection rainfall is also much higher within the urban areas due to the urban area enhances convectional uplift. Storms passing over cities can be ‘refuelled’ by contact with the warm surfaces and the addition of hygroscopic particles from increased pollution. The urban area also produces large amounts of water vapour from industrial sources, meaning that humidity levels are usually higher. Another influence of precipitation over an urban area is also influenced through the topography of the landscape with the largest amount of precipitation occurring over the more hilly regions and lowest values in more low—lying areas. For example, Kenley on the North Downs, at an altitude of 170 metres above mean sea-level has an average annual rainfall of nearly 800 mm whereas London Weather Centre, at 43 metres above mean sea-level, has an average annual rainfall of less than 550 mm. The nature of rainfall varies during the year. In summer, rainfall is often in the type of convectional rainfall, falling over short periods, and is normally more intense than in winter, when rainfall tends to be more frontal in character with falls occurring over longer periods. The influences of precipitation caused by urban areas contributes to urban areas having their own climate. This is due to additional and increasing human activity fuelling these factors which increase the risk of precipitation to these areas.
In conclusion, it is clear to see and analyse that anthropogenic activity has a lot to do with urban climates as it influences the amount of heat absorbed and reflected, leading to a knock on effect of increased precipitation and the distribution and speed of wind, as well as increased temperature compared to the surrounding areas.