Modification of surface characteristics a Airflow

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On average, city wind speeds are lower than those recorded in the surrounding open country owing to the sheltering effect of the buildings. Average city-centre wind speeds are usually at least 5 per cent less than those of the suburbs. However, the urban effect on air motion varies greatly depending on the time of day and the season. During the day, city wind speeds are

Effect Roughness Windspeed

Figure 12.29 Details of urban airflow around two buildings of differing size and shape. Numbers give relative wind speeds; stippled areas are those of high wind velocity and turbulence at street level.

Notes: SP = stagnation point; CS = corner stream; VF = vortex flow; L = lee eddy.

Sources: After Plate (1972) and Oke (1978).

Figure 12.29 Details of urban airflow around two buildings of differing size and shape. Numbers give relative wind speeds; stippled areas are those of high wind velocity and turbulence at street level.

Notes: SP = stagnation point; CS = corner stream; VF = vortex flow; L = lee eddy.

Sources: After Plate (1972) and Oke (1978).

considerably lower than those of surrounding rural areas, but during the night the greater mechanical turbulence over the city means that the higher wind speeds aloft are transferred to the air at lower levels by turbulent mixing. During the day (13:00 hours), the mean annual wind speed for the period 1961 to 1962 at Heathrow Airport (open country within the suburbs) was 2.9 m s-1, compared with 2.1 m s-1 in central London. The comparable figures at night (01:00 hours) were 2.2 m s-1 and 2.5 m s-1. Rural-urban wind speed differences are most marked with strong winds, and the effects are therefore more evident during winter when a higher proportion of strong winds is recorded in mid-latitudes.

Urban structures affect the movement of air both by producing turbulence as a result of their surface roughness and by the channelling effects of the urban canyons. Figure 12.29 gives some idea of the complexity of airflow around urban structures, illustrating the great differences in ground-level wind velocity and direction, the development of vortices and lee eddies, and the reverse flows that may occur. Structures play a major role in the diffusion of pollution within the urban canopy; for example, narrow streets often cannot be flushed by vortices. The formation of high-velocity streams and eddies in the usually dry and dusty urban atmosphere, where there is an ample debris supply, leads to general urban airflows of only 5 m s-1 being annoying, and those of more than 20 m s-1 being dangerous.

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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