Atmo: Climate And The Cities Flashcards
What are the elements of comparison between urban and rural areas? (6)
Temperature, precipitation, solar radiation, relative humidity, cloudiness, wind speed
What factors affect variation in temperature differences?
1) time of day - more pronounced in late evening when more insolation has been absorbed
2) season - more pronounced in winter due to additional sources of heat
3) location - tropical areas have smaller UHI as they are less developed
Size of city and density of population
What are the causes of temperature differences?
1) changes in thermal capacity
2) sensible and latent heat transfer
3) the urban structure
4) heat production by combustion
5) presence of pollutants ‘localised greenhouse effect’
Describe the differences in wind speed between rural and urban areas
Tall buildings alter airflow. They increase frictional drag upon near surface winds and average horizontal wind speeds are reduced. They however cause channeling and local turbulence of winds between buildings, making them more
gusty.
How does changes in thermal capacity affect temperature differences between rural and urban places?
During the day, absorption of solar radiation warms the surface of the ground, and a gradient develops as heat is conducted downward. During the late afternoon, the surface begins to cool due to radiation. The soil temperature profile reverses, with temp increasing with depth. The same process occurs in walls and roofs of buildings. Soil is a poor conductor of heat (Shallow layers) vs construction materials of buildings and roads have a much larger thermal capacity than natural surfaces.
How does differences in sensible and latent heat transfer affect temperature differences between rural and urban areas?
Rural area: moisture allows energy to be transferred by latent heat (evaporation) than sensible heat which increases temperatures.
Urban areas: city streets have low infiltration capacity, leaving little water left for evaporation, sensible heat dominates.
How does the urban structure affect temperature differences between rural and urban areas?
1) Presence of buildings change surface albedo. Urban surfaces (asphalt streets, roofing materials) have lower albedo than the natural surfaces they replace.
2) 3D structure of city cause multiple reflections to occur. Vertical walls do not allow radiation to escape easily. When sunlight penetrates and hits the side of a building, some energy is absorbed and some is scattered back onto adjacent buildings. Process goes on repeatedly and sides of building emit their stored heat, increasing total absorption.
How does heat production by combustion affect temperature differences?
Waste heat from home heating, power generation etc. Almost equal to 1/3 of that received from insolation. Anthropogenic heating greatest in winter.
How does presence of pollutants cause temperature differences? (Hint: pollution domes)
‘Blanket’ of pollutants including particulate matter, water vapour, co2 contributes to UHI by absorbing longways radiation and remitting it. This leads to the ‘localised greenhouse effect’, raising temperatures.
How does precipitation differ between rural and urban areas?
Higher in urban areas
What are the 3 reasons for higher urban precipitation?
1) UHI creates thermally induced upward air motions to increase atmosphere’s instability
2) clouds will be modified due to addition of condensation nuclei from industrial discharges. Condensation will set in with RH as low as 78%. Hygroscopic nuclei cause particles to attract water and trigger the early onset of condensation.
3) rougher urban landscape creates an obstacle effect that impedes progress of weather systems, causing rainfall producing processes to linger over the area longer.
Account for the reduction in solar radiation in urban areas (by about 15%)
Urban pollution adds aerosols like carbon and lead to the atmosphere. This blanket of particulates, called the pollution dome, significantly reduces the amount of solar radiation reaching the surface. Esp when sun angle is low at higher latitudes, the length of path through the aerosol increases.
Account for the lowered relative humidity in urban areas
Temps are higher and evaporation is reduce. Lack of large water bodies and high surface runoff. Lack of vegetation cover decreases evapotranspiration.
