To What Extent Can a City Modify Their Climate

The purpose of this essay is to describe with reference to an Urban microclimate how a city can modify their climate. And then I will attempt to equally explain which the strongest factor to altering the climate is and how cities are dealing with difference. I will also be referencing to case studies of London, Baltimore and los angles.

A microclimate is the distinctive climate of a smaller area. Urban climates differ from rural climates in many ways in terms of heat, precipitation and wind. There are pockets of cool are above parks and bodies of water (eg river or ponds), these are called ‘sinks’. Areas within the city with the same land use generally have the same tempurative which is known as a temperature plateus. Temperature can change rapidly when land use changes (eg from inner city housing to cbd high rise buildings). Rapid changes are referred to as temperature ‘cliffs’.

The phenomenon of urban areas being warmer than rural areas is called the Urban heat island effect. For example, London has a clearly defined UHI being on average 1-2Oc higher. The contrast between urban and rural are greatest under calm, high pressure conditions. The highest temperatures are found in industrial areas and in the most densely built up areas such as the CBD. This is due to industries releasing heat, height and arrangement of buildings, air pollution trapping radiation , building materials absorbing more heat and the small amount of water so little energy used on evapotranspiration.  Anthropogenic heat refers to the heat released by human activity from vehicles, central heating and industrial activities. Cities are, by their nature, very built up areas. Insolation can warm surfaces depending on the amount of absorbed by the surface. This depends on how much is reflected off that surface. This reflectance is the albedo. Due to urban areas having more vertical surfaces, raditions tends to be relected off many surfaces, each one absorbing some of the energy and warming up in the process. With the buildings being so close together, this reduces the sky view factor therefore giving the radiation little chance to escape.

When there is little water at the surface in a city, then less of the surplus heat is used for evaporation (latent heat), and the remainder of the surplus heat will take the form of sensible heat to raise the air temperature. Since cities generally have arrangements – in the form of gutters, drains and sewers – to remove surface water, there is less demand on latent heat to evaporate water, so most of the surplus heat is used as sensible heat to warm the atmosphere. In vegetated rural areas the balance is quite different: twice as much of the surplus heat is used in evaporation as is used in sensible heat. In terms of the urban heat island, pollutants have both a cooling and heating effect. The presence of dust, aerosols and gases such as CO2, SO2 can alter the amount of energy flowing into and out of the urban area. Smokey, dusty cities can reduce the amount of sunshine that reaches the city surface. In some cases where coal is burnt, soot and particulates can reduce this sunshine by 30%. Los Angeles suffers from photochemical smog, but this only reduces sunshine by about 6%. On the other hand, the pollutant dome can absorb heat and prevent some of it escaping. In so doing, the pollutant dome warms up even more, as it was already warmed by the insolation it absorbed in the first place. The pollutant dome can now re-radiate this heat back down to the urban surface – so not only preventing heat escaping, but also adding to the heating load in urban areas.