Geography CUE: 3.2.3.4 Flashcards
Defining climate and weather
The term climate refers to the state of atmosphere over a period of not less than thirty years (an average!). It includes variables such as temperature, rainfall, winds, humidity, cloud cover and pressure. It refers not just to the averages of these variables but to the extremes as well. By contrast, weather refers to the state of the atmosphere at any particular moment and location in time. However, we usually look at the weather over a period of between a few days and a week. Climate and weather are affected by factors such as atmospheric composition, latitude, altitude, distance from the sea, prevailing winds, aspect, cloud cover and increasingly human activities.
What is a microclimate?
The small-scale variations in temperature, precipitation, humidity, wind speed, and evaporation that occur in a particular environment such as an urban area.
Define the Urban Heat Island (UHI)
The term Urban Heat Island is given to describe the typical pattern of temperature in an urban area and its surrounding countryside. The typical heat profile of an Urban Heat Island shows the maximum temperature at the city centre, where building densities are highest, a plateau across the suburbs, and a temperature cliff between the suburbs and rural areas.
Identify the causes of the UHI
- Materials used in abundance in cities, such as stone, concrete and asphalt, tend to shore solar energy during the day but cool more slowly at night compared to rural areas.
- Lack of vegetables and bare soil also reduces heat lose through evapotranspiration
- Enhanced by emission of heat trapping pollutants into atmosphere
List the impacts of the UHI
Positive Impacts: During the winter, warming effects from the UHI can reduce energy needed for heating and may melt ice and snow on roads.
Negative Impacts: In summer, UHI can cause higher energy consumption of air conditioning and a number of health hazards, including evaluate atmosphere pollution and heat-treated illness.
Strategies to reduce the UHI
- Those designed to reduce heat emissions from human activities (e.g. by improving energy efficiency in buildings)
- Those designed to improve urban surfaces (e.g. tree-planting, including roofing and paving materials that are more reflective and therefore have a higher albedo)
- Those designed to improve urban structure (e.g. by creating large green spaces and linking parks, rivers and roads)
Reading UHI Graph
Small scale variations within the Urban Heat Island occur with the distribution of industries, open space, rivers, canals etc. Significant minor peaks in temperature may be located close to areas of manufacturing industry and energy production. By contract, the rest of the suburban area shows lower temperatures, although still higher than the surrounding countryside.
Changes in the nature of heat islands
The nature of heat islands may be hanging. Recent research on London’s heat island suggests that high levels of air pollution are actually decreasing the temperature by day, by blocking out the radiation from the sun. However, by night, the same air pollution is trapping heat within the urban area. Thus, the difference between urban and rural temperatures during the day may be decreasing.
What causes urban areas to be warmer than surrounding rural areas
- Anthropogenic heat
- Height and arrangement of buildings
- Albedo ~ Nature of building material
- The presence of water
- Presence of pollutants
Urban Heat Island
ANTHROPOGENIC HEAT
Released by human activity. Metabolic heat (body heat) is negligible, but the heat released from vehicles, Central heating / air conditioning system in warmer areas and industrial activities is substantial. All of these inject heat into the canopy layer and the upper boundary layer. Some large, Densely packed cities in colder areas can release more heat from human activity then the urban area receives in its net input from the sun.
Urban Heat Island
HEIGHT AND ARRANGEMENT OF BUILDING
Built up areas. It is the vertical aspect of tall buildings close together that is the most important factor in generating the urban Heat Island. Insolation can warm surfaces - the degree of warming depends on the amount of insolation absorbed by the surface. This in turn depends on how much is reflected off that surface. THis reflectance is the albedo. The fact that these tall buildings are so close to each other reduces sky-view factor. Amount of sky we can see without our view being impeded by tall buildings.
Urban Heat Island
ALBEDO - NATURE OF BUILDING MATERIALS
BLACK ABSORBS HEAT WHITE LESS SO
Albedos for selected surfaces:
Grass: 16% - 26% Deciduous forest: 15% - 20%
Asphalt: 5% - 20% Concrete: 10% - 35%
Compared to rural surfaces, urban areas heat up more quickly - the specific heat capacity for concrete is one-third that of wet mud, and so for a given input of energy, concrete will warm more rapidly then wet mud. Also vegetation shades soil, so that it stores less heat. In cities the surface is exposed and the surface area is larger, so it is able to store the heat and release it slowly over the night.
Urban Heat Island
THE PRESENCE OF WATER
The surplus heat can be transmitted as either latent heat or as sensible heat. Latent heat evaporates water. It cannot be which can be felt. After evapotranspiration has taken place, any heat ‘left over’ can take the form of sensible heat, to raise the temperature of the air - the more sensible heat that enters the atmosphere, the warmer it gets. Latent heat does not raise air temp. WAter is important here, since it determines how the surplus heat will be used. If there is little water are the surface, the less of the surplus heat is used for evaporation and the remainder of the surplus heat will take the form of sensible heat to raise the air temperature. Cities –> remover water (drainage) –> less demand on LH to evaporate.
Urban Heat Island
PRESENCE OF POLLUTANTS
In terms of the urban Heat Island, pollutants have both a cooling and heating effect. The presence of dust, aerosols and gases such as C)2, SO2 and pollutants as ozone and PAN (peroxyAce-tylNitrates) which are components of photochemical smog can alter the amount of energy flowing into and out of the urban area. Smokey, dust cities can reduce (attenuate) the amount of sunshine that reaches the city surface. Coal burt, soot and particulates can reduce sunshine by 30%. On the other hand, the pollutant dome can absorb heat & prevent some of it escaping. The build up of CO2, ozone and particulates helps to absorb long-range radiation. In so doing, the pollutant dome warms up even more, as it was already warmed by the isolation it absorbed in the 1st 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.
What is the albedo? How do urban areas influence the albedo?
- The reflectivity of a surface
- A ratio between insolation and amount reflected back into the atmosphere
- Lighter surfaces have a higher a;bedo (more reflective)
Strategies for managing the Urban Heat Island
COOL SURFACES
Cool roofs built from materials with high albedo absorbs and store less solar energy during the day and thus are not major emitters of heat into the urban atmosphere at night. Cool roads and pavements with reflective coats and seals are being trailed around the world
Strategies for managing the Urban Heat Island
SKY VIEW FACTOR
Reduce escape of heat from streets and tall buildings. This can contribute to the accumulation of heat within ‘Street light-coloured and lead to the increase of air temperature’. If streets angled perpendicular to prevailing wind reduce chance of ventilation and removal of heat.
Strategies for managing the Urban Heat Island
GREEN ROAD
Roads with a more porous surface that allows water to seep in and grass to grow through. Reduces the amount of heat absorbed by the road surface.
Strategies for managing the Urban Heat Island
URBAN GREENING VEGETATION
Planting trees and vegetation provides shade (surface peak temperature reductions of 5-20C may be possible) and can have a natural cooling effect as seen by lower temperatures in urban parks around the world. Urban trees also act as carbon stores, can reduce urban flooding by intercepting rainfall and filter pollutants from the air.
Strategies for managing the Urban Heat Island
GREEN ROADS INCREASE VEGETATION
Growing medium planted over a waterproof membrane and can reduce rooftop temperatures by 20-40C on a sunny day. In addition, green roofs reduce run off, act as insulator and increasing biodiversity. Creating habitats for animals.
Strategies for managing the Urban Heat Island
COOL COLOURED CARS
Due to its higher reflectivity, the interior of a light-coloured ear does not heat up as much as a darker one. This reduces the need for air-conditioning and reduces the amount of heat that is emitted into the surrounding air.
What is the impact of urban areas on wind?
WIND SPEED IN RURAL AND URBAN AREAS
Low wind speeds. On the whole, urban areas have lower wind speeds than do outlying suburbs, on average of the land surface, which consists of buildings at a variety of height, all helping to increase surface friction.
What is the impact of urban areas on wind?
THE URBAN CANYON
Straight streets lined with all buildings can cause the Venturi effect which is the effect of squeezing airflow, increasing as buildings tend to get taller towards the city centre causing high velocity winds.
What is the impact of urban areas on wind?
AIRFLOW AROUND A BUILDING
The detailed pattern of airflow around a building is very complex, with the highest pressures experienced in the upper part of the building with air flowing down the front and over the top of the building. Behind the building there is a reverse circulation as suction occurs on the roof, walls and sides of the building. This then leads to turbulence.
