Climates of Urban Areas

The Urban Heat Island Effect...a warm spot in the sea of surrounding cooler, rural air

Building materials (bricks and tarmac) act like rock surfaces and absorb large quantities of heat during the day which is stored and slowly released at night. Some surfaces, like windows, have a high reflective capacity, and multistorey buildings tend to concentrate the heatnig effect into the surrounding streets by reflecting energy downwards.

Heat from industries, buildings and vehicles which all burn fuel. Air conditioning units regulate temepratures indoors but release hot air into the atmosphere. As well as anthropogenic theat

Air pollution from industry/vehicles increase cloud cover, creating a pollution dome which allows in short-wave radiation but absorbs a large amount of outgoing radiation as well as reflectinv it back to space.

Water falling on to the surface is disposed of as quickly as possible. This changes the urban moisture and heat budget - reduced evapotranspiration means more energy is avaliable for heating

The effect is greatest under calm, high pressure particularly with a temperature inversion in the boudary layer above the city. Heat Islands are better developed in winter when there is a bigger impact from heating systems. Urban rural contrasts are more distinct at night when insolation is absent and surfaces that absorbed heat by day slowly release it back into the atmosphere. 

They vary seasonally and diurnally

Surfaces in the city tend to ve less reflective than those is rural areas - lots of tarmac but not a great deal of grass. In winter, rural areas keep snow for longer and so have a greater albedo

The edge oof the island is usually well defined and temperatures change abruptly at the rural-urban boundary/cliff. From here temperatures rise steadily to peak in the city centre where building densities are highest. The rise tends to be gentle - 2-4dc per km. After the cliff, the steady rise has often been referred to as a 'plateau'  - within those, there are variations that reflect the distribution of industry, power stations, water areas and open spaces. 

This is due to albedo. Highly reflective surfaces absorb very little insolation, they reflect it back into the atmosphere and keep cool, or reflect it so that it focusses ina  small area which heats up. Darker surfaces absorb more insolation and reradiate it as long wave energy

Wind speed is reduced in cities by as much as 30% because there is increase frictional drag as the wind is obstructed by buildings. Conversly, very tall buildings can channel wind in between narraw spaces creating a canyon effect where, locally, wind speeds are higher. This effect, combined with turbulence, can proved hazardous to people at street level

Three effects of Urban Areas on Wind Patterns:

THe surface area of cities is unknown because of the varying height of the buildings. Builgins exert frictional drag on air moving over and around them, creating turbulence, giving rapid and arbrupt changes in both wind direction and speed. Average wind speeds are lower in cities than in the surrounding areas and they are also lower in city centres than in suburbs.

High-rise buildings may slow air movement but they also channel air into the 'canyons' between them. Winds in such places can be so powerful that they make buildings sway and knock pedestrians off their feet

On calm and clear nights when the urban heat island effect is at its greatest, there is a surface inflow from the cooler areas outside the city to warmer areas in the city centre.

As the air usually warmer in cities it can hold more moisture and relavite humidity levels are upto 6% lower. In addition there is less vegetation which, in conjuctions with the lack of exposed bodies of water means evapotranspiration is reduced. In terms of cloud cover, cities appear to experience mor frequent and thicker cloud cover than surrounding rural areas, mainly as a result of convection currents above urban areas and the higher concentration of cloud-forming nucleii in the air. The higher levels of cloud cover help to explain why precipitation is arouund 5-15% greater in terms of the mean annual totals in large urban areas. In addition, and partly as a result of the urban heat island effect, monthly rainfall is about 28% higher 30-60km downwind of cities, when compared with upwind precipitation.

Added to this effeect is the upto 400% greater incidence of hailstroms, as strong heating, particularly in summer gives rise to very strong convection, and a 25% greater chance of thunderstorms. If precipitation falls as snow, it tends to lie on the grouund for less time as the higher temperatures lead to more rapid snow melt than in th countryside

Fogs and mists tend to be thicker and persist for longer over cities, particularly when anticyclonic conditions mean winds are too weak to blow them away. This is largely because the greater concentration of condensation nuclie encourages condensation and enables mist/fog to  form more easily

Winds are therefore affected by the size and shape of buildings. Air is displaced upwards and around the sides of buildings and is also pushed downwards in the lee of the structure. The air pushes against the wall here with relatively high pressures. As the air flows around the sides of the building it becomes separated from the walls and roof and sets up suction in these areas.

On the windward side the overpressure, which increases with height, causes a descending flow. This forms a vortex when it reaches theground and sweeps around the windward corners. This vortex is considerably increased if there is a small building to windward.

In the lee of the building tere is a zone of lower pressure, causing vortices behind it.If two sepaarate buildings allow airflow between them, then the movement may be subject to the venturi effect in which the pressure within the gap causes the wind to pick up speed and reach high velocities. Some buildings have gaps in them, or are built on stilts, to avoid the problem, but a reasonable flow of air at street level is essential to remove pollution

Usually buildings are part of a group and the disturbance to the airflow depends upon the height of the buildings and the spacing between them. IF they are widely spaced, each building wiil act as an isolated block, but if they are closer, the wake of each building interferes with the airflow around the next structure and this produces a complex pattern of airflow

When buildings are designed it is important that pollution emitters (like Chimneys) are high enough to ensure that pollutants are released into the undistrubed flow above the building and not into the lee eddy or the downward-flowing air near the walls

Fog in cities incereased along with industrialisation. Records in London show that in the 1700s, there were around 20 days of fog a year but by the 1800s this had risen to over 50days. It was discovered in the1950s that the average number of particles in a city in the more developed world is much greater in rural areas. THere particles act as condensation nuclei and encourage gor formation at night (usually under high-pressure weather conditions)

In the UK, the clean air act of the 1950's resulted in a daramtic reduction in smoke production and particulate emissions and a decrease in the number of foggy days. As cities in LDCs industrialise they are experiencing more fog. Cities such as New Dehli and Beijing suffer regular winter fogs. These can cripple transportation networks and become hazardous to human health when they trap pollutants.

The mixture of fog and smoke particulates produces smog. This was common ni European cities through the the 19th & 20th centuries because of the high incidence of coal burning. Britain suffered badly as man were so thick, they know as 'pea soupers'

In December 1952, smog in London killed 4000 people. This presuaded the government that legislation was needed. Air pollution varies with the time of year and with air pressure - The concentrration of pollutants may increase 5/6 fold in winter because temoeratures incersions trap over the city.

More recently there has been an increase in photochemical smog. The action of sunlight of nitrous oxides and hydrocarbons in vehicle exhaust gases causes a chemical reaction, producing ozone. LA has had a seriosu problem with photochemical smog because of its high density of vehicles, frequent sunshine and topography that traps photooxidant gases at low levels

Photochemical smog is a particular hazard during anticyclonic conditions because once the air has descended it is relatively static owing to the absence of wind. Such weather sstems tend to be stable and persist for weeks during the summer months.Many cities are located in river basins so the relief ensures a sheltered location, perfect for the formation of photochemical smog. InLondon, high levels of No2 have been shown to occur at certain times of the day. These come from vehicle exhaust emissions during the rush hour.

Suspended Particulate Matter:- The solid matter in the urban atmosphere, which derives mainly from the power stations and vehicle exhausts (diesel). Particles are usually less than 25um in diameter and respinsible for fog/smog, respiratory problems, soiling buildings nd may contain carcinogens. Other particulates include cement dust, tobacco ash, ash , coal, dust and pollen - in coastal cities there is a vast number of sea salt particulates. 

Particulates are sometimes referred to as PM10s as the majority have a diameter of less than 10um.

Sulphur Dioxide:- produces haze, acid rain, repsiratory problems, damdge to lichens and plants and corrodes buildings

Nitrogen Oxides:- cause accelerated weathering of buildings, photochemical reactions, respiratory problems, acid rain and haze

Carbon monoxide:- associated with heart problems, headaches and fatigue

Photochemical Oxidants:- Ozone and peroxyacetyl nitrate (PAN) are associated with smog, damage to plants, headaches, coughs, eye irritation and chest pains.