21 terms

The impact of urban forms and processes on local climate and weather.

Urban microclimate
Human activity chemically and physically alters air and weather characteristics over and around urban areas, making it different from air and weather over rural areas
Climate within a small area that differs significantly from the climate of the surrounding area
Venturi effect
Acceleration as winds are forced to move through narrowing terrain, buildings in this case
Urban heat island
The concept that on average, both maximum and minimum temperatures are higher in urban areas than in nearby rural settings.
Absorbs lots of heat and minimises albedo
The fraction of solar radiation that is reflected off the surface of an object, it tends to be lower in urban areas where surfaces are more likely to absorb radiation
Heat pollution
Heat escaping from buildings and industrial processes raises the temperature of the air locally
Pressure gradient
Creates winds between slightly warmer and hence lower pressure urban areas and cooler higher pressure rural areas
Wind turbulence
Increases in urban areas as buildings cause eddies in wind flow
Air pollution by a mixture of smoke and fog
Condensation nuclei
Increase in urban areas as more smoke and dust are emitted by power stations and industrial processes. Leads to increased condensation and cloud development
Relative Humidity
Is often lower in urban areas as warm air can hold more water vapour
Temperature inversion
a reversal of the normal decrease of air temperature with altitude, or of water temperature with depth. Can happen in cities trapping pollutants.
Physical Geography and temperature inversions
Cities are closely surrounded by hills and mountains, or on plains which are surrounded by mountain chains, which makes an inversion trap the air in the city.
During a severe inversion
trapped air pollutants form a brownish haze that can cause respiratory problems.
The Great Smog of 1952
In London, England, is one of the most serious examples of such an inversion. It was blamed for an estimated 11,000 to 12,000 deaths.
Temperatures in urban areas
can be 1 to 3°C warmer than its surrounding and up to 12°C warmer on a calm, clear night.
Amount Relative humidity can be lower in urban areas
Wind speeds
See an overall 20 to 30% decrease in urban areas due to an increase if obstructions and friction
Urban Canyons
a place where the street is flanked by buildings on both sides creating a canyon-like environment, can increase wind speeds as wind is funnelled down the canyon.
can increase by up to 10% as there are more hygroscopic nucleii