City Climate

City Climate

The Heat Island Effect

HO Pui-sing

City Climate

City Climate as a micro-climateMicro-climate is a climate conditions of the nearest 100m of the ground surface.

Heat Island

Temperature of cities is higher than the temperature of the surrounding areas.

Showing by isotherm, it looks like a “warm island” surrounded by a “cool sea”.

The Heat Island Effect

Causes of the heat islandMagnitude of the head islandTemperature distribution within a city

Causes of the Heat Island

Changes of surfaceHeat ProductionChanges in the composition of atmos.Increase Turbidity

Causes of the Heat Island

Changes of surfaceWaterproof cement surfaceThermal AdmittanceRoughness of the surface

Causes of the Heat Island

Heat ProductionIndustrial ActivitiesCooling and HeatingConcentration of People

Causes of the Heat Island

Changes in the composition of atmosphere

Increase green-house gases – CO2, COEnhance green-house effect

Green House Effect

Causes of the Heat Island

Increase TurbidityIndustrial and transport activities introduce a great quantity of fine particles into the air.Increase the counter-radiationHinder heat dispersion

Magnitude of Heat Island

Size of citiesLocal Micro-climate conditionTopographyDiurnal and Weekly variationSeasonal variationBuilding densityDistance from city

Magnitude of Heat Island

Size of citiesLarge city – strong heat island effectSmall city – weak heat island effect

Magnitude of Heat Island

Local Micro-climate conditionWind speed• fast – weak effect• Slow – strong effect

Cloud cover• High - strong effect• Low – weak effect

Magnitude of Heat Island

TopographyThe effect will be less pronounced among ocean, rivers, lakes, wood land, pastures and areas of high elevation

Magnitude of Heat Island

Diurnal and weekly variationMore pronounced at night and during week days

Seasonal variationMore pronounced during winter

Magnitude of Heat Island

Building densityMagnitude increases with increasing building density

Distance from city centreMagnitude increases with decreasing distance from city centre

Magnitude of Heat Island

Temperature distribution within a city

Variation within a city / Horizontal distribution

Street intersection – (the coldest place)Heavily traveled main roads (2 degree )Around stop light (2-3 degree )

Temperature distribution within a city

Vertical distribution

Normal Inversion

Temperature distribution within a city

Inversion and Pollution

Temperature distribution within a city

Dust Dome / Pollution Dome

The End