- The phenomenon of urban areas being warmer than rural areas is called the urban heat island effect.
- Urban areas with higher air temperatures than the surrounding rural areas are called urban heat islands(UHIs). For example, London has a clearly defined UHI.
- The highest temperaturesare found in industrial areas and in the most densely built up areas such as the
- There are pocketsof cool air above parks and bodies of water. These are called temperature ‘sinks’.
- Areas within the city with the same land usegenerally have the same temperature. These are called temperature ‘plateaus’.
- Temperature can change rapidlywhen land use changes. Rapid changes are referred to as temperature ‘cliffs’.
Four Main Causes of the UHI Effect
- Absorption of heat by urban surfaces: concrete, brick and tarmac surfaces absorband store heat from the sun during the
- They slowly releasethe heat as long wave radiation, this is most noticeable at night, when it warms the air.
- Air pollution: air pollution from cars and factories increases cloud coverover the city. It also creates a ‘pollution dome’, a layer of pollution over the city.
- Both these things trap outgoing heat radiationand reflect it back to the surface
- Heat from human activity: cars, factories, offices, central heating, air conditioning units and people themselves all release heat.
- Less evapotranspiration: when it rains the waters quickly removed by drainage systems, so there’s little surface waterto
- Also, there isn’t much vegetation,so there’s little transpiration. Evapotranspiration uses heat energy, so less evapotranspiration means higher temperatures.
UHI Effect Explanatory
- The UHI effect is stronger at Urban daytime temperatures are on average 0.6 degrees warmer than surrounding rural areas, but night time temperatures can be 3-4 degrees warmer.
- This is because rural areas cool down at night, but urban areas don’t cool as muchbecause urban surfaces continue to release heat that they’ve absorbed during the day.
- It’s stronger in Average winter temperatures can be two degrees warmer, but average summer temperatures can be up to five degrees warmer. This is because there’s more solar radiation in summer, so urban areas absorb more heat.
- It’s stronger when there’s an Anticyclones cause clear skies and low winds. If there are no clouds, more solar radiation reaches and heats the ground. Low winds mean warm air isn’t blown away.
- Average wind speedis usually lower in cities than in rural areas. This is because tall buildings create friction that slows down the moving air.
- There are areas where wind speedis zero, because some areas are totally sheltered from wind by
- You get turbulence around buildings. This happens when wind hitsthe face of a building with some of its deflected down, some around the sides and some over the top. When these winds hit other buildings or the ground, they cause
- You get powerful gustof wind when wind is channelled down streets, this is known as the canyon effect.
- It rains more oftenin urban areas than in the surrounding countryside.
- The rain is more intenseand there are more thunderstorms.
- There are twomain reasons for these things:
- The UHI effectmeans the air in urban areas is warm, and warm air can hold more water. The warm, moist air rises – this is called convectional uplift.
- As it rises, it cools, the watervapour condenses, and it This type of rain is called convectional rainfall.
- Urban areas generate vast amounts of dustand Particles of dust and pollution floating about in the air act as condensation nuclei.
- This encourages cloudsto form, rather than allowing the warm, moist air to disperse.
- It doesn’t snow as oftenin urban areas, and when it does, the snow melts faster. This is because it’s warmer due to the UHI effect.
- Urban areas have fewer days of frostfor the same reason.