Urban Climate Characteristics Urban areas are warmer than the surrounding rural areas The phenomenon of urban areas being warmer than rural areas is called 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 temperatures are found in industrial areas and in the most densely built up areas, e.g. the CBD. There are pockets of cool air above parks and bodies of water (e.g. rivers or ponds). These are called temperature ‘sinks’. Areas within the city with the same land use (e.g. industry) generally have the same temperature. These are called temperature ‘plateaus’. Temperature can change rapidly when land use changes (e.g. from inner city housing to CBD high rise buildings). Rapid changes are referred to as temperature ‘cliffs’.

There are four main causes of the UHI effect ABSORRPTION OF HEAT BY URBAN SURFACES: Concrete, brick and tarmac surfaces absorb and store heat from the sun during the day. They slowly release the 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 cover over the city. It also creates a ‘pollution dome’ – a layer of pollution over the city. Both these things trap outgoing heat radiation and 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 water’s quickly removed by drainage systems, so there’s little surface water to evaporate. Also, there isn’t much vegetation, so there’s little transpiration. Evapotranspiration uses heat energy, so less evapotranspiration means higher temperatures.

The effect varies seasonally and diurnally (between day and night) The UHI effect is stronger at NIGHT. Urban daytime temperatures are on average 0.6°C warmer than surrounding rural areas, but night time temperatures can be 3-4°C warmer. This is because rural areas cool down at night, but urban areas don’t cool as much because urban surfaces continue to release heat that they’ve absorbed during the day. It’s stronger in SUMMER (in mid-latitude cities like London). Average winter temperatures can be 2°C warmer, but average summer temperatures can be up to 5°C warmer. This is because there’s more solar radiation in summer, so urban areas absorb more heat. It’s stronger when there’s an ANTICYCLONE. 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.

Winds are affected by buildings in urban areas Average wind speed is 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 speed is zero, because some areas are totally sheltered from wind by buildings. You get turbulence around buildings. This happens when wind hits the face of a building – some of it’s deflected down, some around the sides and some over the top. When these winds hit other buildings or the ground they cause vortices (bodies of swirling air). You get powerful gusts of wind when wind is channelled down streets – this is known as the canyon effect. There’s more rain, fog and thunderstorms… It rains more often I urban areas than in the surrounding countryside. The rain is also more intense and there are more thunderstorms. There are two main reasons for these things: The UHI effect means 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 water vapour condenses and it rains. This type of rain is called convectional rainfall. Urban areas generate huge amounts of dust and pollution. Particles of dust and pollution floating about in the air act as condensation nuclei (they trigger water to condense around them). This encourages clouds to form, rather than allowing the warm, moist air to disperse. The higher concentration of condensation nuclei in urban areas also increases the frequency of fog.

…but less snow and frost It doesn’t snow as often in urban areas, and when it does snow the snow melts faster. This is because it’s warmer due to the UHI effect. Urban areas have fewer days of frost for the same reason. Exam question City A City B Annual average rainfall (mm) 1200 1300 Average July temp (°C) 21 23 Average number of thunderstorms per year 18 25 Average number of days with fog 17 Average number of days without cloud cover 180 120 Study the table, which shows climate data for two cities. Describe and suggest reasons for the differences between the two urban climates. (7 marks)