Urban climate

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  • Urban climate
    • Urban areas are warmer than the surrounding rural areas
      • 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).
      • The highest temperatures are found in industrial areas and in the most densely built up areas.
      • There are pockets of cool air above parks and bodies of water. These are called temperature 'sinks'.
      • Areas within the city with the same land use generally have the same temperature. These are called temperature 'plateaus'.
      • Temperatures can change rapidly when and use changes. Rapid changes are referred to as temperature 'cliffs'.
    • There are four main causes of the UHI effect
      • Absorption 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. There isn't much vegetation, so there's little transpiration. Evapotranspiration uses heat energy, so less means higher temperatures.
    • The effect varies seasonally and diurnally
      • The UHI effect is stronger at night. 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 much because urban surfaces continue to release heat that they've absorbed during the day.
      • It's stronger in summer. Average winter temperatures can be 2 degrees warmer, but average summer temperatures can be up to 5 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 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.
      • 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 in 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:
        • 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. 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
      • Urban areas have fewer days of frost for the same reason.
  • This is because it's warmer due to the UHI effect.
    • It doesn't snow as often in urban areas, and when it does, the snow melts faster.
      • But less snow and frost
        • Urban areas have fewer days of frost for the same reason.

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