Download L8 Urban Climate Charateristics

Urban Climate Characteristics
Urban areas are warmer than the surrounding rural areas
1) The phenomenon of urban areas being warmer than
rural areas is called urban heat island effect
2) 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.
3) The highest temperatures are found in industrial
areas and in the most densely built up areas, e.g.
the CBD.
4) There are pockets of cool air above parks and
bodies of water (e.g. rivers or ponds). These are
called temperature ‘sinks’.
5) Areas within the city with the same land use (e.g.
industry) generally have the same temperature.
These are called temperature ‘plateaus’.
6) 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
1) 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.
2) 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.
3) HEAT FROM HUMAN ACTIVITY:
Cars, factories, offices, central heating, air conditioning units and people themselves all release heat.
4) 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)
1) 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.
2) 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.
3) 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
1) 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.
2) There are areas where wind speed is zero, because some areas are totally sheltered from wind
by buildings.
3) 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).
4) 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…
1) It rains more often I urban areas than in the surrounding countryside.
2) The rain is also more intense and there are more thunderstorms.
3) 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.
4) The higher concentration of condensation nuclei in urban areas also increases the frequency of
fog.
…but less snow and frost
1) 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.
2) Urban areas have fewer days of frost for the same reason.
Exam question
Study the table,
which shows climate
data for two cities.
Describe and suggest
reasons for the
differences between
the two urban
climates. (7 marks)
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
23
Average number of days
without cloud cover
180
120