Weather in cities Flashcards
Temperature in cities
- buildings retain and release heat more strongly than in rural areas
- temperatures are higher
- tarmac surfaces are heated intensely and then strongly heat the air above it
Winds in cities
- winds can be channelled between high-rise buildings increasing velocities
- buildings also act as windbreaks, reducing velocities
Sunlight in cities
- actual sunlight amounts are lower
- high-rise buildings block out light
- high amounts of dust reflect and absorb sunlight
Precipitation in cities
- greater amounts than in other locations
- concentrate surfaces lead to convection currents and cloud formation
Humidity in cities
-lower in urban areas as higher temperatures can hold greater amounts of water vapour
Clouds in cities
-thicker and more frequent
The percentage of reflected insolation is…
1.9 per cent in rural and 0.4 per cent in urban areas(a lot of is absorbed).
Anthropogenic heat is…
heat released by human activity(driving, industry). Some cities in colder areas can release more heat from human activity than the area receives from the sun.
The percentage of outgoing infrared is…
8.5 per cent in rural and 10.3 per cent in urban areas.
Outgoing infrared is also called…
terrestrial radiation.
The percentage of storage heat is…
0.3 per cent in rural and 1.3 in urban areas.
Asphalt albedo
5-20 per cent
Concentrate albedo
10-35 per cent
White paint
50-90 per cent
Coloured paint
15-35 per cent
Grass albedo
25-30 per cent
The percentage heat loss due evaporation is
24 per cent in rural and 1 per cent in urban areas(water is stored mainly underground)
The percentage of heat given up by the ground at night is…
twice the amount as in the rural areas.
There is temperature…
inversion in urban areas at some point. (ground is colder than the air above)
In which ways the urban area modifies the climate?
- wind speeds
- temperature
- clouds and precipitation
- pollution
The greater the urban area…
the greater the modification of a climate will be.
Winds in urban areas are modified:
- lower speeds
- greater variability
- large-scale convection
Winds lower speeds
- have lower wind speeds on average per cent less in the city centre
- due to the roughness of the land surface(different heights of buildings), which increases the surface friction
Greater variability of winds in urban areas
- due to layout and height of the buildings
- urban canyons
- venturi effect
Urban canyons
Straight streets lined with tall buildings can produce it. This funnel wind down them producing high velocities thanks to the venturi effect.
Venturi effect
It is the effect of squeezing the airflow, increased as the buildings tend to get taller towards the city centre.
The highest pressure is experienced in…
the upper part of the building.
Building shape and spacing all affect…
the behaviour of the wind.
Densely packed buildings allow…
air to move over the top, leaving the lower urban canopy layer relatively without turbulence.
If no mixing by turbulence occurs…
then the pollutants at street level cannot be flushed away, causing smog situations.
Large-scale convections
The heat island effect also produces large-scale convection and uplift over the whole urban area, leading to lowering the pressure. Air is drawn in from surrounding areas.
The urban heat island is on average…
1-2 degrees warmer per year than surrounding area.
The intensity of the urban heat island is…
the maximum difference between the temperature of the rural area and the peak urban temperature. Greater at night.
High-pressure systems with clear nights and sunny days tend to…
increase the intensity.
Heat islands are the result of:
- anthropogenic heat
- height and the arrangement of buildings
- the nature of the building materials
- the presence of water
- pollutants
Height and arrangements of buildings
- tall buildings close together is the most important factor in generating urban heat island
- more vertical surfaces so radiation tend to be reflected more
- reduces the sky-view factor
The sky-view factor is…
the amount of sky we can see without being impeded by a tall building.
The sky-view factor tends to…
trap radiation, which reduces the chance of it escaping.
In cities, there is a good chance of longwave radiation being…
intercepted by another surface and absorbed, reducing heat loss. The sky-view factor has a similar effect.
Building height and arrangement is the key factor in creating urban heat islands as…
it is a way how the inputs and outputs of heat are controlled.
Nature of building materials
- the specific heat capacity of materials effects how much heat can be stored
- e.g. concentrate will warm up more rapidly and store more heat than wet mud
Most of the surplus heat in urban areas is lost as…
sensible heat, heating up the atmosphere.
Vegetation shades…
the soil so that stores less heat.
The presence of water
- if there is little water at the surface, less surplus heat is lost as latent heat
- cities have arrangements in the form of drains and sewers to remove surface water
- twice as much heat is used in evaporation than used in sensible
Presence of pollutants
-have both cooling and heating effect
Sensible heat index gives…
the percentage of the total heat energy at the surface to raise the temperature of the air above. 85% in urban and 25% in rural.
The presence of dust…
aerosols and gases can alter the amount of energy flowing in or out of the city.
Smoky, dusty cities can reduce…
the amount of sunlight that reaches the city surface.
Pollutant dome can absorb…
heat and prevent it from escaping. The build-up of ozone, CO2 and particulates help to absorb LW radiation and re-radiate it back.
Clouds and precipitation
- urban areas have 5-10% greater cloud cover
- due to a greater concentration of hygroscopic nuclei (100x more than in rural areas)
Precipitation is more common in…
cities 5-30% more.
Fog is less common in…
cities due to higher temperatures and a higher concentration of hygroscopic nuclei, forming smaller droplets, which tend to discourage the formation of which fogs.
