Urban Climate Flashcards
What is Urban Heat Island
A metropolitan area that is a lot warmer than surrounding rural areas
Factors that contribute to Urban Heat Island
Land Use, Construction Materials, Canyon Geoemetry, Wind movement, Anthropogenic factors
What is Thermal Admittance
Ability of surface to accept/release heat per unit time
Higher thermal admittance would mean that the surface stores and releases more heat
What is Albedo
an expression of the ability of surfaces to reflect sunlight
Reflective surfaces within a canopy however reduces urban surface albedo
Longwave radiation loss is restricted at night hence increasing temperatures
What is Canyon Geometry
Higher Height/Width Ratio increases UHI intensity
Heat is trapped within the Urban Canopy layer
Narrow streets and alleys are like canyons, and we call this the canyon geometry of the city
When the sun shines on the city, the buildings absorb the heat, and because they are so close together, they trap heat in these “urban canyons”. This makes the city warmer than the surrounding areas, which is what we call the urban heat island effect
If the canyons are deeper(taller buildings) and narrower, they can trap more heat. If they are oriented in a way that doesnt get much airflow, the heat cant escape easily, making the area even warmer. On the other hand, wider streets or canyons with more vegetation can help reduce the heat
So the geometry of the Urban canyons plays a big role in how much heat is trapped in the city, contributing to the urban heat island effect
Explain the Sky View Factor
Sky View Factor is a measure of how much the sky is visible from a certain point on the ground
Generally high Sky View Factor means that more of the sky can be seen, which usually happens in areas with lower buildings or wider streets
When the SVF is high, there’s more opportunity for heat to escape into the sky, and more sunlight can reach the ground which can help to evaporate water and cool the area
This can lead to a lower Urban Heat Island effect because there is less heat trapped between the buildings
Explain Wind movement
In rural areas, wind speeds dont change much with height as there is less friction at the surface
Urban areas have lower wind speeds in general as buildings produce more friction and slow down the wind and alter its direction
Explain Anthropogenic Factors affecting rising air temperatures in Urban Areas
Less green and water space/More artificial structures and pavements → Decreasing reflection coefficients → Increasing amount of heat absorbed by buildings → Rising temperatures of buildings → Extensive use of air conditioners → Increasing anthropogenic heat release → Rising air temperatures in urban areas
Decreasing reflection coefficients → Rising surface temperatures → Increasing heat transfer from the surface to the air → Rising air temperatures in urban areas
Rising surface temperatures → Increasing radiant heat → Rising air temperatures in urban areas
Less green and water space/More artificial structures and pavements → Decreasing evaporation from the surface → rising surface temperatures → Increasing heat transfer from the surface to the air → Rising air temperatures in urban areas
What is the Maximum-Minimum Thermometer
Used to measure temperature
Lowest point of each metal indicates temperature
Diurnal temperature range: 6 degrees celsius
Placed in Stevenson screen
Kept away from direct sunlight
1.5 metres above ground to prevent heat absorption from the ground
Explain the Stevenson Screen
A white wooden box with louvres to shelter thermometers from direct heat of the Sun and allow air circulation
Thermometer should not have contact with ground or warmed object.
They should not be under direct sunlight
Temperature should not be recorded at the same time throughout the fieldwork
Take readings at eye level to avoid parallax error
Take at least 3 readings to take the average and reduce error
Why are thermometers kept in the Stevenson Screen
Keeps out direct sunlight
Elevated at least 1.5 m above the ground as the ground can radiate heat which results in inaccurate readings
Painted white to reflect heat thus not affected by the Sun’s direct heat
Louvred sides allow free ventilation of air
Explain rain guage
Used to measure rainfall
Record rainfall amount over a certain period of time
Water collected in gauge is poured into measuring cylinder to record
Place in an open area away from the buildings and trees so that water is not intercepted by these objects and make the measurement inaccurate as well as to avoid obstruction
Place in an area where it is protected from the wind as wind may blow rain past the gauge resulting in underestimation
Prevent gauge from falling over by sinking it into the ground so that water from the ground cannot splash back into the rain gauge and so that it is stable and when wind blows it wont topple
Explain Anemometer and Wind Vane
Anemometer: Read off the display or the digital recorder
Wind Vane: Wind Vane points to the direction where the wind wind is blowing from
Anemometer: Position it at a location high up and away from obstructions where the wind flows freely
Wind Vane: Ensure wind vane is not obstructed by objects
Explain the sling psychrometer
It is used to measure Relative Humidity
Dip wick in water
Swing sling psychrometer at steady and comfortable pace after 1 min, record temp on Wet Bulb thermometer
Continue swinging and repeat until same temperature is recorded consecutively
Measure Dry Bulb thermometer
Calculate the difference and use the Conversion table to find out the RH
Precautions
While swinging, psychrometer should be far from body to prevent it from picking up body heat
Explain Barometer
Used to measure air pressure
Measured in millibars
2 Needles
Moveable Pointer must be arranged over the initial
Measuring Hand to mark current pressure
As you move along, the pressure falls if the measuring hand moves to the left, rises if it moves to the right of the Moveable Pointer
