CAMBRIDGE AS GEOGRAPHY - CASE STUDY: URBAN AREA CHICAGO. Heat island effect, precipitation, temperature, humidity winds. Albedo effect and its decline.

1. URBAN CLIMATE
CHICAGO
EXAM CASE STUDY
AS GEOGRAPHY - ATMOSPHERE AND WEATHER

2. INTRODUCTION
This case study follows precisely the Cambridge AS 2019 syllabus requests
for the “Atmosphere and Weather” case study.
Candidates must study an urban area which shows the effects of human
activity on climate:
temperature (heat island), humidity, precipitation and winds.

3. CHICAGO
The terrain of the Midwest and Lake Michigan is flat and this aspect is making
Chicago’s weather unpredictable and very often extreme.
Summer is very warm and humid.
The highest temperatures occur during July and August and can reach
35-38 degrees Celsius.
The coldest month is January, with temperatures around
-18 or -20 degrees Celsius.
Chicago receives an average annual rainfall of 80.5cm per year.

4. TEMPERATURE

5. TEMPERATURE
Chicago, with an estimate population of 2,705,994 (2018), is warmer than
the surrounding rural areas.
Because of that, Chicago is considered to be an urban heat island.
The temperature difference can be as great as 3 degrees Celsius on a sunny
day and 6 degrees Celsius at night in winter.
Temperature differences have a number of causes.

7. CAUSES FOR DIFFERENCES IN TEMPERATURE
BUILDING MATERIALS
Building materials have a lower specific heat capacity than grass and trees.
Specific heat capacity is the amount of heat required to raise one unit of the
material 1 degree Celsius.
The specific heat capacity of concrete is 800 Joules/Kg degrees Celsius and
for the soil it can be 2000 Joules/kg degrees Celsius.
So concrete heats up more quickly in the day, warming the air around it.

8. CAUSES FOR DIFFERENCES IN TEMPERATURE
WET SURFACES
Rural areas have more wet surfaces (in towns, rivers are often covered and
water runs off into drains).
Evaporation from these wet surfaces produces cooling because heat is
absorbed by the process of evaporation.

9. CAUSES FOR DIFFERENCES IN TEMPERATURE
BUILDINGS
Buildings are heated, and vehicles and air conditioning systems
generate heat.

10. CAUSES FOR DIFFERENCES IN TEMPERATURE
WINDS
Buildings act as a barrier to winds.
As winds could convey cooler temperatures from Lake Michigan, this has the
effect of keeping the city warmer.

11. CAUSES FOR DIFFERENCES IN TEMPERATURE
ALBEDO
Buildings with dark surfaces such as roof tiles have a lower albedo – they
reflect less solar radiation, absorb more and become hotter.
Dark coloured tarmac, often used as a road surface, has a low albedo.

12. CBD – THE WARMEST PART OF THE CITY
The denser the built-up area, the hotter it tends to be, so the central
business district is the warmest part of the city.
Temperature contrasts are greatest when wind speeds are low because
wind will carry in cool air from outside the city.
Wind speeds are always lowest when air pressure is high.

13. THE URBAN HEAT ISLAND EFFECT
The urban heat island effect tends to be greater in winter because more
heat is being generated to keep buildings warm.
The effect is also greater at night because buildings store heat generated
during the day and lowly emit it at night.
A field in a rural area is pointing towards the sky and will re-radiate long-
wave radiation back into space if the sky is clear.
A canyon of buildings does not point towards the sky and this lack of a “sky
view” impedes reradiation.

14. DECLINE IN ALBEDO
The City of Chicago
government aims to
mitigate the urban heat
island effect by giving
grants to residents and
firms who plant rooftop
gardens and trees in their
gardens.
They have also resurfaces
pathways in the city with
surfaces that have a
higher albedo and reflect
more solar energy.

15. HUMIDITY

16. HUMIDITY
Chicago is less humid than the surrounding rural areas because:
- The city is warmer, and higher temperatures lower the relative humidity
- Precipitation is quickly removed from the surface into drains. Surfaces in
towns are impermeable and water runs off quickly into drains. In rural areas
water is held in the soil and this raises the humidity of the air above.
- Vegetation in rural areas emits water vapour by the process of
transpiration.

17. PRECIPITATION

18. PRECIPITATION
Because Chicago is warmer, the hot air is more likely to rise and if it has a
high humidity it will cause convectional rainfall – short intense bursts of rain
and thunderstorms.
Urban areas produce particles of dust and these act as condensation nuclei,
which encourage rain production.
Because of the warmer temperatures there is less snow in the city than
surrounding areas.

19. WINDS

20. PRECIPITATION
Winds are lower in Chicago than the countryside because the buildings act
as barriers (wind beaks).
Long streets with tall buildings can act as wind tunnels in which winds are
funnelled down the street, and Chicago is notoriously gusty as winds are
channelled round buildings (eddying).

21. EXAMPLE
DIFFERENTIATION FOR A*

22. CHICAGO’S CITY HALL
Chicago’s most famous rooftop garden sits atop the City Hall, an 11-storey
office building in the Loop.
The City Hall and the adjacent Cook County buildings appear to most people as
one building spanning a city block bounded by LaSalle, Randolph, Clark and
Washington streets.
Initiated in 2000, the City Hall rooftop garden was conceived as a
demonstration project—a part of the City’s Urban Heat Island Initiative—to test
the benefits of green roofs and how they affect temperature and air quality.
The garden consists of 20,000 plants of more than 150 species, including shrubs,
vines and two trees. The plants were selected for their ability to thrive in
conditions on the roof, which is exposed to the sun and can be windy and arid.
Most are prairie plants native to the region.

24. MITIGATING THE URBAN HEAT ISLAND EFFECT
The rooftop garden mitigates the urban heat island effect by replacing what
was a ballasted, black tar roof with green plants.
The garden absorbs less heat from the sun than the tar roof, keeping the City
Hall cooler during summers and requiring less energy for air conditioning.
The garden also absorbs and uses rainwater.
It can retain 75% of a 1-inch rainfall before there is stormwater runoff into
sewers.