Transcript Urban Heat Island - Cal State L.A. - Cal State LA

Urban Heat Island
04_21.JPG
05_03.JPG
05_02.JPG
Air temperature measurements:
Thermometer is located in the shade at
about 1.5 meters above a short grass
surface in an open field.
Three temperature units: ºC, ºF, º K.
05_01.JPG
Diurnal Air Temperature
04_14.JPG
Surplus means absorbed insolation is higher than outgoing longwave radiation
And thus air temperature increases;
Deficit means absorbed insolation is lower than outgoing longwave radiation and
thus air temperature decreases.
UHI: Urban heat island Intensity
difference between maximum urban center air temperature
and background rural air temperature
04_22.JPG
UHI varies in time and space as result of
atmospheric conditions, location, and
urban characteristics.
(1) UHI reaches maximum at a few hours
after sunset, and minimum in the middle
of the day
(2) Seasonal patterns of UHI in US cities
(summer seems to be the strongest)
(3) UHI Increases with population increase
Why N. America has higher rate of increase?
(4) UHI is less during weekend than week days in winter,
no difference in summer
Urban night time temperature is much more likely to be higher
Urban day time temperature could be lower in warm season
Vertical structure:
Inversion (air temperature increases with height) is more
common during night over rural areas while the urban
area is almost isothermal and less stable
Steep lapse rate (air temperature decreases with height) is
found in strong UHI city during day.
Urban
Rural
Impact of wind (Fig 5.21)
UHI decreases with increasing wind speed. Different city has
different threshold for urban heat island to disappear. For
example, critical wind speed for breaking up UHI is about
10m/sec at 10 meter rural site (for 1 million population)
(smaller for smaller town)
Ideal condition for UHI: weak wind, cloudless sky.
Causes of canopy layer of urban heat isle
a) increased RL↓ due to absorption of RL↑ and reemission by pollution
b) decreased net RL ↑ loss from canyons due to reduction
in sky-view factor by building.
c) greater shortwave absorption due to effect of canyon
geometry on albedo
d) greater day heat storage due to thermal properties of
urban materials and its nocturnal release.
e) anthropogenic heat from buildings
f) decreased evaporation due to removal of vegetation
and surface “water proofing” of city
g) decreased loss of sensible heat due to reduced winds
in canopy.
High Pressure and Urban Heat Island Spells
Disaster!
The heat wave in July 1995 in Chicago was one
of the worst weather-related disasters in Illinois
history with approximately 525 deaths over a
5-day period.
STATION: CHICAGO_MIDWAY_AP_3_SW, IL
(Station ID: 111577)
Year
1995
1995
1995
1995
1995
1995
1995
1995
1995
1995
1995
1995
1995
1995
1995
1995
1995
Mo
07
07
07
07
07
07
07
07
07
07
07
07
07
07
07
07
07
Dy
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
Precipitation
(in)
0.00
0.00
0.00
0.28
0.09
0.00
0.00
0.00
0.04
0.00
0.00
0.00
0.00
0.00
0.47
0.00
0.00
|<--temperature->|
High
Low
Mean
(F)
(F)
(F)
77
60
69
78
57
68
83
63
73
86
69
78
87
67
77
81
69
75
81
63
72
84
60
72
85
66
76
90
64
77
90
73
82
98
76
87
106
81
94
102
84
93
99
77
88
94
76
85
89
73
81
Heating and cooling degree days
Heating degree days: accumulation of all
degrees below a certain threshold (65ºF)
throughout of a year.
Cooling degree days: accumulation of all
degrees above a certain threshold (65ºF)
throughout of a year.
• On average, HDD is 8% less in cities (46%
difference in LA urban and airport; Table 5.8).
• CDD is 12% more on Average in cities.
(LA 48%, NY 11%).
Last Freezing date and freeze-thaw cycles
Urban has half of the frequency of freeze-thaw cycles in Baltimore, Maryland
Mitigation Measures
• Reflective roofing (increase albedo)
• Green roofs (increase latent heat)