Urban Heat Island. Air temperature measurements: Thermometer is located in the shade at about 1.5 meters above a short grass surface in an open field.

Urban Heat Island

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Air temperature measurements:Thermometer is located in the shade at about 1.5 meters above a short grass surface in an open field.

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Three temperature units: ºC, ºF, º K.

Diurnal Air Temperature

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Surplus means absorbed insolation is higher than outgoing longwave radiationAnd thus air temperature increases;Deficit means absorbed insolation is lower than outgoing longwave radiation and thus air temperature decreases.

UHI: Urban heat island Intensitydifference between maximum urban center air temperature and background rural air temperature

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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 higherUrban 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.

Rural

Urban

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 re-emission 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 buildingsf) decreased evaporation due to removal of vegetation

and surface “water proofing” of cityg) decreased loss of sensible heat due to reduced winds

in canopy.

STATION: CHICAGO_MIDWAY_AP_3_SW, IL (Station ID: 111577)

Precip- |<--temperature->| itation High Low MeanYear Mo Dy (in) (F) (F) (F)1995 07 01 0.00 77 60 691995 07 02 0.00 78 57 681995 07 03 0.00 83 63 73 1995 07 04 0.28 86 69 781995 07 05 0.09 87 67 771995 07 06 0.00 81 69 751995 07 07 0.00 81 63 721995 07 08 0.00 84 60 721995 07 09 0.04 85 66 761995 07 10 0.00 90 64 771995 07 11 0.00 90 73 821995 07 12 0.00 98 76 871995 07 13 0.00 106 81 941995 07 14 0.00 102 84 93 1995 07 15 0.47 99 77 881995 07 16 0.00 94 76 851995 07 17 0.00 89 73 81

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.

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)