Week 2 & 3: The Urban Heat Island
1. Most obvious climatological manifestation of urbanization is trend towards higher air
temperatures in cities. Also it is the theme most frequently discussed in literature since its
discovery by Luke Howard, 1818, in book on urban climatology. He also created term,
“city fog”. Howard wrote about London temperatures being warmer than the country.
Footnote: ..”night is 3.70o warmer, and day .34o cooler in city than in country.” The
literature is filled with other comparisons for all major cities, some towns and even malls.
2. Closed isotherms separate city from general temperature field, which forms the UHI.
3. Even single building complex (King Hall) shows different microclimate from equalsized land in natural state.
4. UHI varies in time and space as result of meteorology, location and urban
characteristics. UHI consists of ‘peak’, ‘plateau’, and ‘cliff’, where rural/urban boundary
exhibits a steep temp. gradient or cliff to UHI.
Especially N. American cities show final peak at core where
urban max temperature is reached. Difference between this
max value and background rural T defines the UHI, or
ΔTu-r.
6. Population. Oke found that as city pop. Increases so does its UHI max.
7. Winds, clouds. ΔTu-r is inversely related to wind speed (u) and cloud cover. Ideal
conditions for max UHI are weak winds, cloudless sky. Critical wind speed for breaking
up UHI is about 10 m/sec at 10 m. rural site (for 1 mill population). Critical speed less for
smaller towns.
8. Diurnal variation. UHI max usually a few hours after sunset and smallest in the middle
of the day. Some cities, ΔT even negative around mid-day. Growth of UHI particularly
rapid following sunset due to difference between urban and rural cooling rates.
9. Causes of canopy layer urban heat isle (Oke):
a. increased RL↓ due to absorption of outgoing RL and re-emission by pollution.
b. decreased net RL↑ loss from canyons due to reduction in sky-view factor (SVF) by
buildings.
c. greater shortwave radiation absorbed 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 (QF) from building sides.
f. decreased evaporation (QE) due to removal of vegetation and surface ‘water proofing’
of city.
g. decreased loss of sensible heat (H) due to reduced winds in canopy.
In summer, c & d, and to lesser extent e & f may combine to make canopy store sensible
heat by day, and after sunset, b & g prevent rapid dissipation and hence keep urban temps
higher than country.
In winter, role of e likely becomes more important.
Daytime negative UHI may be restricted to cities with deep and narrow canyons near
core. This provides shade so that c only works above ground.
Its unlikely a is important within the canopy.
10. City temperatures increase with population. Example, after WWII Tokyo destroyed,
but rapidly rebuilt and urban temp rose nearly 1oC. Daily Tmax rose at rate of o.036/yr,
but Tmin rose faster by 0.047o/yr during reconstruction interval (1946-1963).
11. Weekdays vs. weekend. Weekend UHI usually less.
12. Vertical temperatures. Less stable temperature profile over cities due to UHI. Also
UHI stronger when atmosphere more stable. At night, rural areas often have inversion,
while city heat breaks up inversions.
13. Consequences of UHI. (a) heating, cooling degree days. LA 46% difference in
heating degree days (HDD) between urban and airport. NY 7%. Cities average about
12% difference in CDD, LA-48%, NY-11%. (b) Frost-free season or growing season is
longer in cities. Munich has 60 more days than rural, Moscow 30. Also snow melts
earlier, sticks later in cities than rural.
Bibliography
Comrie, A.C., 2000: Mapping a wind-modified urban heat island in Tucson, Arizona.
Bull. of Amer. Meteorol. Soc., 81, 2417-2431.
Klysik, K. and Fortuniak, K., 1999: Temporal and spatial characteristics of the urban heat
island of Lodz, Poland. Atmospheric Environment, 33, 3885-3895.
Magee, N., Curtis, J. and Wendler, G., 1999: The urban heat island effect at Fairbanks,
Alaska. Theoretical and Applied Climatology, 64, 39-47.
Oke, T.R., 1973: City size and the urban heat island. Atmospheric Environment, 7, 769779.
Runnalls, K.E. and Oke, T.R., 2000. Dynamics and controls of the near surface heat
island of Vancouver, British Columbia. Physical Geography, 21, 283-304.
Torok, S.J., Morris, C.J.G., Skinner, C. and Plummer, N., 2001: Urban heat island
features of southeast Australian towns. Australian Meteorology Magazine, 50, 1-13.
Project Atlanta (UHI): http://wwwghcc.msfc.nasa.gov .