Thoughts on urban warmth
E. Linacre and B. Geerts
Some useful criticisms of Section 3.7 have been made by Professor Tim Oke, the leading expert on the study of urban temperatures. In view of these, readers might like to note the following points -
- The term 'urban heating' on p.72 of the book could be taken to imply that the reason a city centre is warmer than the surroundings, is the heat generated within the city. Of course, heat is indeed liberated there, but it is not the main reason for the temperature difference, as explained below. A better term for the observed temperature difference might be 'urban warmth'.
- Though the book refers to the maximum of the temperature difference 'often occurring at about 9pm' (note the qualifiers 'often' and 'about'), the fact is, obviously, that the time is relative to the time of sunset. So it depends on latitude and season of the year. Urban warmth is most visible between sunset and midnight.
- It is hard to judge the effect of Brisbane's buildings on the pattern of isotherms in Fig 3.17, without some indication of the city's limits and local topography. Better would be a map showing building densities in the early 1960's when the temperature measurements were taken. At that time the city was quite small, and Ipswich was clearly separated. Possibly the building density could be expressed in terms of the ratio of building height to road width; this ratio is pertinent to radiation geometry noted in point 5 below.
- The figure quoted as 'astonishing' for heat within Manhattan is now known to be wrong. The correct values given by Prof. Oke (1): annual 159 W m-2, summer 53 W m-2 and winter 265 W m-2.
- The list of causes of urban warmth omits what can be the most important for urban warmth at screen-level - that is the reduction of longwave-radiation loss from street level, caused by obstruction of the cold sky by the warmer buildings. The longwave-radiation cooling in a city canyon may be only two thirds that from an open, rural site, for instance.
That last figure (two thirds) is merely an example. A more exact calculation can be made (2). In the simplest case of a long roadway of width 2 W between buildings which are uniformly of height H, the fraction (of the longwave radiation lost in the canyon, to that lost in flat terrain) is cos(b
), where b
is the angle whose tangent is H/W. Thus a 20 m wide roadway between 10 m buildings (2-3 stories high) implies that half of the sky is visible from the middle of the roadway, so longwave-radiation cooling to the sky at night will be 71% (cos(45)) of what it would be in the open.
(1) Oke, T., 1987: Boundary Layer Climates (2nd edition, Routledge).
(2) Johnson, G.T. & I.D. Watson, 1984: The determination of view factors in an urban canyon. J. Climate Appl. Meteor. 23, 329-35.