City trees and the urban climate

E. Linacre and B. Geerts

10/'97

Trees occur in cities at various scales (Table 1). They suppress surface winds, either by the formation and shedding of eddies in the lee of isolated trees, or, in the case of a close-planted canopy, by containing mechanical and buoyant turbulence to the top of the canopy. Some turbulence results from hydrodynamic instability (3), associated with the roughness of the urban profile. Trees change the urban profile, increasing the roughness if they protrude above the buildings, or decreasing it by smoothing the outline as a result of filling in gaps. The roughness is quantified by a 'roughness length' z_o, which depends on the frontal area of the average element divided by the ground area it occupies. Trees within an industrial park or a region of apartments may reduce z_o from 1.2 m to 0.7 m, whereas in a suburb of (low) detached houses the effect is to raise z_o from 0.25 m to 0.40 m (1). Thus trees slightly increase winds (at 20 m above ground) in the first case and reduce them in the second. However, these changes are negligible at ground level.

Table 1. Typical dimensions of various kinds of urban tree canopy in a city of about a million people (1,2)

Unit	Built features	Tree features	Climate phenomena	height: m	width: m	length: m
building	garden	single tree	wake, shadow	10	10	10
canyon	street	avenue, boulevard, shelterbelt	street vortex, shade, temperature	10	30	300
block	city block, factory	park, wood	local breeze	-	500 m	5 km
land-use zone	residence, industry, city centre	greenbelt, suburban forest	air quality, topoclimates	-	5 km	5 km
city	built-up area	urban forest	heat, humidity, city breeze, smog dome, rainfall modification	-	25 km	25 km

Trees differ from buildings in that heat is generated within the latter by human occupancy, and temperatures are not kept close to the air temperature by evaporation. The temperatures of leaves of well-watered plants rarely differ more than two or three degrees from air temperature (4, 5, 6). More than 70% of incoming energy is commonly used in evaporation.

Another difference is the albedo. That of building materials is often higher than the 22% typical of green foliage, but the multiple surfaces of city buildings tend to trap reflected radiation, effectively decreasing the albedo overall. Consequently, there is usually less than 5% albedo difference between built-up and foliated surfaces.

Taking into account the various differences, it is possible to compare the net effect by calculation of the surface temperatures at 3pm on a clear day in summer, within canopies of either trees or buildings entirely. The forest is typically 8 K cooler, and the difference of air temperatures in the same circumstance is estimated as 3.5 K (1). With 50% tree cover, the differences would be about half as much. Hence, trees can improve daytime conditions significantly in hot climates.

Also, the reduced heating of a city by trees decreases the 'country breeze', which blows sometimes in the evening from the rural environs towards the warmer city centre.

Apart from the modest effects of trees on topoclimates within city regions, the vegetation can considerably modify winds around individual buildings. There one can differentiate between vertical winds down the face of an isolated tall building, and the cyclone generated within an almost enclosed courtyard, and the accelerated horizontal winds caused by funnelling down a city canyon or round the sides of a tall building. In the case of a single tall building, winds impacting at below 0.7 times its height (called the 'stagnation point') are directed downwards, with occasional pulses of winds appropriate to that height (1). If these gusts exceed 22 m/s (ie 80 km/h) at least once a year, the situation at ground level is regarded as unsatisfactory, since such winds can blow a pedestrian into the path of traffic (7). In some cases of courtyard spaces, as in shopping complexes, the induced whirling winds have caused commercial failure. Spaced trees along streets or beside tall buildings can substantially reduce wind speeds there (8). However, one has to balance the advantage of reduced cold winds in winter against the disadvantage of poorer ventilation in summer.

Often trees are planted near houses to give shade in summer, on the side facing the Sun - the north side in the southern hemisphere. Unfortunately, this also entails less radiation onto the house in winter, when the heat would be welcome. Even deciduous trees in winter cut out about a third of the radiation.

It is concluded that care is required when planting trees to improve the environment within cities.

References

(1) Oke, T.R. 1989. The micrometeorology of the urban forest. Phil. Trans. Royal Soc., London. B324, 335-49.

(2) Finnigan, J. 1994. Improving the physical urban environment with trees. Proc 1994 Second National Urban Tree Seminar. (Royal Australian Inst. Parks an Recreation) 22pp.

(3) Raupach, M.R. 1992. Drag and drag partition on rough surfaces. Boundary-Layer Meteor. ,60, 375-95.

(4) Linacre, E.T. 1964. A note on a feature of leaf and air temperatures. Agric. Meteor., 1, 66-72.

(5) Linacre, E.T. 1967. Further notes on a feature of leaf and air temperatures. Archiv. Meteor. Geophys. Bioklim. B15, 422-36.

(6) Linacre, E.T. 1972. Leaf temperatures, diffusion resistances and transpiration. Agric. Meteor. 10, 365-82.

(7) Melbourne, W.H. 1978. Criteria for environmental wind conditions. J. Wind Engineering an Indust. Aerodynamics, 3, 241-9.

(8) Gandemer, J. 1981. The aerodynamic characteristics of windbreaks, resulting in empirical design rules. J. Wind Engineering an Indust. Aerodynamics , 4, 15-36.