|
E. Linacre |
11/'97 |
Diagrams published by Sturman & Tapper (1) show temperatures at 2pm and 10.30pm on a calm winter’s day in Christchurch, New Zealand, in 1979. The centre of town was about 16° C in the afternoon, i.e. 2 K warmer than the coast 5 km away, but 9° C at night, i.e. 4 K warmer than the outskirts of the city. The night was clear and a 500 m deep inversion was present over the outskirts of the city. Measurements above the city at night show that lapse conditions, due to urban heating, were present to a height of about 300 m. There was a weak inversion to about 600 m and lapse conditions prevailed above that level. We conclude that on calm nights urban heating causes pollutant mixing in a deeper layer over cities than over the surrounding countryside.
The figures above are consistent with a relationship between the maximum urban heating observed in New Zealand and Australia, and city population. The maximum is about 2 K for populations of 1,000 people, 4 K for 100,000, 6 K for a city like Melbourne with about 3 million (2). The corresponding temperatures in North America are about 2 K, 8 K and 12 K, implying greater concentrations of energy consumption per unit area. The magnitude of urban heating in European cities is intermediate between Australian and US figures.
References
(1) Sturman, A.P. & N. Tapper, 1996: The Weather & Climate of Australia & New Zealand (Oxford), on page 332.
(2) as above but page 334, from Torok et al 1996.