KS1.1 LUKE HOWARD, TIM OKE AND THE STUDY OF URBAN CLIMATES Gerald Mills UCD, Dublin, Ireland. 1. INTRODUCTION 2. THE CLIMATE OF LONDON Urban climate research is concerned with the Luke Howard (1772-1864) is most famous for impacts that urbanized areas have on climate at all his classification of clouds (Hamblyn, 2001). scales. The bulk of this research work has been However, he is also the first to describe the carried out in the last sixty years. During this period development of the urban heat island effect over the progress has been hesitant with periods of rapid course of a year and to suggest its causes (Mills, progress separated by periods of consolidation. The 2007). Both of these contributions are evident in his reasons for this pattern are manifold and are monumental work, The Climate of London, the associated with asynchronous developments in second edition of which was published in 1833. The theory and instrumentation. In the urban context importance of this book for urban climatology is these problems have perhaps been accentuated by such that it has been republished by the the complexity of the urban atmospheric International Association for Urban Climate (Howard, environment, the lack of dedicated researchers and 2007). their relative isolation from one another. From 1806 to 1830 Howard made daily To a considerable extent many of the barriers to recordings of the pressure, temperature, humidity, progress have been removed. Advances in theory, precipitation and evaporation at locations outside observation and models now allow urban effects to London. In addition he maintained a diary of his be explored at a very detailed level. Moreover, observations and collated newspaper articles on grappling with such a complex, spatially any event of meteorological interest. The first heterogeneous, landscape has stimulated research volume of both editions contains his analysis of that has advanced boundary-layer studies generally. climate and is organized according to each This has occurred as interest in the urban atmospheric property. The preface contains an environment has increased because of the dramatic extensive discussion of the instrumentation increase in the planet’s urban population and the employed in making observations and includes his realization that urban-based activities have effects famous Essay on Clouds. at the local, regional and global scales. As a It is important to note that Howard was not consequence there is now a growing community of concerned about the climate of the city of London scholars focused on urban climate. This is evident but rather with climate generally as viewed from the in some of the large scale city projects that draw vantage point of London. As such, he became upon international teams to jointly examine urban concerned at the deviation between his recordings effects (e.g. Rotach et al., 2005). and those gathered at the Royal Society in central Nevertheless, there remain significant ‘gaps’ in London (Figure 1). From his analysis he concludes the field. Oke (2006) outlined the stages in the that the: Mean Temperature of the Climate, … is development of an urban climate science: strictly about 48.50° Fahr.: but in the denser parts of Conceptualization; Theory; Observation; Modelling; the metropolis, the heat is raised, by the effect of Validation; Application and; Evaluation. While the the population and fires, to 50.50°; and it must be first three have advanced significantly, the others proportionately affected in the suburban parts. The are less developed. In particular, the challenge of excess of the Temperature of the city varies through incorporating urban climate knowledge into planning the year, being least in spring, and greatest in and design practice remains. winter; and it belongs, in strictness, to the nights; which average three degrees and seven-tenths In this paper I will look at the development of urban warmer than in the country; while the heat of the climate research with a particular focus on the study day, owing without doubt to the interception of a of the urban temperature effect. In doing so, I will portion of the solar rays by a veil of smoke, falls, on discuss the respective approaches and a mean of years, about a third of a degree short of contributions of Luke Howard and Tim Oke. that in the open plain. Whereas Howard’s research on the climate of London represents the beginning of the field, Oke’s Howard identifies the UHI as the difference research is synonymous with the modern history of between the air temperature in the city and that in urban climate research. the country (∆Tu-r) and hypothesizes that this difference must increase from the edge of the city *Corresponding author address: Gerald Mills, toward its centre. His analysis also showed that School of Geography, Planning & Environmental Policy, ∆Tu-r is primarily a night-time phenomenon which is Newman Building, UCD, Dublin 4, Ireland; greatest in winter months. email: [email protected] buildings: the country has an almost inexhaustible store of moisture to supply its evaporation ― that of the city is very speedily exhausted, even after heavy rain. When we consider that radiation to the sky, the contact of fresh breezes, and evaporation, are the three principal impediments to the daily accumulation of heat at the surface, we shall perceive that a city like London ought to be more heated by the summer sun than the country around it. Figure 1. The difference between the average His analysis is superb as he has identified most of monthly mean temperatures for London and the the causes for the UHI: country based on data for 1807-1816. 1. Anthropogenic sources of heat, particularly in winter. Howard goes on to suggest the causes of the 2. The geometry of urban surfaces that ‘traps’ UHI: radiation and obstructs ‘free radiation to the sky’. That the superior temperature of the bodies of men 3. The effect of urban ‘roughness’ in impeding the and animals is capable of elevating, in a small passage of ‘the light winds of summer’. proportion, the Mean heat of a city or populous 4. The lack of evaporation in the country. tract of country in a temperate latitude, is a The only aspect of urban areas that he did not proposition which will scarcely be disputed…But consider is the thermal properties of urban materials. the proportion of warmth which is induced in a city by the Population, must be far less considerable Elsewhere, Howard considers the rate at which than that which emanates from the fires: the the urban area warms and cools relative to the greater part of which are kept up for the very surrounding country: But this effect is not produced purpose of preventing the sensation attending the suddenly. For while, in the forenoon, a proportion of escape of heat from our bodies. A temperature the walls are exposed to the sun, the remainder are equal to that of Spring is hence maintained, in the in shade, and casting a shadow on the intervening depth of Winter, in the included part of the ground. These are receiving, however, in the wider atmosphere, which, as it escapes from the houses, streets, the reflected rays from the walls opposed to is continually renewed: another and more them; which they return to the former, when visited considerable portion of heated air is continually in their turn by the sun. Hence in the narrow streets, poured into the common mass from the chimnies; especially those that run East and West, it is to which, lastly, we have to add the heat diffused in generally cooler than in the larger ones, and in the all directions, from founderies, breweries, steam squares. Hence too, in the morning of a hot day, it engines, and other manufacturing and culinary fires. is sensibly cooler in London than in the country; and The real matter of surprise, when we contemplate in the evening sensibly warmer. For the hottest time so many sources of heat in a city is, that the effect in a city, relatively to the hour of the day, must be on the Thermometer is not more considerable. that, when the second set of vertical surfaces To return to the proportions held by the excess having become heated by the Western sun, the of London, it is greater in winter than in summer, passenger is placed between two skreens, the one and it sinks gradually to its lowest amount as the reflecting the heat it is receiving, the other radiating temperature advances in the spring, all which is that which it has received. Many of my readers must consistent with the supposition, that in winter it is recollect having felt the heat of a Western wall, in principally due to the heat diffused by the fires. passing under it long after sunset. It appears that London does not wholly lose its In this passage he identifies the processes of superiority of temperature, by the extinction of most multiple reflection of solar radiation and the of the fires in Spring: on the contrary, it is resumed exchange of terrestrial radiation within streets. in a large proportion in the Sixth month, and Critically, Howard has correctly linked observations continues through the warm season. It is probable, to the three-dimensional, micro-scale, urban context therefore, that the Sun in summer actually warms within which they are situated. the air of the city more than it does that of the country around. Several causes may be supposed Unfortunately Howard did not examine the to contribute to this: the country presents for the correspondence between clouds and the magnitude most part a plain surface, which radiates freely to of the UHI (∆Tu-r).