Trends in Mean Annual Minimum and Maximum Near Surface Temperature in Nairobi City, Kenya

This paper examines the long-term urban modification of mean annual conditions of near surface temperature in Nairobi City. Data from four weather stations situated in Nairobi were collected from the Kenya Meteorological Department for the period from 1966 to 1999 inclusive. The data included mean annual maximum and minimum temperatures, and was first subjected to homogeneity test before analysis. Both linear regression and Mann-Kendall rank test were used to discern the mean annual trends. Results show that the change of temperature over the thirty-four years study period is higher for minimum temperature than maximum temperature. The warming trends began earlier and are more significant at the urban stations than is the case at the sub-urban stations, an indication of the spread of urbanisation from the built-up Central Business District (CBD) to the suburbs. The established significant warming trends in minimum temperature, which are likely to reach higher proportions in future, pose serious challenges on climate and urban planning of the city. In particular the effect of increased minimum temperature on human physiological comfort, building and urban design, wind circulation and air pollution needs to be incorporated in future urban planning programmes of the city. 1. Introduction The concentration of socioeconomic activities in urban areas can be attributed to rapid expansion of industrialisation and urbanisation. In developing countries, unplanned and fast urbanisation in cities has caused environmental problems including increase in energy consumption, alteration of the local climate, and higher amounts of air pollution [1]. One of the consequences of these problems is a consistent rise in surface temperature within the urban environment [2–4]. Such noticeable surface temperature rise in urban environment is known as the “urban heat island”. The heat island effect is the basic climatic response to urbanisation, and it begins being noticed in small to medium size cities with a population of less than 100,000 inhabitants [5–7]. Nairobi’s population has increased from 120,000 when the first census was conducted in 1948 to 2, 137,570 in the 1999 population census [8]. The city’s growth rate of about seven percent per annum is one of the fastest city growth in Africa and is projected to grow faster in future [9]. The factors causing the increase in urban temperatures are outlined by Todhunter [10] and Oke [11]. The largest increase in temperatures occurs during calm and cloudless nights [12]. During windy periods, the urban-rural temperature