The higher temperature in the city was ascertained in Nairobi, a city of the equatorial region, by analyzing the climatological data and by conducting temperature measurements in and around the city.
Nairobi is a city with the population of 267, 000 (1962). It is located at 1°17' S on the East African Plateau gently sloping toward the east. The altitude ranges from 1, 800 meters in the west to 1, 600 meters in the east. The climate seems to be modified locally by the presence of forests in the wetter west where the average annual precipitation exceeds 1, 000 mm., and by the extensive grasslands in the drier east where trees are totally lacking with about 840 mm. of annual precipitation. The climate of the built-up area may be affected mainly by the density of buildings, the greater friction near the ground surface, and the thermal characteristics of concrete or asphalt. The warming effect through heating by combustion is thought to be negligible in an equatorial city. Dust is not thought to be enough to give such an effect to reduce insolation as is expected in a large city.
The climatological data for three stations in and around Nairobi are available. The East African Meteorological Department at Dagoretti Corner (1, 798 meters above sea level) represents the western suburban area. The Eastleigh Air Port (1, 634 meters) is taken as a representative station for the urban area. Although it is located off the center of the city, there is no appropriate station in the city. The third station is the Nairobi International Air Port (1, 624 meters) representing the eastern suburban area.
The city temperature is characteristically higher than the suburban areas in so far as minimum temperature is concerned. The difference between Eastleigh and the International Air Port reaches 1.3°C (Table 1). When compared with large cities in the higher latitudes, it is rather small. But the city center would make a greater difference.
As for maximum temperature, there does not seem to be a conspicuous difference between the city and the suburbs. The city center has therefore a small diurnal range of temperature.
Dagoretti is cooler than the International Air Port by 2.2°C for maximum temperature and by 1.3°C for minimum temperature. But the temperatures for the two stations can be plotted on the altitude-temperature line calculated for all the stations in Kenya (Fig. 2). The low temperature at Dagoretti may be accounted for by the altitudinal difference.
It is also seen on the graph that the minimum temperature at Eastleigh is appreciably higher for the altitude, implying the urban effect.
In order to know the simultaneous distribution of temperature in Nairobi, temperature measurements were conducted with a thermister-thermometer installed on a landrover. Temperatures were read at some 120 places in the city. Reduction had to be made to a certain time, for it took the author about one and half hours to complete a round trip of measurement. Figs. 3-6 are the distribution maps of temperatures thus obtained.
Two warmest areas are always found on the maps. One is near the city center. But it does not always coincide with the area of the highest density of buildings. The vicinity of the Nairobi railway station is likely to be the warmest for some unknown reason. The other warm area is in the northeastern part of the city. Why this place is particularly warm-satisfactory explanations are not given for it.
The high city temperatures are discernible even in the daytime. There is a relatively steep gradient of temperatures in the northern and western periphery of the built-up area. It is interesting to note, however, that the temperature gradient becomes extremely gentle at 9 p. m., when, it has been reported, nocturnal radiation begins to make a marked city-suburbs, temperature difference.
