an engineering geological characterisation of tropical clays - GBV

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1.6 Climate
The Nairobi area and its immediate environments, including the current study area, experience a
type of climate characterised by temperatures and rainfall patterns that vary according to location,
altitude and season of the year (Survey of Kenya, 1990). The generally high altitude (about 1700 m
above sea level) results in a climate that is not quite typically tropical, despite the closeness of these
areas to the Equator. Generally, temperatures are neither uncomfortably high during the days nor
low at nights (Kenya Meteorological Department, 2001). However, exceptional weather conditions
are also experienced during the hot, dry period shortly before the start of rains in March, and is
characterised by relatively high temperatures (up to 28 °C at mid-day), low relative humidity (down
to 10%), and occasional dust-storms caused by the moderately strong easterly wind blowing across
the area at this time of the year. The weather is also affected by the occurrance of the south-east
winds in the coastal region of Kenya during the period of June to September. The latter winds
frequently result in the formation of a cap of cloud over the high ground immediately east of the
Great Rift Valley (including Nairobi and the study area). The persistance of the cloud cover causes
low temperatures during the day (up to a maximum of 18° C); and also at nights as well as during
early morning hours (as low as 6° to 8° C). Very cold nights may also occur in the months of
January and February when the sky is clear (Kenya Meteorological Department, 2001).
Table 1.1. Mean maximum and minimum temperatures for each month of the year over a period of
50 years, for Nairobi city and the present study area (Kenya Meteorological Department, 2001).
Mean
Mean
Months
maximum minimum Mean range
(°C)
(°C)
(°C)
January 26,8 13,1 13,7
February 28 13,4 14,6
March 27,4 14,4 13
April 24,6 14,3 10,3
May 24,1 14,2 9,9
June 23,1 12,6 10,5
July 22,3 11,5 10,8
August 22,7 11,8 10,9
September 25,3 12,2 13,1
October 26,2 13,7 12,5
November 23,6 14,4 9,2
December 25,1 13,8 11,6
Year 24,9 13,3 11,6
Nairobi and the current study area are characterised by mean minimum temperatures of 11,5° to
14,4 °C; and mean maximum temperatures ranging from 22,3° to 28 °C. Low temperatures are
registered during the cool, dry and rather cloudy months of June to mid - October; with mean
maximum temperatures of 22,3° to 25,3 °C. High temperatures are recorded in the months of mid -
December to mid - March, with mean maximum temperatures in the range of 26,8° to 28 °C. The
weather could be generally described as warm to hot, and dry with sunny days during this latter
period of the year. The coldest month is July with an average monthly temperature of 16,9 °C;
while the hottest month is March with an average monthly temperature of 20,9 °C. The mean
monthly range across the year is from 9,2 °C in November to 14,6 °C in February. A mean annual
temperature of 19,5 °C is recorded for the study area; with a mean annual range of 11,6 °C. No
large seasonal changes in temperature therefore occur. The highest and lowest temperatures