SOME FEATURES OF DIURNAL RAINSTORMS OVER IBADAN METROPOLIS, NIGERIA

Abstract

The seasonal characteristics-sequence, amount, duration and intensity of rainstorms during various periods of the day, which determine the exact amount of moisture available have received little attention. This study investigated the diurnal cycles of rainstorms over Ibadan, Nigeria. A two-year hourly data on the characteristics of rainstorms for 50 stations were processed to generate statistics on the amplitude and phases of diurnal cycles of rainstorms in Ibadan. Their temporal variations were analysed using descriptive statistics. Of the 154 rainstorms studied, 14.3%, 30.7%, 21.7% and 33.9% occurred during dry, early, late and rainy seasons. The highest frequency of rainstorms during the dry season was recorded in February and November, in the late afternoon and early evening between 1500 h and 1800 h, with values of 22.2 % and 33.4%, respectively. Over 50% of the frequency of rainstorms was recorded in the midnight and early morning between 0000 h and 0600 h during the early rainy season, especially in April. The highest frequency was recorded at midnight between 0000 h and 0300 h, with a value of 27.2%. During rainy season months, the highest frequency of rainstorms was recorded in June, in the afternoon between 1200 h and 1500 h, with a value of 17.7%, and 29.5% of the total frequency of rainstorms was recorded as the highest in the afternoon between 1200 h and 1500 h during late rainy season months, other rainstorms characteristics studied showed a similar pattern. There was a general tendency for rainstorms to decrease from midnight to late morning between 0000 h to 0900 h. Most water in rainstorms occurring late in the afternoon is available for soil moisture replenishment since little evaporation happens at night.