Daily climatic records have been kept at Danum Valley Field Centre since July 1985. The climate at Danum is equatorial with hot and wet conditions prevailing throughout the year. The area, however, is subject to the seasonal wind and other changes associated with the Indo-Australian Monsoon System and seasonal migrations of the Intertropical Convergence Zone (ITCZ). Thus the area lies under the influence of the northeast monsoon from November to March and the southwest monsoon from June to August, with transitional periods of variable winds associated with the ITCZ in months around the equinoxes in April-May and September-October.
The mean annual temperature is 26.9oC with an annual range of just 1.7oC between the coldest (January: 26.0oC) and warmest (May: 27.7oC) monthly means. Mean maxima range from 32.4oC in May to 29.5oC in January. The mean diurnal range between maximum and minimum temperatures is 8.6oC, varying from 9.4oC in the SW monsoon months to 7.1oC in the wettest month of January. Temperatures in excess of 34ºC are rare, occurring only during prolonged dry periods. The highest temperatures recorded at Danum Valley have been 36.5oC in April 1992 and 36.3ºC in April 1998, both during ENSO events. Minimum temperatures rarely fall below 19oC. Mean relative humidity at 1400 hours averages 79% and 96% at 0800 hours. In prolonged sunny weather in the 1998 ENSO drought, afternoon relative humidities as low as 61 % were recorded, but in general relative humidities of less than 70% are rare. Danum receives between 1800 and 2000 hours of bright sunshine per year (as recorded with a Campbell-Stokes sunshine recorder).
The mean annual rainfall (mid-1985 to mid-2011) at Danum is 2881 mm. The lowest annual rainfall of 1918mm occurred in 1997, which was an ENSO year, and the highest annual total of 3539 mm was recorded in 2003. Mean monthly rainfall ranges from 167mm in April to 318 mm in January down to 167 mm in April. Rainfall tends to be highest during the northeast monsoon months from November to February and also in the transition months following the equinoxes (May and October). The highest recorded monthly total to date has been 798 mm in February 2006 (with the highest daily fall of 182 mm recorded on 9th February 2006). Rainfall is generally lowest during March and April, mainly because they are the most drought-prone months during ENSO events, and also in August and September when the south-westerly monsoon is at its height.
Daily rainfalls of 50 mm or greater occur on average 9.0 times per annum. Of the 23 daily rainfalls exceeding 100 mm in the 26-year period mid-1985 to mid-2011, 15 occurred in the NE monsoon months of December, January and February. The highest recorded daily falls are 182.2 mm (9th February 2006), 177.2 mm (27th March 1988) and 162.5 mm (January 19th 1996). On February 5th 2011, 112 mm of rain fell at Danum in less than two hours.
Although rain occurs throughout the year, Danum is subject to occasional dry periods usually associated with El Niño-Southern Oscillation (ENSO) events, during which pressure is higher than normal over Indonesia and Malaysia, convectional activity is dampened and cloudiness and rainfall is reduced. In northern Borneo, dry periods tend to be most intense from February to mid-May in both Years 1 and 2 of an ENSO event, which is typically around 18 months in duration. At Danum, such droughts are intermediate in length and intensity compared with the severe droughts of eastern Borneo and Sulawesi and the short dry periods of Sarawak, Brunei and south-west Sabah. The main ENSO events to affect Danum since 1980 occurred in 1982-83, 1991-94, 1997-98 and 2010. The 1997-98 event was particularly intense and resulted in extensive partial defoliation of canopy trees by late April and early May 1998
There is growing evidence of recent climatic change both at Danum and elsewhere in Sabah. At DVFC the five-year running mean of annual temperature has risen by 0.5oC from 26.7oC in 1986-90 to 27.2oC in 2006-10, with the highest annual mean of 27.6oC recorded in 2010. At Kota Kinabalu the five-year mean has risen by 0.8oC from 26.4o in 1968-72 to 27.3-27.5oC since 1990, though some of the increase might be due to expansion of the airport and urbanization.
At Danum, annual rainfall in the years 1999-2010 was 14 % higher than in the years 1985-1998 and the frequencies of large daily rainfalls in excess of 50 mm, 80 mm and 100 mm thresholds have shown substantial increases (by 33%, 40% and 13 % respectively). Longer-term daily records at Sandakan, Kota Kinabalu and Tawau extending back to 1906 also indicate a recent upswing in large rainstorms that has intensified since 1999. There has also been an increase in dry period magnitude-frequency in Sabah since 1967, though the period 1877-1915 was equally drought-prone. The increase in annual rainfall and in the magnitude-frequency of large rainstorms are in line with the latest IPCC predictions for northern Borneo.
