The climate of Mount Kenya has played a critical role in the development of the mountain, influencing the topography and ecology amongst other factors. The area around Mount Kenya is covered by a comparably large number of weather station data with long measurements series and thus the climate is well recorded.[1] It has a typical equatorial mountain climate which Hedberg described as winter every night and summer every day.[2]
The year is divided into two distinct wet seasons and two distinct dry seasons which mirror the wet and dry seasons in the Kenyan lowlands. As Mount Kenya ranges in height from 1,374 metres (4,508 ft) to 5,199 metres (17,057 ft) the climate varies considerably over the mountain and has different zones of influence. The lower, south eastern slopes are the wettest as the predominant weather system comes from the Indian ocean. This leads to very dense montane forest on these slopes. High on the mountain most of the precipitation falls as snow, but the most important water source is frost. Combined, these feed 11 glaciers.
The current climate on Mount Kenya is wet, but drier than it has been in the past. The temperatures span a wide range, which diminishes with altitude. In the lower alpine zone they usually don't go below 12 °C (54 °F).[3] Snow and rain are common from March to December, but especially in the two wet seasons. The wet seasons combined account for 5/6 of the annual precipitation. The monsoon, which controls the wet and dry seasons, means that most of the year there are south-easterly winds, but during January and February the dominant wind direction is north-easterly.
Mount Kenya, like most locations in the tropics, has two wet seasons and two dry seasons as a result of the monsoon. From mid-March to June the heavy rain season, known as the long rains, brings approximately half of the annual rainfall on the mountain.[4] This is followed by the wetter of the two dry seasons which lasts until September. October to December are the short rains when the mountain receives approximately a third of its rainfall total. Finally from December to mid-March is the dry, dry season when the mountain experiences the least rain.
Mount Kenya straddles the equator. This means during the northern hemisphere summer the Sun is to the north of the mountain. The altitude and aspect of the watersheds and main peaks results in the north side of the upper mountain being in summer conditions. Simultaneously, the southern side is experiencing winter conditions. Once it is the southern hemisphere summer, the situation reverses.
The low pressure belt around the equator known as the Intertropical Convergence Zone (ITCZ) is responsible for the wet and dry seasons of Mount Kenya.[5] During the two dry seasons the ITCZ is over Arabia during July and southern Tanzania and northern Zambia in March. The belt of low pressure passes over Kenya as it swaps between its two extremes and Mount Kenya, and Kenya, experience wet seasons. The amount of rain fluctuates from year to year and is dependent on sea surface temperatures in the Atlantic and Indian Ocean as well as El Niño amongst other factors.[6] Warmer seas and El Niño both result in enhanced rainfall.[7]