The climate of Mount Kenya has played a critical role in the development of the mountain, influencing the topography and ecology amongst other factors.
It has a typical equatorial mountain climate which Hedberg described as winter every night and summer every day. Mount Kenya is home to one of the Global Atmosphere Watch’s atmospheric monitoring stations.
Seasons
In January the Inter tropical Convergence Zone is at its southern extreme over the Indian Ocean. In July it is at its northern extreme over Tibet and Arabia. As it passes over the equator, Mount Kenya experiences a wet season.
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 m (4,508 ft) to 5,199 m (17,057 ft) the climate varies considerably over the mountain and has different zones of influence.
The lower, southeastern slopes are the wettest as the predominant weather system comes from the Indian Ocean. This rainfall supports dense montane forests on these slopes. High on the mountain most of the precipitation falls as snow. Combined, these water sources 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 temperature usually do not go below 12 °C (54 °F). Snow and rain are common from March to December, but especially in the two wet seasons.
The wet season’s 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 dominate 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.
This is followed by the wetter of the two dry seasons which lasts until September. October to December is the short rain 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.
Daily Weather Pattern
During the dry season, the mountain almost always follows the same daily weather pattern. Large daily temperature fluctuations occur which led Hedberg to exclaim winter every night and summer every day.
There is variation in minimum and maximum temperatures day to day, but the standard deviation of the mean hourly pattern is small.
In the dry season, mornings are typically clear and cool, but the mountain is hidden in clouds by mid-day. A typical day is clear and cool in the morning with low humidity.
The mountain is in direct sunlight which causes the temperatures to rise quickly with the warmest temperatures occurring between 0900 and 1200. This corresponds to a maximum in the pressure, usually around 10 am.
Low on the mountain, between 2,400 m (7,874 ft) and 3,900 m (12,795 ft), clouds begin to form over the western forest zone, due to moist air from Lake Victoria.
The anabatic winds caused by warm rising air gradually bring these clouds to the summit region in the afternoon. Around 1500 there is a minimum in sunlight and a maximum in humidity causing the actual and perceived temperature to drop.
At 1600 there is a minimum of pressure. This daily cover of clouds protects the glaciers on the southwest of the mountain which would otherwise get direct sun every day, enhancing their melt.
The upwelling cloud eventually reaches the dry easterly air streams and dissipates, leading to a clear sky by 5 pm. There is another maximum temperature associated with this.
Being an equatorial mountain the daylight hours are constant with twelve-hour days. Sunrise is about 0530 with the sun setting at 1730. Over the course of the year, there is a one-minute difference between the shortest and longest days.
At night, the sky is usually clear with katabatic winds blowing down the valleys. Above the lower alpine zone, there is usually frost every night