Map from Google - Kenya

Kenya is bisected by the Equator, yet has three types of climate: hot and humid along the coast (zone 1 on the map), temperate in the west and south-west, where there are mountains and plateaus (zone 2 on the map), and finally hot and dry in the north and east (zone 3 on the map).

Climates in Kenya

In general, the warmest period in Kenya goes from February to March, while the coolest goes from July to August, although the seasonal variations in temperature are small. The temperatures and rainfall vary according to the prevailing winds, which determine two seasons: from October to March, the hot and originally dry winds coming from Arabia (called kaskazi) prevail, while from April to September the prevailing winds are cooler and wetter and come from the Indian Ocean (called Kuzi). At the beginning of the two periods, and in correspondence with the two passages of the sun at its zenith, there are the two rainy seasons: the "long rains" from March to May, and the less intense "short rains" from October to December, while in the western highlands it rains a lot even between June and September. The rainfall is more abundant along the coast and in the south-west (zone 1 and 2), where it typically ranges from 800 to 2,000 millimetres per year, while in the arid zone (3 on the map) it drops below 500 mm. The rains occur mainly in the form of downpour or thunderstorm in the afternoon or evening, and are pretty irregular: on the one hand, with the global warming the rainfall in some areas is declining, on the other hand, during the years of El Niño, the chance of heavy rains increases.

On the coast, it's hot from November to March, with average highs around 31/32 °C, but with peaks of 36/37 °C, and high humidity. The period from June to August is more pleasant, when the maximum temperature goes down to around 28 °C.
Here are the average temperatures in Malindi.
Average temperatures - Malindi
MalindiJanFebMarAprMayJunJulAugSepOctNovDec
MalindiJFMAMJJASOND
Min (°C)232424242323222222222323
Max (°C)313132312928272728303131

In January and February there is little rain, while in March and December, some showers may occur in the southern part of the coast (see Mombasa). The long rains season here occurs later than in the rest of Kenya, and runs from April to June, with May as the wettest month. The best months are August and September, the coolest ones, which are sunny and with not abundant rains, that come in the form of brief showers, and leave room for the sun. The short rains season is not pronounced in the northern part of the coast (see Lamu), and it's a little more evident in Malindi and Mombasa, with some showers in October and November, usually not intense. However, it must be said that it depends on the years: sometimes it can rain heavily even between October and December, and especially in November. On average, however, on the coast 1,000/1,100 millimetres of rain per year fall.
Here is the average precipitation in Malindi.
Average precipitation - Malindi
MalindiJanFebMarAprMayJunJulAugSepOctNovDecYear
MalindiJFMAMJJASONDY
Prec. (mm)1015351653001559065457075351060
Days223111712129766390

The sea is warm enough for swimming all year round: the water temperature goes from 25 °C in August to 29 °C in March and April.
Sea temperature - Malindi
MalindiJanFebMarAprMayJunJulAugSepOctNovDec
MalindiJFMAMJJASOND
Sea (°C)282829292827262526262828

Malindi

In the plateau, the climate is generally temperate. The temperature and rainfall vary according to altitude and slope exposure, in general, however, the mountains make easier the formation of thunderclouds, so the higher you climb, the higher the risk of rain, and even in dry seasons, some thunderstorms may break out. At higher altitudes, above 3,000 metres, night temperatures can drop below freezing, and occasionally this can happen above 2,500 metres.

The capital Nairobi is located near the Equator, at 1,600 metres above sea level. The altitude makes the temperature pleasant, but with cool nights, so that highs hover around 21/22 °C in the cooler months (June, July and August), and around 25/26 °C in the hottest months (January, February and March), while lows down to around 10/11 °C from June to September, and go up to 12/14 °C from January to April. In July and August, the sky is often cloudy even though there is little rain, it's cool at night and it can be even cold, in fact sometimes the temperature can drop to around 5 °C.
Here are the average temperatures.
Average temperatures - Nairobi
NairobiJanFebMarAprMayJunJulAugSepOctNovDec
NairobiJFMAMJJASOND
Min (°C)121213141311101011131313
Max (°C)252626242322212124252323

The average annual rainfall in Nairobi amounts to 1,000 mm. During the two rainy seasons, from March to May and October to December, it often rains, though even here the rains come in the form of thunderstorm, not affecting too much the sunshine hours. The sunniest months are January and February, although there can be a few showers even in this period.
Here is the average precipitation.
Average precipitation - Nairobi
NairobiJanFebMarAprMayJunJulAugSepOctNovDecYear
NairobiJFMAMJJASONDY
Prec. (mm)65559522017535202530551551001025
Days45916135344715893

The Masai Mara National Reserve, which borders the Serengeti in Tanzania, is located at an average altitude of around 1,500 metres. Here, more than 100 mm of rain per month fall from March to May and from October to December, and April is the rainiest month with 200 mm. However, there is not a completely dry season, although July is the driest month with about 60 mm.
Here are the average temperatures.
Average temperatures - Masai Mara
Masai MaraJanFebMarAprMayJunJulAugSepOctNovDec
Masai MaraJFMAMJJASOND
Min (°C)121212131312111112121212
Max (°C)282827262525252527282727

Lions in Masai Mara
The parks of the south-east of the country are generally warmer and more arid than the heart of the plateau. The Amboseli National Park is located in the south, at 1,100 metres above sea level, and a short distance from the border with Tanzania, beyond which lies the Kilimanjaro, looming huge on the horizon. Here there's a dry savannah, where elephants roam freely among many other species of animals such as lions, cheetahs, impalas, hyenas, giraffes, zebras, and there are Masai villages as well. Similar situation is found in the huge Tsavo National Park, which lies to the east, and is divided into two. Here, the altitude varies from 500 to 1,700 metres in the western park, and from 200 to 800 metres in the eastern park. Here are the average temperatures of Voi, located in this area, at 580 metres above sea level.
Average temperatures - Voi
VoiJanFebMarAprMayJunJulAugSepOctNovDec
VoiJFMAMJJASOND
Min (°C)202021202018181718192020
Max (°C)323333323029282829313131

As you can see from the average rainfall, in this area and at these lower middle altitudes, rainfall is not abundant, since it amounts to 550 mm per year.
Here is the average precipitation.
Average precipitation - Voi
VoiJanFebMarAprMayJunJulAugSepOctNovDecYear
VoiJFMAMJJASONDY
Prec. (mm)3530801003055101525105120555
Days538752123410858

The Chyulu Hills National Park is located at a higher elevation, ranging from 900 to 2,100 metres: above 1,800 metres there is a montane forest, but also a series of volcanic peaks and lava tunnels that make the landscape lunar.

In the highlands north-west of Nairobi, above 2,000 metres, the rain pattern is different than the rest of the country: in July and August, which are typically not too rainy in the rest of Kenya, the rains are plentiful, so much so that they go from 100 to 200 mm per month, mostly in the form of thunderstorm in the afternoon or evening. In this area (see Eldoret, Nakuru, or parks like Kakamega, South Nandi, Mount Elgon, Mau Forest), there's a peak in rainfall from April to September, while the relatively dry season goes from October to March.
Here is the average rainfall in Eldoret, at 2,100 metres above sea level.
Average precipitation - Eldoret
EldoretJanFebMarAprMayJunJulAugSepOctNovDecYear
EldoretJFMAMJJASONDY
Prec. (mm)35406515512095160175805055451080
Days4561312917168884110

And here is the average temperature.
Average temperatures - Eldoret
EldoretJanFebMarAprMayJunJulAugSepOctNovDec
EldoretJFMAMJJASOND
Min (°C)910111211910109101110
Max (°C)252626242423222223242324

Lake Nakuru, where you can watch pink flamingos, is located at 1,750 metres. Here are the average temperatures.
Average temperatures - Nakuru
NakuruJanFebMarAprMayJunJulAugSepOctNovDec
NakuruJFMAMJJASOND
Min (°C)9910121110101099109
Max (°C)272828262524242426252426

Lake Nakuru, rhino and flamingos

The highest peak in the country is Mount Kenya, 5,199 metres high, which has different climatic zones according to altitude and slope exposure (the south-east is the most humid): above 2,000 metres the montane forest is found, around 2,500 metres the bamboo on the north side which is more arid, above 3,000 metres the heath, above 4,000 m the high altitude desert. Above 4,700 metres there are also some glaciers, the largest of which is the Lewis Glacier, unfortunately retreating because of global warming.
West of Mount Kenya, the Aberdare Mountains reach 4,000 metres. Even Mount Elgon is very high, being 4,321 metres high, however, its top is located in Uganda.

Also the eastern portion of Lake Victoria, located at 1,100 metres above sea level, belongs to Kenya. Here are the average temperatures of Kisumu.
Average temperatures - Kisumu
KisumuJanFebMarAprMayJunJulAugSepOctNovDec
KisumuJFMAMJJASOND
Min (°C)181919191817171717181818
Max (°C)292929272726272728282828

We speak now of the hot and dry area.
The north is the driest and hottest area of Kenya, especially the areas located at lower altitudes. Lake Turkana (or Rudolph) is located at 365 metres above sea level; here the landscape is semi-desert in the south part and desert in the north, where the annual precipitation drops below 100 mm per year. In this area the weather is really hot all year round: in Lodwar, the daily average is around 30 °C in almost all months, except in July and August when it drops to 28/29 °C.
Average temperatures - Lodwar
LodwarJanFebMarAprMayJunJulAugSepOctNovDec
LodwarJFMAMJJASOND
Min (°C)222324252524242424252423
Max (°C)363636353534333435353535

The rains are sporadic and irregular, however it is more likely to see some showers in the two canonical rainy seasons of Kenya, from March to May and October to December. Here is the average precipitation.
Average precipitation - Lodwar
LodwarJanFebMarAprMayJunJulAugSepOctNovDecYear
LodwarJFMAMJJASONDY
Prec. (mm)852040258139381013162
Days11353131111223

Lake Turkana

Even the north-east, near the border with Ethiopia and Somalia is torrid and dry, and even here the poor and irregular rains occur during the two periods from March to May and October to November. In this torrid area, July and August are the best months, because they are less hot, although not by much.

When to go

For a beach holiday, on the coast the best months are August and September, because they are cooler and are quite sunny, with little rainfall. If you want, you can also choose January and February, which are the least rainy months of the year, but they are also hot and muggy. On the other hand, they are those in which the sea is calmer and clearer, therefore, the most suitable for diving.
To visit the arid region, July and August are the cooler months, but in reality there is not much difference compared with the rest of the year: the heat is still intense.
For a safari in national parks and in the highlands, you can choose from two periods: June to September, and January-February. In the first period there is a greater risk of showers in the afternoon and evening at high altitudes, in the second the chance of intense heat at lower altitudes. However, you should dress in layers, because in the early hours of the day and at higher altitudes, it can be cool throughout the year.
Recall that the equatorial sun is very strong, and is even stronger at high altitudes: the two months when the sun is strongest, because it passes overhead, are March and September.
The days last 12 hours throughout the year, so the sun rises at about six in the morning and sets at six pm, and is used by the local population as a clock.
The least advisable period is that of the long rains, from March to May in the interior, and from April to June on the coast, being typically the rainiest and the one presenting the greatest risk of flooding, particularly April in inland areas and May on the coast.

What to pack

For the plateau, in general, spring/autumn clothes, in layers, warmer as you go up in altitude, sun hat, sunscreen, raincoat for showers in rainy periods, lighter in the warm season and at low altitudes; for mountain climbing, hiking shoes; for the summit of Mount Kenya, throughout the year, clothing for cold weather, down jacket, Gore-Tex jacket, hat, gloves, scarf, sunglasses, sunscreen. For parks and reserves, neutral-coloured clothes, comfortable canvas shoes.
For the reef, equipment for snorkeling, water shoes or rubber soled shoes.

In winter (June to August): on the coast (Malindi, Mombasa) and Tsavo National Park, light clothing, a sweatshirt for the evening, a scarf for the sea breeze, light raincoat; for Lake Victoria, Nairobi, Masai Mara and the plateau at intermediate altitudes, light clothes for the day, sweater and jacket for the evening, raincoat or umbrella; for the highlands above 2,000 metres, spring/autumn clothes; above 3,000 metres, warm jacket and hat. For the north and Lake Turkana, loose fitting, light-coloured clothing, light and long shirt and pants of natural fibres (cotton, linen), a wide-brimmed hat, sunglasses, desert turban, sun hat.

In summer (December to February): on the coast (Malindi, Mombasa) and Tsavo National Park, light clothing of natural fibres, a light sweatshirt for the evening; for Lake Victoria, light clothing made of natural fibres, sun hat, a light sweatshirt for the evening, light raincoat or umbrella; for Nairobi, Masai Mara and the plateau at intermediate altitudes, light clothes for the day, jacket and sweater for the evening; for the highlands above 2,000 metres, spring/autumn clothes, raincoat or umbrella; above 3,000 metres, warm jacket and hat. For the north and Lake Turkana, loose fitting, light-coloured clothing, light and long shirt and pants of natural fibres (cotton, linen), a wide-brimmed hat, sunglasses, desert turban, sun hat.