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 31 to 80 inches per year, while in the arid zone (3 on the map) it drops below 20 in. 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 88/90 °F, but with peaks of 97/100 °F, and high humidity. The period from June to August is more pleasant, when the maximum temperature goes down to around 82 °F.
Here are the average temperatures in Malindi.
Average temperatures - Malindi
MalindiJanFebMarAprMayJunJulAugSepOctNovDec
MalindiJFMAMJJASOND
Min (°F)737575757373727272727373
Max (°F)888890888482818182868888

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 40/43 inches of rain per year fall.
Here is the average precipitation in Malindi.
Average precipitation - Malindi
MalindiJanFebMarAprMayJunJulAugSepOctNovDecYear
MalindiJFMAMJJASONDY
Prec. (in)0.40.61.46.511.86.13.52.61.82.831.441.7
Days223111712129766390

The sea is warm enough for swimming all year round: the water temperature goes from 77 °F in August to 84 °F in March and April.
Sea temperature - Malindi
MalindiJanFebMarAprMayJunJulAugSepOctNovDec
MalindiJFMAMJJASOND
Sea (°F)828284848281797779798282

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 10,000 feet, night temperatures can drop below freezing, and occasionally this can happen above 8,000 feet.

The capital Nairobi is located near the Equator, at 5,000 feet above sea level. The altitude makes the temperature pleasant, but with cool nights, so that highs hover around 70/72 °F in the cooler months (June, July and August), and around 77/79 °F in the hottest months (January, February and March), while lows down to around 50/52 °F from June to September, and go up to 54/57 °F 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 41 °F.
Here are the average temperatures.
Average temperatures - Nairobi
NairobiJanFebMarAprMayJunJulAugSepOctNovDec
NairobiJFMAMJJASOND
Min (°F)545455575552505052555555
Max (°F)777979757372707075777373

The average annual rainfall in Nairobi amounts to 40 inches. 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. (in)2.62.23.78.76.91.40.811.22.26.13.940.4
Days45916135344715893

The Masai Mara National Reserve, which borders the Serengeti in Tanzania, is located at an average altitude of around 5,000 feet. Here, more than 4 in of rain per month fall from March to May and from October to December, and April is the rainiest month with 8 in. However, there is not a completely dry season, although July is the driest month with about 2.4 in.
Here are the average temperatures.
Average temperatures - Masai Mara
Masai MaraJanFebMarAprMayJunJulAugSepOctNovDec
Masai MaraJFMAMJJASOND
Min (°F)545454555554525254545454
Max (°F)828281797777777781828181

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 3,600 feet above sea level, and at 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 1,600 to 5,600 feet in the western park, and from 600 to 2,600 feet in the eastern park. Here are the average temperatures of Voi, located in this area, at 1,900 feet above sea level.
Average temperatures - Voi
VoiJanFebMarAprMayJunJulAugSepOctNovDec
VoiJFMAMJJASOND
Min (°F)686870686864646364666868
Max (°F)909191908684828284888888

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 22 inches per year.
Here is the average precipitation.
Average precipitation - Voi
VoiJanFebMarAprMayJunJulAugSepOctNovDecYear
VoiJFMAMJJASONDY
Prec. (in)1.41.23.13.91.20.20.20.40.614.14.721.9
Days538752123410858

The Chyulu Hills National Park is located at a higher elevation, ranging from 2,900 to 6,900 feet: above 5,900 feet 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 6,500 feet, 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 4 to 8 inches 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 7,000 feet above sea level.
Average precipitation - Eldoret
EldoretJanFebMarAprMayJunJulAugSepOctNovDecYear
EldoretJFMAMJJASONDY
Prec. (in)1.41.62.66.14.73.76.36.93.122.21.842.5
Days4561312917168884110

And here is the average temperature.
Average temperatures - Eldoret
EldoretJanFebMarAprMayJunJulAugSepOctNovDec
EldoretJFMAMJJASOND
Min (°F)485052545248505048505250
Max (°F)777979757573727273757375

Lake Nakuru, where you can watch pink flamingos, is located at 5,740 feet. Here are the average temperatures.
Average temperatures - Nakuru
NakuruJanFebMarAprMayJunJulAugSepOctNovDec
NakuruJFMAMJJASOND
Min (°F)484850545250505048485048
Max (°F)818282797775757579777579

Lake Nakuru, rhino and flamingos

The highest peak in the country is Mount Kenya, 17,057 feet high, which has different climatic zones according to altitude and slope exposure (the south-east is the most humid): above 6,500 feet the montane forest is found, around 8,200 feet the bamboo on the north side which is more arid, above 10,000 feet the heath, above 13,100 ft the high altitude desert. Above 15,400 feet 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 13,100 feet. Even Mount Elgon is very high, being 14,177 feet high, however, its top is located in Uganda.

Also the eastern portion of Lake Victoria, located at 3,600 feet above sea level, belongs to Kenya. Here are the average temperatures of Kisumu.
Average temperatures - Kisumu
KisumuJanFebMarAprMayJunJulAugSepOctNovDec
KisumuJFMAMJJASOND
Min (°F)646666666463636363646464
Max (°F)848484818179818182828282

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 1,200 feet above sea level; here the landscape is semi-desert in the south part and desert in the north, where the annual precipitation drops below 4 inches per year. In this area the weather is really hot all year round: in Lodwar, the daily average is around 86 °F in almost all months, except in July and August when it drops to 82/84 °F.
Average temperatures - Lodwar
LodwarJanFebMarAprMayJunJulAugSepOctNovDec
LodwarJFMAMJJASOND
Min (°F)727375777775757575777573
Max (°F)979797959593919395959595

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. (in)0.30.20.81.610.30.50.40.10.30.40.56.4
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 vacation, 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-colored 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 6,500 feet, spring/autumn clothes; above 10,000 feet, warm jacket and hat. For the north and Lake Turkana, loose fitting, light-colored clothing, light and long shirt and pants of natural fibers (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 fibers, a light sweatshirt for the evening; for Lake Victoria, light clothing made of natural fibers, 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 6,500 feet, spring/autumn clothes, raincoat or umbrella; above 10,000 feet, warm jacket and hat. For the north and Lake Turkana, loose fitting, light-colored clothing, light and long shirt and pants of natural fibers (cotton, linen), a wide-brimmed hat, sunglasses, desert turban, sun hat.