Map from Google - Tanzania

Tanzania is a big African country, overlooking the Indian Ocean, and lying just south of the Equator. Much of the country is covered by a plateau, in which a large number of world-famous tourist attractions are found, with several parks and nature reserves, in the savannah environment. This plateau has a tropical climate, warm but not too hot because of altitude. On the contrary, the thin coastal stretch is hot and humid throughout the year, especially from November to April.
As regards the rain pattern, the country can be divided into four zones. In the north and east, except the region of Lake Victoria, there are two rainy seasons: one less intense, known as short rains season, between October and December, and the other more intense, known as long rains season, from March to May, with the peak in April. In January and February, in the period between the two rainy seasons, the weather is hot and sultry, at least below a thousand metres (3,300 feet). In the far north-west, the two rainy seasons are practically connected, because it rains even in January and February, when rainfall amounts to 100/150 millimetres (4/6 inches) per month, depending on the area: therefore in this region in practice there is a single rainy season from October to May. Such a situation can be found occasionally also in some areas of the north-east, in particular along the southern slopes of Mount Kilimanjaro. In the centre and south, there's only one rainy season, from December to April in the south and south-east, and from November to April in the west. The following map shows the areas where there are one or two rainy seasons, and the months in which they occur.
Rainy seasons in Tanzania
The tropical rains occur mainly in the form of downpour or thunderstorm in the afternoon, so that in the morning the sky is often clear even during the rainy periods.

Dar es Salaam, the largest city and former capital of Tanzania, is located on the coast, in the area with two rainy seasons, and is hot all year round, with a slightly hotter and wetter period from November to April, when daytime temperatures are around 31/32 °C (88/90 °F), but with peaks of 35/36 °C (95/97 °F), and relative humidity is high, especially in March and April, the rainiest months. Here are the average temperatures.
Average temperatures - Dar Es Salaam
Dar Es SalaamJanFebMarAprMayJunJulAugSepOctNovDec
Dar Es SalaamJFMAMJJASOND
Min (°C)242323222119181818202123
Max (°C)323232313029292930313132
Min (°F)757373727066646464687073
Max (°F)909090888684848486888890

In Dar Es Salaam, annual precipitation amounts to 1,150 mm (45 in), with a maximum from March to May; April is the rainiest month with 255 mm (10 in), and a secondary maximum, that of the short rains, from October to December. However, the rainfall pattern is quite irregular, so that in some years, the latter period can be very rainy as well. From June to September there is little rain; this period, in addition to being the driest, is also the coolest, with highs around 29/30 °C (84/86 °F), and lows around 18 °C (64 °F), though sometimes at night the temperature can drop to around 15 °C (59 °F). The amount of sunshine is good throughout the year, except during the long rains season, from March to May.
Here is the average precipitation.
Average precipitation - Dar Es Salaam
Dar Es SalaamJanFebMarAprMayJunJulAugSepOctNovDecYear
Dar Es SalaamJFMAMJJASONDY
Prec. (mm)755514025520045252525701251201150
Prec.(in)32.25.5107.91.81112.84.94.745.3
Days74111813544358991

The sea in Dar es Salaam (as in the rest of Tanzania) is always warm enough for swimming: the water temperature ranges from 25 °C (77 °F) in August, to 29 °C (84 °F) in the summer months.
Sea temperature - Dar Es Salaam
Dar Es SalaamJanFebMarAprMayJunJulAugSepOctNovDec
Dar Es SalaamJFMAMJJASOND
Sea (°C)292929292827262526262829
Sea (°F)848484848281797779798284

Both the climate and the rain pattern are similar in the islands of Zanzibar, where, however, it rains more, about 1,600 mm (63 in) per year in Zanzibar, and even 1,900 mm (75 in) in the northern island of Pemba.

In the plateau, temperatures vary with altitude, but they are usually mild or warm without excesses for most of the year. The rainfall amount tends to increase with altitude, but it also depends on slope exposure and the proximity to the highest mountains, so there are more or less rainy areas.

Dodoma, the capital of Tanzania, is located at 1,100 metres (3,600 feet) above sea level, in an inland dry area, and has a single rainy season. In an average year, only 600 mm (23.5 in) of rain fall, most of which occur from December to mid-April, while it almost never rains from May to October.
Here is the average precipitation.
Average precipitation - Dodoma
DodomaJanFebMarAprMayJunJulAugSepOctNovDecYear
DodomaJFMAMJJASONDY
Prec. (mm)1351451156050000225125605
Prec.(in)5.35.74.52.40.200000.114.923.8
Days109751000002741

In October and November, before the rains, it can get hot, but peaks of 35 °C (95 °F) are possible until February. So the best time here goes from May to September, when the daytime temperature is about 27/29 °C (81/84 °F), while at night it drops to around 14/15 °C (27/29 °F), with some colder nights, around 8/10 °C (46/50 °F).
Average temperatures - Dodoma
DodomaJanFebMarAprMayJunJulAugSepOctNovDec
DodomaJFMAMJJASOND
Min (°C)191918181714141415171819
Max (°C)292929292827272729313030
Min (°F)666664646357575759636466
Max (°F)848484848281818184888686

In Iringa the climate is similar, but it is a bit cooler, since the city is located at 1,400 metres (4,600 feet) above sea level, and therefore it's never excessively hot, and at night it can be a bit cold, especially from May to September.

In the north-east, in the area with two rainy seasons, we find a series of towering mountains, including Kilimanjaro, a volcano, 5,895 metres (19,340 feet) high, which is also the highest mountain in Africa, and has permanent snow above 5,000 metres (16,400 ft), unfortunately in retreat because of global warming. Climbing the slopes, we find different climates and biomes: between 1,800 and 2,800 metres (5,900 and 9,200 ft), the rainforest; between 2,800 and 4,000 metres (9,200 and 13,100 ft), where at night the temperature can drop below freezing (0 °C or 32 °F), a dry mountain vegetation; between 4,000 and 5,000 metres (13,000 and 16,500 ft), a cold desert, where, however, during the day the high altitude equatorial sun is very strong, and it's able to considerably warm up the bare rocks, while at night it gets very cold. On the summit, which is often above the cloud layer, the night temperature may drop to -15/-25 °C (5/-13 °F). Considering both the temperature (which is higher in January and February, and lower from June to August) and precipitation (which is higher from March to June, and lower from August to October), it's generally preferable to climb the mountain in January, February and September, while the worst months are April and May, due to the high rainfall (and even snowfall at high elevations), and frequent cloudiness. Some visitors choose the days of the full moon, which occur around once per month, during which the moonlit glaciers offer a wonderful view at night.

Kilimanjaro

West of Kilimangiaro, we find another mountain, the volcano Meru, 4,565 metres (14,977 feet) high.
At the foot of Mount Meru, the city of Arusha is located, at 1,400 metres (4,600 feet) of altitude. Here the temperature is pleasant all year round, in fact the daily averages go from 19 °C (66 °F) in the period from January to April, to 14.5 °C (58 °F) in June and July. Sometimes, however, between November and March, during the day it can get hot. Nights are cool throughout the year, especially from June to September, when lows can drop below 10 °C (50 °F), while during the day the temperature fluctuates around 20/22 °C (68/72 °F).
Average temperatures - Arusha
ArushaJanFebMarAprMayJunJulAugSepOctNovDec
ArushaJFMAMJJASOND
Min (°C)10101113118988101010
Max (°C)282827252221202224262727
Min (°F)505052555246484646505050
Max (°F)828281777270687275798181

The rainfall in Arusha amounts to 1,100 mm (43 in) per year. Here is the average rainfall.
Average precipitation - Arusha
ArushaJanFebMarAprMayJunJulAugSepOctNovDecYear
ArushaJFMAMJJASONDY
Prec. (mm)65651453401704020151540125901125
Prec.(in)2.62.65.713.46.71.60.80.60.61.64.93.544.3
Days1212152114554361515127

The Serengeti National Park is located north-west of Arusha, towards Lake Victoria and the border with Kenya; here the altitude varies between 1,100 and 2,000 metres (3,600/6,600 feet). This is the area of the savanna, a kind of grassland becoming greener and higher in the rainy season, with rare trees such as the huge baobabs, and animals such as rhinos, hippos, giraffes, elephants, lions, cheetahs and leopards. The driest part of the Serengeti, where the trees are a rarity, is the south-eastern one.
Here are the average temperatures recorded at 1,400 metres (4,600 feet) above sea level.
Average temperatures - Serengeti
SerengetiJanFebMarAprMayJunJulAugSepOctNovDec
SerengetiJFMAMJJASOND
Min (°C)161616161615141515161616
Max (°C)292929282727262728292828
Min (°F)616161616159575959616161
Max (°F)848484828181798182848282

The Olduvai Gorge, the cradle of humankind, where fossil remains of early hominids were found, is located between the Serengeti and the Ngorongoro Conservation Area, at 1,400 metres (4,600 feet) above sea level. Here the climate is more arid.
On the contrary, the Ngorongoro Conservation Area is rainier, at least in the higher areas, which exceed 2,000 metres (6,500 feet). In the city of the same name, which lies at 2,400 metres (7,900 ft), the weather is spring-like all year round, with cool nights, sometimes even a bit cold, while the days are pleasantly warm from October to March, and mild from May to September. Here, as in the far north-west, some thunderstorms may occur even in the dry season, in January and February.
Average temperatures - Ngorongoro
NgorongoroJanFebMarAprMayJunJulAugSepOctNovDec
NgorongoroJFMAMJJASOND
Min (°C)1010101110888891010
Max (°C)232322212019192021222222
Min (°F)505050525046464646485050
Max (°F)737372706866666870727272

In Tanzania there are several large lakes. In the north-west there is the huge Lake Victoria, at 1,100 metres (3,600 feet) above sea level, where the climate is warm all year round, as we can see from the temperatures of Mwanza.
Average temperatures - Mwanza
MwanzaJanFebMarAprMayJunJulAugSepOctNovDec
MwanzaJFMAMJJASOND
Min (°C)181818181816151617181818
Max (°C)282829282828282929292827
Min (°F)646464646461596163646464
Max (°F)828284828282828484848281

Here the rainfall amounts to 1,050 mm (41 in) per year; we are in the area with a single rainy season, from October to May. Here is the average precipitation.
Average precipitation - Mwanza
MwanzaJanFebMarAprMayJunJulAugSepOctNovDecYear
MwanzaJFMAMJJASONDY
Prec. (mm)1051101401707520102025851551401050
Prec.(in)4.14.35.56.730.80.40.813.36.15.541.3
Days1081114821238131292

Further south, Lake Tanganyika, along the Rift Valley, is located at 770 metres (2,500 feet) above sea level, and still further south, Lake Malawi is at 480 metres (1,570 feet), therefore along its banks the weather is hotter due to the lower altitude. There are several national parks and nature reserves in the centre-south as well.
Tanzania is too close to the Equator to be affected by tropical cyclones, because the Coriolis effect, which causes winds to swirl, at the Equator is zero.

When to go

The best time to visit Tanzania as a whole runs from July to August, being the coolest, the sunniest and driest. June is a good month almost everywhere, but in the north-east and in the island of Pemba, the long rains can linger on until this month. September is also a good month, but at lower elevations the days can be hot. In October, the season of the short rains begins in the north, though in the first half of the month there is usually little rain, however, it can get hot at low altitudes.
In January, it rains a lot in the south, in the midst of the rainy season. Wanting to visit the country in the Northern Hemisphere winter (January and February), you can choose the northern areas, which in this period are in the pause between the two rainy seasons, remembering that on the coast and at lower altitudes it's hot, and that some showers in the afternoon may still occur especially on the shores of Lake Victoria and in mountains like Kilimanjaro.
In the Serengeti you can go from June to September, with a preference for June, in the period of the wildebeest migration through the rivers, before the animals move to Kenya, or even in January and February, to watch them in the southern part, towards the Olduvai Gorge.

What to pack

In inland areas, in general, it's better to dress in layers, warmer at high altitudes, lighter in the warm season and at low altitudes; sun hat, sunscreen, raincoat for the rain showers in rainy periods; for mountain climbing, hiking shoes. In parks and reserves, neutral-coloured clothes, comfortable canvas shoes. For the summit of Kilimanjaro, throughout the year, cold weather clothing, synthetic thermal long underwear, fleece, parka, wind jacket, warm boots, sunglasses, sunscreen.
For the reef, equipment for snorkeling, water shoes or rubber soled shoes.

In winter (June to August): on the coast (Dar es Salaam, Zanzibar), light clothing, a sweatshirt for the evening, a light scarf for the breeze. On the shores of the lakes Malawi and Tanganyika, light clothing, a sweatshirt and a light jacket, a light scarf for the breeze; in Arusha, Dodoma, Serengeti and Lake Victoria, light clothes for the day, sweater and jacket for the evening. In Ngorongoro and the plateau above 2,000 metres (6,500 feet), spring/autumn clothes, with jacket and sweater; above 3,000 metres (9,800 ft), warm jacket and hat.

In summer (December to February): on the coast (Dar es Salaam, Zanzibar), light clothing, made of natural fibres, a light sweatshirt for the evening, light raincoat or umbrella, especially in the south (see Mtwara). On the shores of the lakes Malawi, Tanganyika and Victoria, and in Dodoma, light clothing, made of natural fibres, sun hat, a light sweatshirt for the evening, light raincoat or umbrella; in Arusha, Serengeti and the plateau at intermediate altitudes, you can add a jacket and a sweater for the evening. In Ngorongoro and the plateau above 2,000 metres (6,500 feet), spring/autumn clothes, with jacket and sweater, raincoat or umbrella; above 3,000 metres (9,800 ft), down jacket and hat.