Tanzania's weather in general
Tanzania has a pleasant tropical climate but has large regional climatic variations influenced by several factors, including altitude. Temperatures drop by about 6.5°C for every 1,000m you climb (or 3.5°F per 1,000ft). The hottest and most humid part of the country is on the coast. Other low lying areas, such as the western and southern parks, are also hot but much less humid. The rest of the interior is much milder and cools down significantly at night. Tanzania is too close to the equator to experience a real winter and summer. There is, however, a dry and Wet season.
Dry season - June to October
There is virtually no rainfall during the whole winter and humidity is very low. When water becomes scarce wildlife will gather around predictable water resources.
June, July, August, September & October - Afternoon temperatures are usually between 20°C/68°F and 30°C/86°F and vary greatly according to altitude and location. Most days have a fine, clear sky and sunny weather. It cools off at night; be sure to pack warm clothing because morning game drives in open vehicles will be cold- especially in the northern parks. These are the coldest and least humid months and the most pleasant time of the year to visit the coastal areas. There is very little rain in most of the country.
Wet season - November to May
During most of the Wet season, afternoon temperatures are more consistent throughout the country (a bit above or below 30°C/86°F) but will be colder above 1,300m/4,265ft. Morning temperatures are more moderate, but still cold in most northern parks due to the high altitude. It is advised to pack a windproof jacket and warm sweater for early morning game drives in open vehicles.
November & December - 'Short rains' - A period of about a month of short rains which occurs sometime between November and December. Its timing is unpredictable and, in Northern Tanzania, it can even happen in October. The rains will rarely interfere with your safari.
January & February - The northern parks and coastal areas tend to have a break in the rainy season with less rain around this time, but the exact timing of the dry spell is unpredictable. The other parks don't really experience a dry spell. In Selous, Ruaha and Mikumi the rain gradually builds from November to its peak in April. Gombe, Mahale and Katavi have one continuous Wet season.
March, April & May - 'Long rains' - These are the wettest months. It tends to rain heavily almost every day, although not often for the whole day. It is often cloudy, especially in the northern parks. Humidity tends to be high, especially in the hotter southern and western parks.
Best Time to Visit
The best wildlife viewing months in Tanzania are during the Dry season from late June to October. The best chance of seeing the wildebeest migration in the Serengeti is during June and July and the time to see the wildebeest calving is late January to February. The southern and western circuit parks are best visited during the dry-season (June to October), unlike the more popular northern circuit parks that can be visited year-round. Tarangire is the only exception, since its wildlife viewing is considerably better in the dry-season as well.