Tanzania has a tropical climate with large climatic variations due to the region and season. Higher regions are much cooler than the lower ones. Every 1000m of elevation sees a pretty drastic temperature drop. Coastal territories are the hottest and most humid parts. The western and southern areas are also hot but less humid.
The dry season runs from June to October. During this period you won't meet any rain at all and the humidity is low as well. This is the best time to visit the country, see the wildlife gathering at the water resources, and enjoy pleasant days on the coast. Though days are sunny and warm, nights can get really cool in the Noth so don't forget to pack some warm clothes as well. The islands will also have similar weather conditions.