Months of July and August are the winter seasons in Tanzania /
Kenya...however it is advisable to carry with you sweaters and warm
clothing's. At Zanzibar, the temperatures don't get all that low
compared to the northern parts of Tanzania. Generally, the cold isn't
terribly bad as many people survive it. Additionally, it's the dry
season and animals can easily be spotted without driving much distances
in search for them so I suggest you go ahead with your plans. I did a
safari with Time Warner Safaris (Tanzania) and they were amazing.
Their services were all excellent including their very knowledgeable
driver guide. I highly recommend them to organize everything for you.
Get in touch with them on and they will be
of great help in arranging everything...

All the best.