Even though global warming is making weather predictions harder each year, in theory East Africa has one "dry season" which coincides with "winter", and then a "green season" which has a rainy period at either end of it - the bigger one being around April and May.
December, January, February and March are the warmest months of the year and all coincide with "summer". During these months, you can hope for clear mornings and evenings, with clouds building up during the day, surrounding the summit in the early afternoon hours. The predictable patterns of weather make it a good time to climb.
April and May is the primary rainy season and bring dense cloud formations over the mountain, reducing visibility and comfort. You can expect frequent snowstorms on the summit and heavy rains on the lower slopes during this time.
Mid June to mid October is considered the long "dry" season which can get to be very cold at night. During this “winter” time, there is a persistent dripping belt of cloud around the whole mountain above the forests and scrub zones, leaving the summit and a few other distant peaks poking out the top. Above the cloud belt however, it is usually clear and dry, and can be one of the best times to plan a climb. In late October, the clouds finally give way to the small rain, and until early December the mountain is shrouded in heavy clouds again, with persistent showers from base to summit.
Therefore, we believe that the best months to climb Mt Kilimanjaro are January, February, July, August and September (though Kilimanjaro can still be climbed throughout the year!).
Please note the above weather patterns are simply a guideline and cannot be guaranteed.