April - June
The main rainy season lasts from the end of March through to mid June. As elsewhere in the world, when exactly it rains and when it stops is impossible to predict. It's the warmest time of the year in Tanzania, but those months are so wet that many operators simply do not offer climbs in April/May at all.
June - August
The rain gradually decreases, and so do the temperatures on Mount Meru. The weather on Mount Meru is fairly dry and clear but the nights will be bitter cold. June is quiet, but the number of climbers increases as the year progresses.