The southwest monsoon starts mid-May and peaks from July to September.
The dry zone between Mandalay and Pyay gets the least rain. Rain can make roads impassable anywhere (especially in the delta region).
(Oct & Nov, Mar & Apr)
March to May, Yangon often reaches 104ºF (40ºC). Areas around Bagan and Mandalay are hotter.
Cooler in the hill towns of Shan State.
All forms of transport booked solid during Thingyan in April.
Rains least (if at all, in some places) and is not so hot.
Book accommodation and transport well ahead for this busy travel season.