No part of Northeast India receives less than of rainfall a year. Areas in the Brahmputra valley receive of rainfall a year while mountainous areas receive a year. The southwest monsoon is responsible for bringing 90% of the annual rainfall to the region. April to late October are the months where most of the rainfall in Northeast India occurs with June and July being the rainiest months. In most parts of the region, the average date of onset of the monsoons is 1 June. Southern areas are the first to receive the monsoon (May or June) with the Brahmaputra valley and the mountainous north receiving later (later May or June). In the hilly parts of Mizoram, the closer proximity to the Bay of Bengal causes it to experience early monsoons with June being the wettest season.