The weather in Malaysia is characterised by two monsoon regimes, namely, the Southwest Monsoon from late May to September, and the Northeast Monsoon from November to March. The Northeast Monsoon brings heavy rainfall, particularly to the east coast states of Peninsular Malaysia and western Sarawak, whereas the Southwest Monsoon normally signifies relatively drier weather. The transition period in between the monsoons is known as the intermonsoon period. Ancient traders plying in the Indian Ocean and adjoining Arabian Sea used it to describe a system of alternating winds which blow persistently from the northeast during the northern winter and from the opposite direction, the southwest, during the northern summer.
Monsoon is caused by land-sea temperature differences due to heating by the sun's radiation.
In winter, the continental landmass cools rapidly resulting in extremely low temperatures over central Asia. Cold air flows out of Siberia as northwesterlies and turns into northeasterlies on reaching the coastal waters of China before heading towards Southeast Asia. In summer, intense solar heating leads to scorching temperatures over the Asian landmass. Moist southeasterlies originating from the southern Indian Ocean and the Indonesian-Australian region transforms into southwesterlies on crossing the equator and flow across Southeast Asia before converging towards Indochina, China and Northwest Pacific. The northeast monsoon is the major rainy season in the country.