The East Asian monsoon has two parts: the subtropical monsoon and the tropical monsoon. They start at different times and are not dependent on each other. The subtropical monsoon begins in mid-March, earlier than the tropical monsoon. The subtropical monsoon is not a branch of the tropical monsoon, but actually helps it start. The subtropical monsoon sets up gradually and retreats quickly, while the tropical monsoon starts suddenly and ends slowly. The differences in temperature between land and sea, especially over the Tibetan Plateau, are important for both monsoons.