taking an excerpt from http://en.wikipedia.org/wiki/Amihan:

In the Philippines, Amihan refers to the season dominated by the trade winds, which are experienced in the Philippines as a cool northeast wind.^[1] It is characterized by moderate temperatures, little or no rainfall, and a prevailing wind from the east. As a general rule of thumb, the Philippines’ Amihan weather pattern begins sometime in September or October and ends sometime in May or June. There may, however, be wide variations from year to year.^[2] Throughout the rest of the year, the Philippines experiences the west or southwest wind; south-west monsoon,^[1] which in turn is referred to as the Habagat. The Habagat season is characterized by hot and humid weather, frequent heavy rainfall, and a prevailing wind from the west. Habagat is also known as monsoon or southwest monsoon in the Philippines. The main indicator of the switch between the Amihan and Habagat seasonal patterns is the switch in wind direction. In most years this transition is abrupt and occurs overnight. In some years there is a period of perhaps a week or two where the wind will switch between Amihan and Habagat patterns several times before settling into the pattern for the new season.