Philippines' weather systems: Amihan, shear line, easterlies

The Philippines’ weather is shaped by different systems that bring shifting winds, temperatures, and rainfall throughout the year. This quick guide breaks down several weather systems in the Philippines, explaining what they are, how they form and the kind of conditions Filipinos can expect when they are in play.

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Amihan (Northeast Monsoon)

Amihan refers to the northeast monsoon season in the Philippines, characterized by cool and dry winds coming from the northeast.

Characteristics of amihan:

• Typically occurs from October to late March.
• Brings cooler temperatures and less rainfall to most parts of the Philippines.
• Associated with the high-pressure system over Siberia.
• Can cause drizzling rainfall and squalls on the east coast of Luzon.
• Often brings colder weather associated with the Christmas season.

Easterlies

Easterlies are winds that blow from east to west, predominantly in the Earth's equatorial regions. In the Philippines, easterlies are often mentioned as one of the weather systems affecting the country.

Characteristics of easterlies:

• Part of the global wind patterns, also known as trade winds.
• Formed due to the Hadley cell circulation and the Coriolis effect.
• In the Northern Hemisphere, they blow from the northeast towards the southwest.
• Can bring warm and moist air, potentially causing scattered rains and thunderstorms.

Shear line

A shear line is a narrow zone where there is an abrupt change in the horizontal wind component parallel to this line. In the Philippines, it's often associated with the northeast monsoon (amihan) season and can cause significant rainfall, especially in the eastern parts of the country.

Characteristics of a shear line:

• Formed when cold and warm winds converge.
• Can cause prolonged periods of rain, though not always continuous.
• Shear line-induced rainfall events typically last around four days.
• They generally occur during the amihan season, with peak activity in December-January.

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Editor's Note: While an editorially approved generative AI model assisted in research and information processing of this glossary, the content remains the product of human reporting, writing and editorial judgment. The final versions are thoroughly reviewed and verified by our team of senior journalists.