Monsoon

Also known as: southwest monsoon · northeast monsoon · seasonal reversing wind · seasonal wind system

Definition

A monsoon is a seasonal reversing wind accompanied by corresponding changes in precipitation — traditionally defined by the wind’s reversal, and more broadly by the seasonal shifts in atmospheric circulation and rainfall that follow the annual migration of the Intertropical Convergence Zone (ITCZ) north and south of the equator. The physical engine is the different way land and ocean heat: water’s high heat capacity and convective mixing keep sea temperatures stable, while land heats and cools quickly, so in the warm months a thermal low forms over the continents and moist maritime air flows inland, rises, and rains; in the cold months the pattern reverses, with subsiding, drier air flowing from land to sea. The word itself entered English through Portuguese monção and Dutch monson from the Arabic mawsim, “season.” (Wikipedia — Monsoon)

For the Philippines the monsoon is not one wind but two alternating regimes that structure the whole year: the southwest monsoon, known in Filipino as habagat, which brings heavy rains to most of the archipelago from roughly May or June to October, and the northeast monsoon, amihan, the cool, relatively dry flow that prevails from about mid-October or November to March or early April. The reversal between them is usually abrupt — recorded as occurring “overnight” in some years — and PAGASA, the national weather service, operationalizes it through public declarations of the rainy season’s onset and of each monsoon’s termination, most recently announcing the termination of the southwest monsoon on October 5, 2026. The two-monsoon rhythm, its ENSO modulation, and the roughly twenty tropical cyclones that enter the Philippine Area of Responsibility each year are documented in this site’s Climate of the Philippines, El Niño-Southern Oscillation, and Typhoon entries. (Wikipedia — Climate of the Philippines, Wikipedia — Amihan, PAGASA — Termination of the Southwest Monsoon)

Identities

Source	Identifier	URL
Wikipedia	Monsoon	https://en.wikipedia.org/wiki/Monsoon
Wikidata	Monsoon (Q42967)	https://www.wikidata.org/wiki/Q42967
DBpedia	Monsoon	https://dbpedia.org/page/Monsoon
ProductOntology	N/A	N/A
Wiktionary	monsoon	https://en.wiktionary.org/wiki/monsoon
Library of Congress Subject Headings (LCSH)	Monsoons (sh85086993)	https://id.loc.gov/authorities/subjects/sh85086993
MeSH	N/A	N/A
NCBI Taxonomy	N/A	N/A
AGROVOC	N/A	N/A
Google Scholar	monsoon Philippines habagat amihan southwest monsoon rainfall PAGASA flood	N/A
ConceptNet	N/A	N/A
OpenCyc	N/A	N/A

Also Known As

southwest monsoon
northeast monsoon
seasonal reversing wind
seasonal wind system

Examples and Analogies

A wind that changes address with the sun: the monsoon behaves like a giant sea breeze planetaryized — as the sun’s heating migrates, the pressure contrast between continent and ocean flips, and the moist onshore flow of summer becomes the dry offshore flow of winter, the explanation that replaced the older pure sea-breeze account with an ITCZ-driven, planetary-scale mechanism. (Wikipedia — Monsoon)
Two landlords of the Philippine year: habagat and amihan are best understood as two landlords who exchange the property annually — the habagat’s warm, rain-bearing southwest flow owns roughly May or June to October, then hands the keys to the amihan’s cool northeast flow for November to March or April, an exchange the record describes as sometimes abrupt enough to occur overnight. (Wikipedia — Climate of the Philippines, Wikipedia — Amihan)
The borrowed rain of an enhanced habagat: in July 2024 Super Typhoon Carina (Gaemi) never made Philippine landfall, yet its moisture was drawn into the southwest monsoon and flooded Metro Manila on July 24 — a typhoon elsewhere delivering a monsoon’s disaster at home, the mechanism also documented in the Typhoon entry on this site. (Wikipedia — Typhoon Gaemi)
Verified data (definition, seasons, events):
Definition: seasonal reversing wind with corresponding precipitation change; broader sense tied to ITCZ migration; from Arabic mawsim, “season” (Wikipedia — Monsoon, Wiktionary — monsoon)
Habagat: heavy rain over most of the archipelago roughly May–October, onset typically May–June, terminating between late September and mid-October (Wikipedia — Climate of the Philippines)
Amihan: cool, mostly dry northeast flow from about mid-/late October or November to March or early/mid-April, with drizzle on Luzon’s east coast (Wikipedia — Amihan, Wikipedia — Climate of the Philippines)
2012 habagat floods: August 1–8, 2012, the Hagupít ng Habagat; 1,007 mm of rain in 72 hours over parts of Metro Manila; Marikina River at 20.6 m; 70–90 percent of Metro Manila affected; 95 deaths; at least ₱604.63 million in losses (Wikipedia — 2012 Luzon southwest monsoon floods)
2024 habagat floods: July 24, 2024, Carina-enhanced monsoon; Marikina River peak 20.7 m; up to 512.8 mm of rain; 48 deaths; ₱10.37 billion in damage (Wikipedia — Typhoon Gaemi)
Operational declarations: rainy-season onset declared June 3, 2025; habagat termination declared October 5, 2026 (Wikipedia — Climate of the Philippines, PAGASA — Termination of the Southwest Monsoon)

Usage Scenarios

1. Seasonal Forecasting and Monsoon Declarations

PAGASA converts the monsoon from a climatology into a running public service: it declares the rainy season’s onset (June 3, 2025 in the current record), monitors the habagat through the season, and announces the wind regime’s termination — the October 5, 2026 release cited the strengthening high-pressure system over East Asia and the southward shift of the ITCZ as the markers ending the southwest monsoon and the rainy season over the western sections of Luzon and the Visayas. (Wikipedia — Climate of the Philippines, PAGASA — Termination of the Southwest Monsoon)

2. Flood Preparedness During Habagat Events

Disaster managers treat the habagat season as a standing flood scenario independent of landfalling typhoons: the August 2012 monsoon floods put Metro Manila and nine provinces under a state of calamity without any typhoon striking the capital, and the July 24, 2024 Carina-enhanced habagat did so again, so river-level protocols for the Marikina, dam-spillway watch at La Mesa, and pre-emptive evacuation belong to the monsoon calendar itself. (Wikipedia — 2012 Luzon southwest monsoon floods, Wikipedia — Typhoon Gaemi)

3. Agriculture and Water Planning Around the Two Monsoons

Farmers and water agencies schedule against the reversal: planting and impoundment ride the habagat’s rains, while the amihan months — cooler and, in the western sections, dry — are the season of withdrawal and irrigation release, the rhythm the four PAGASA rainfall climate types (Type I’s pronounced November–April dry season through Type IV’s evenly distributed rain) map onto local planning. (Wikipedia — Climate of the Philippines)

4. Climate Research on Monsoon-Cyclone Interaction

Researchers use the Philippine record to study how the monsoon and tropical cyclones compound: the 2012 event paired the habagat with Typhoons Saola (Gener) and Haikui passing north of the country, and the 2024 event with Gaemi — documented cases of cyclone-enhanced monsoon rainfall delivered far from the cyclone itself. (Wikipedia — 2012 Luzon southwest monsoon floods, Wikipedia — Typhoon Gaemi)

Strategies

Plan by wind regime, not by calendar month: because monsoon onsets and terminations vary from year to year, anchor planting, construction, and preparedness to PAGASA’s event-based declarations — the rainy-season onset of June 3, 2025 and the habagat termination of October 5, 2026 — rather than fixed dates. (Wikipedia — Climate of the Philippines, PAGASA — Termination of the Southwest Monsoon)
Monitor the habagat even when no typhoon threatens landfall: the 2012 and 2024 floods were caused by monsoon flow enhanced by distant cyclones, so the southwest monsoon deserves its own watch separate from the landfall forecast. (Wikipedia — 2012 Luzon southwest monsoon floods, Wikipedia — Typhoon Gaemi)
Treat rainfall, not wind, as the habagat’s threat metric: the monsoon’s damage comes from water — 1,007 mm in 72 hours in 2012, up to 512.8 mm in the 2024 episode — so river levels and dam operations, not wind signals, drive its response. (Wikipedia — 2012 Luzon southwest monsoon floods, Wikipedia — Typhoon Gaemi)
Expect the transition to be fast: the amihan-habagat handover can flip the prevailing wind abruptly, in some years overnight, so late-season outdoor schedules and coastal operations should build margin around the late-September-to-October window. (Wikipedia — Amihan, Wikipedia — Climate of the Philippines)
Read the monsoon through the local climate type: the same habagat delivers Type I Manila’s pronounced wet season and Type IV General Santos’s even rainfall, so national monsoon advisories must be translated locally. (Wikipedia — Climate of the Philippines)

Security and Safety Measures

Prepare western Luzon and the Visayas first: the habagat’s rains fall heaviest on the western sections — the October 2026 termination release explicitly ended the rainy season “over the western sections of Luzon and Visayas” — so flood assets belong westward during the season. (PAGASA — Termination of the Southwest Monsoon)
Respect river thresholds during monsoon rain: the Marikina River’s 20.6 m in 2012 and 20.7 m in 2024 against a 16-meter flood level are the documented triggers for evacuation of riverbank communities; both events also overtopped La Mesa Dam and flooded downstream Tullahan areas. (Wikipedia — 2012 Luzon southwest monsoon floods, Wikipedia — Typhoon Gaemi)
Heed states of calamity as protection windows: Metro Manila and nine provinces were placed under a state of calamity in August 2012 and Metro Manila again in July 2024 — declarations that unlock evacuation, price controls, and relief funds while the monsoon is still raining. (Wikipedia — 2012 Luzon southwest monsoon floods, Wikipedia — Typhoon Gaemi)
Guard against landslide and dam-spill hazards in sustained monsoon rain: the 2012 event killed nine in a single Quezon City barangay landslide and overflowed Laguna de Bay onto Pasig, Taguig, San Pedro, and Calamba — hazards that persist hours after rainfall peaks. (Wikipedia — 2012 Luzon southwest monsoon floods)
Follow official monsoon advisories at source: PAGASA’s seasonal declarations and advisories — not unofficial forecasts — are the documented channel for onset, enhancement, and termination information. (PAGASA — Termination of the Southwest Monsoon)

Historical Context

The monsoon’s scientific understanding evolved from an older description as a large-scale sea breeze to the current view of a planetary-scale phenomenon driven by the ITCZ’s annual migration and the thermal contrast between land and sea — a shift that turned the word from a regional wind of the Indian Ocean world, via the Arabic mawsim, into a global family of systems spanning West Africa, South and East Asia, Australia, and the Americas. (Wikipedia — Monsoon)

The Philippine record gives the monsoon its modern disaster chronology. In August 2012, eight days of habagat rain enhanced by Typhoons Saola (Gener) and Haikui — remembered as the Hagupít ng Habagat, the “wrath of the monsoon” — flooded 70 to 90 percent of Metro Manila, killed 95 people, destroyed 8,428 homes, and caused at least ₱604.63 million in losses, the worst rain damage since Typhoon Ondoy (Ketsana) in 2009 despite no typhoon landfall. In July 2024 the pattern repeated: Super Typhoon Carina (Gaemi), passing north of Luzon, pumped moisture into the southwest monsoon and flooded Metro Manila on July 24 with up to 512.8 mm of rain, 48 deaths, and ₱10.37 billion in damage, in an event PAGASA itself compared to Ondoy. Operationally, PAGASA now marks the regime publicly each year — declaring the 2025 rainy season’s onset on June 3 and announcing the habagat’s termination on October 5, 2026, when a strengthening East Asian high and the southward-shifting ITCZ ended the rainy season over western Luzon and the Visayas. (Wikipedia — 2012 Luzon southwest monsoon floods, Wikipedia — Typhoon Gaemi, Wikipedia — Climate of the Philippines, PAGASA — Termination of the Southwest Monsoon)

Challenges and Controversies

Warning for a Disaster Without a Storm

Both landmark habagat floods were caused by cyclones that never made landfall, exposing a standing communication problem: publics conditioned to respond to typhoon wind signals must also be moved by monsoon advisories alone. The 2012 event struck between named storms, and the 2024 flooding arrived while Carina stayed offshore — cases cited in the debate over how the Philippines communicates “enhanced habagat” risk, a question this site’s Typhoon entry documents from the cyclone side. (Wikipedia — 2012 Luzon southwest monsoon floods, Wikipedia — Typhoon Gaemi)

The Drifting Seasonal Calendar

Monsoon onsets and terminations vary widely year to year — the amihan’s start can swing between mid-October and November, its end between March and mid-April — putting pressure on school calendars, planting dates, and water-release schedules built on fixed months. PAGASA’s move to event-based declarations, such as the June 3, 2025 onset and the October 5, 2026 termination, is the institutional answer to that drift, and itself part of the adaptation debate. (Wikipedia — Amihan, Wikipedia — Climate of the Philippines, PAGASA — Termination of the Southwest Monsoon)

Urban Drainage Against Monsoon Rainfall

The 2012 and 2024 floods renewed scrutiny of Metro Manila’s drainage capacity: rainfall of 1,007 mm in 72 hours (2012) and up to 512.8 mm (2024) ran through the same watersheds — Marikina River, La Mesa Dam, Laguna de Bay, Tullahan — in both events, and in both a state of calamity followed. Whether the recurring inundation is attributed to the monsoon’s intensity or to the metropolis’ drainage deficit remains the standing dispute over habagat-season losses. (Wikipedia — 2012 Luzon southwest monsoon floods, Wikipedia — Typhoon Gaemi)