Monsoons play an outsized role in world literature – notably in E.M. Forster’s 1924 novel “A Passage to India’. There, the Indian monsoon is a metaphor for the cultural divide between the British colonizers and the indigenous Indian population.
In “The God of Small Things” by Arundhati Roy, published in 1997, the monsoon is a symbol of loss, social change and forbidden love.
There are more examples.
While both novels are set in India, monsoons occur worldwide. This is true even of Phoenix, Arizona, in the United States, which is situated in the vast Sonoran Desert. There, the monsoon usually occurs from June 15 to September 30, bringing sandstorms and torrential rains.
Locally, the Bicol Region experiences two monsoons every year – the Southwest Monsoon and the Northeast Monsoon.
These two weather phenomena also have roles in literature, namely as characters in pre-colonial Philippine folklore – Habagat and Amihan.
Habagat, the Southwest Monsoon, is the legendary Filipino god of wind and rain. He is a force of nature, bringing life-sustaining rain, but he can be destructive as well, causing typhoons and floods.
Amihan, the Northeast Monsoon, is Habagat’s counterpart, bringing peace and cool winds. She is a complementary force to Habagat.
Generally worldwide, national meteorological departments use several criteria for determining the date of onset of a monsoon. These include wind patterns, rainfall, atmospheric pressure, humidity, cloud cover and sea surface temperature.
Among the community of scientists, weather has a reputation for being notoriously difficult to predict. The European Centre for Medium-Range Weather Forecasts (ECMWF) alone collects 287 terabytes of data daily from weather stations, satellites and remote sensors like weather balloons and microwave sensors that measure cloud size and density.
These data are fed into an Atos BullSequana X series supercomputer in Bologna, Italy, for processing.
By comparison, to predict the position of Mars in the night sky requires only about six variables and a few kilobytes of data that can be processed on a cell phone.
In the Philippines, due to their importance to rice farmers, the exact dates of the onset of Habagat and Amihan are announced in the nation’s press by the Philippine Atmospheric, Geophysical and Atmospheric Services Administration (PAGASA).
In the Philippines PAGASA leans on the ECMWF and other international agencies to determine the exact dates of the onset of both Habagat and Amihan, but different variables come into play for the two monsoons.
For Habagat, the most important variable arguably is the cumulative rainfall over a 5-day period, or pentad.
Specifically, the exact date of the onset of Habagat is the middle day of the first of three consecutive pentads that exceed annual mean rainfall, as measured in pentads.
PAGASA also includes measures of wind patterns and geographical coverage in at least seven out of 13 PAGASA monitoring stations.
So, it is scientifically well established that the onset of Habagat normally occurs from mid-May or early June to October.
According to a study published in the Philippine Journal of Science, the date of the onset of Habagat is fairly consistent. For example, the onset of Habagat was May 29 in 2024 and June 2 in 2023 and May 18 in 2022.
The study reports that the average duration of Habagat is 148 days, with the most common withdrawal, or ending, date being Oct. 21. However, the study adds that the withdrawal date is highly variable, suggesting that the actual length of Habagat in any given year is determined more by its date of withdrawal rather than its date of onset.
Amihan typically begins in late October to early November and lasts until March or mid-April. It begins when meteorologists detect a cold high pressure system over Eurasia called the Siberian High.
This anticyclone – at 2 million square kilometers, the hugest on Earth — forms over northeastern Eurasia from September to April due to intense cooling of the land. Cold, dense air as low as minus 40 degrees Celsius at the center of the anticyclone accumulates and spins clockwise, causing northeast winds over much of Southeast Asia, including Catanduanes.
To determine the exact date of the onset of Amihan, PAGASA looks for a shift in prevailing wind patterns from the southeast to the northwest.
If Amihan is arriving, this shift will be accompanied by a drop in temperatures and an increase in air pressure. A drop in sea surface temperatures may also be detected.
Bryce McIntyre, PhD, resides in San Andres. He holds a doctoral degree from Stanford University, Palo Alto, California, USA