EXPLAINER: How did the Kanlaon ashfall reach Cebu and how long will it last?

CEBU CITY, Philippines — Ashfall from Kanlaon Volcano surprised many Cebuanos on Thursday, July 9, after fine volcanic ash reached several parts of Cebu despite the volcano being located on neighboring Negros Island.

Experts, however, said that the combined effects of the southwest monsoon, or habagat, and the enhanced winds brought by Typhoon Inday (international name: Bavi) made the event possible.

They also said the main ash plume was expected to move beyond Cebu within hours if Kanlaon does not erupt again. Some fine ash may remain suspended in the atmosphere longer depending on weather conditions.

Why Kanlaon ashfall reached Cebu

According to the Philippine Institute of Volcanology and Seismology (Phivolcs), the weather did not cause the eruption itself.

Instead, Phivolcs said that prevailing wind and atmospheric conditions determined where the volcanic ash drifted after the eruption.

During a press conference on Thursday, July 9, Mariton Antonia Bornas said that strong winds associated with the southwest monsoon carried the ash eastward toward Cebu. Bornas leads Phivolcs’ Volcano Monitoring and Eruption Prediction Division.

Habagat spread the ash farther

She explained that Typhoon Inday enhanced the habagat, allowing volcanic ash to spread farther than it normally would.

“Dahil po ang ihip ng hangin ay papunta po ng silangan at ang meron po tayong nanaig na hanging Habagat, ang Southwest Monsoon. Bunsod na rin po ng pagdaan ng bagyong Bavi o yung Inday ay nadamay na rin po talaga ang Cebu sa pagkalat po ng ash at ang ashfall,” Bornas said.

(Because the wind is blowing toward the east and we have the prevailing habagat winds, the southwest monsoon. Also, the approach of typhoon Bavi, or Inday, spread the ashfall toward Cebu.)

Bornas also said that the cool, dense atmosphere caused by the rainy weather slowed the settling of the ash, allowing wind to carry finer ash particles over longer distances.

How fast the ash traveled

As a result, traces of ash and even the smell of sulfur dioxide showed up in different parts of Cebu.

According to PAGASA Visayas Regional Services Division Chief Alfredo Quiblat Jr., wind observations at PAGASA Mactan showed southwest winds reaching around 50 kilometers per hour on Thursday morning.

Using PAGASA’s air parcel trajectory model, forecasters determined that volcanic ash from Kanlaon would be carried toward Central Visayas.

“Malakas ’yung hangin natin at 50 km/h base sa sukat namin dito sa PAGASA Mactan. At ang direksyon po ay southwest ’yung hangin, nahatak po nung habagat. And base din dito sa air parcel trajectory na information sa PAGASA na in-issue kaninang umaga ay ’yung direksyon talaga nung air parcel na nanggaling doon sa Kanlaon ay papunta dito sa Central Visayas,” Quiblat said.

Kanlaon ashfall drifted over Cebu 2 hours later

(The wind blew strong at 50 km/h based on our measurements at PAGASA Mactan. It went southwest, carried by the habagat. And based also on the air parcel trajectory information from PAGASA this morning, the air coming from Kanlaon moved toward Central Visayas.)

Since Kanlaon erupted at 7:33 a.m., Quiblat said that the ash was expected to reach Cebu in two hours. Ashfall observations began around 11 a.m.

He said the wind’s direction and speed determined both where the ash would travel and how quickly it reached Cebu.

How long the ashfall will last

PAGASA said it expected the main ash plume to move beyond Cebu and toward Leyte roughly six hours after Kanlaon’s eruption.

If the volcano does not erupt again, Quiblat said that Cebu can expect to be largely clear of ashfall by Friday.

He noted, however, that while ashfall may stop, determining when air quality fully returns to normal falls under the Department of Environment and Natural Resources–Environmental Management Bureau (DENR-EMB).

Could ashfall continue?

Phivolcs said that the answer depends largely on Kanlaon’s activity and prevailing weather conditions.

Bornas explained that larger ash particles usually settle within minutes to a few hours. However, finer ash can remain suspended in the atmosphere much longer, especially during cool, rainy conditions when the atmosphere becomes denser.

She also said that strong winds could trigger what experts call “remobilization.” Previously deposited ash around the volcano may be lifted back into the air and transported again.

She said remobilized ash is more likely to affect communities nearer Kanlaon in Negros, although authorities will continue monitoring whether prevailing weather conditions could allow ash to reach Cebu again.

Is the air quality still safe?

The Department of Environment and Natural Resources–Environmental Management Bureau (DENR-EMB) said that air quality in Cebu remained within guideline values as of 12 noon Thursday, although monitoring stations had detected an increase in particulate matter from the ashfall.

Air Quality Monitoring Expert Arthur Niño Calupig said that the agency observed an increasing trend in PM10, or particulate matter measuring 10 microns or smaller.

“It’s within the guideline values. Meaning, it won’t affect the public that much. But we noticed an increase in PM10 levels because of the ashfall in Cebu,” Calupig said.

He said that Cebu could continue experiencing ashfall if Kanlaon continues erupting and the prevailing wind pattern remains unchanged.

“Kung patuloy po ang pag-alburoto ng Kanlaon at kung pareho pa rin ang wind patterns na papunta dito sa Cebu, projected po natin ay maaari pa rin nating maranasan yung ganitong ashfall sa mga susunod na araw,” he added.

What residents should do

The Cebu City Disaster Risk Reduction and Management Office (CCDRRMO) continues to advise residents to wear face masks when going outdoors, limit exposure to volcanic ash, protect food and drinking water from contamination, and monitor official advisories from Phivolcs, PAGASA, and local disaster management offices.

Meanwhile, the Office of Civil Defense (OCD) in Central Visayas activated its virtual Emergency Operations Center and continues coordinating with local government units to monitor affected communities.

According to OCD Regional Director Edgar Posadas, the agency convened an emergency meeting after ashfall reached Cebu to gather information that would guide local governments in issuing public advisories.

Authorities said that they will continue monitoring Kanlaon’s activity and prevailing weather conditions to determine whether additional ashfall could affect Central Visayas in the coming days.

Your subscription could not be saved. Please try again.

Your subscription has been successful.