Airports across Java and Sumatra are operating again, but the volcano that closed them has not been downgraded. NASA's Earth Observatory published satellite imagery of Anak Krakatau's early September eruption on September 9 and noted that the volcano remained at the second-highest level on Indonesia's four-level alert scale.

That gap between the reopening and the underlying hazard is what travelers and air carriers should keep in view. Indonesia's meteorological agency, BMKG, dated the first eruption of this episode to September 4, and NASA described an eruption in early September that lasted more than 24 hours. Ash from that eruption forced the temporary closure of eight airports and disrupted nearly 3,000 flights, according to news reports cited by NASA. The continuous explosive phase subsided on September 6. The volcano kept erupting.

Anak Krakatau sits in the Sunda Strait between Java and Sumatra, close to the airports that serve Jakarta and southern Sumatra. For readers far from Indonesia, the relevance is less about ash on the ground than about ash in the air, where it can disrupt flights well beyond the volcano itself.

Ash at Cruising Altitude Posed a Different Hazard Than Ash on the Ground

BMKG reported that ash had reached very different altitudes on either side of the volcano by September 6: roughly 6,000 meters, or about 20,000 feet, to the east, and roughly 15,000 meters, or about 50,000 feet, to the west.

Ashfall reached populated areas east of the volcano, including Jakarta and other parts of West Java. Ash to the west climbed above the altitudes where airliners typically cruise. Volcanic ash consists of pulverized rock and volcanic glass rather than soot, and when it is drawn into a jet engine it can melt on hot engine parts. That property, more than the size of any single explosion, is why ash can close airspace far from an eruption.

BMKG confirmed the temporary closure of eight airports on Java and Sumatra, including Soekarno-Hatta and Halim Perdanakusuma, which serve the Jakarta area, Husein Sastranegara in Bandung and Radin Inten II in Lampung. The agency said it had issued 39 SIGMET aviation hazard warnings since the first eruption on September 4. BMKG head Teuku Faisal Fathani urged the public not to panic and described the closures as a proactive safety measure.

On September 8, BMKG said field paper tests had come back negative for ash and that all eight airports had reopened, several of them shortly after midnight local time. The agency and its partners also began weather modification flights on September 7, seeding clouds and spraying water mist in an effort to clear ash from the atmosphere.

Landsat and VIIRS Captured the Eruption at Two Scales

NASA's images come from two instruments that observed the eruption on September 5. The Operational Land Imager on the NASA-USGS Landsat 8 satellite recorded a close view showing a white plume of volcanic gas billowing over a brown ash cloud. The Visible Infrared Imaging Radiometer Suite on the Suomi NPP satellite captured a wider view of volcanic material dispersing over Indonesia and the Indian Ocean.

Orbital views help fill gaps in ground observation. In weekly summaries published this summer, Indonesia's volcanology agency, PVMBG, noted that fog and cloudy weather at times obscured continuous visual observation of the volcano.

The eruption also revives a common confusion about Krakatau's history. According to the Smithsonian Institution's Global Volcanism Program, caldera collapse during the catastrophic 1883 eruption destroyed the Danan and Perbuwatan cones and left only a remnant of Rakata. That eruption caused more than 36,000 deaths, most of them from tsunamis. Anak Krakatau, whose name means Child of Krakatau, was built later inside the 1883 caldera and has erupted frequently since 1927. It is the successor to the volcano that erupted in 1883, not the same cone.

Alert Level 3 Carries a 3-Kilometer Exclusion Zone

Indonesia's volcano alert scale runs from 1 to 4, so Level 3 is the second-highest level, not the top. PVMBG raised Anak Krakatau to Level 3 on July 2, the same day the aviation color code was raised to Orange, the second-highest color on a four-color scale.

The level has not held perfectly steady. NASA describes the volcano as having been at the second-highest level since early July, but Smithsonian weekly summaries list it at Level 2 in reports covering late July and the first days of August, and at Level 3 again in the report covering August 6 through 12.

The most recent published summary, covering August 27 through September 2, reported ash-and-gas plumes rising 100 to 400 meters above the summit, with the alert level unchanged at 3 and the public advised to stay 3 kilometers, about 1.9 miles, from the summit. That report predates the September eruption. The Smithsonian and USGS update the weekly report by 2300 UTC every Thursday, so a newer summary was due on September 10.

Since September 6, the volcano has returned to a more typical pattern of Strombolian activity, marked by intermittent bursts of ash and volcanic material, and BMKG cited PVMBG in reporting a small Strombolian eruption on the morning of September 8. In its September 9 image release, NASA said that after airports reopened, "the volcano remained at the second-highest alert level on the country's scale."

Volcanic Ash Calls for Different Precautions Than Fire Haze

The Indonesian Humanitarian Coordination Platform reported on September 6 that ashfall affected populated areas, particularly to the east in Jakarta and other parts of West Java. Volcanic ash can irritate the respiratory tract, eyes and skin. The group noted that this hazard differs from the peatland fire smoke affecting other parts of Indonesia in particle characteristics, dispersal patterns, and protective measures.

BMKG advised people in affected areas to limit outdoor activity and to wear masks and protective eyewear if they must go outside. Ash exposure near the volcano was not new this season. Residents of Tejang Village on Sebesi Island, about 20 kilometers north-northeast of Anak Krakatau, reported ashfall and health complaints in mid-August, according to a news report cited in the Smithsonian's weekly summary.

Travelers with flights through Jakarta, Bandung, Lampung or other affected hubs in the coming weeks should confirm status with their carrier rather than relying on last week's reopening. Aviation ash warnings are issued separately from Indonesia's ground-based alert level, so flights can be affected by new ash emissions without any change to Level 3.

The Next Confirmed Data Points

PVMBG posts daily observations and issues Volcano Observatory Notices for Aviation when ash is detected. The Smithsonian and USGS Weekly Volcanic Activity Report updates every Thursday by 2300 UTC. BMKG issues its own bulletins when ash affects Indonesian airspace.

The most recent published reports show no change from Level 3. Nature World News will follow the weekly reports and any change to the aviation color code.

The practical takeaway is narrow. The airports reopened, but the volcano did not stand down, and a vent at Alert Level 3 producing Strombolian activity can still put ash into flight corridors with little notice. Carrier notifications, not last week's reopening news, are the best guide to current conditions.

What Readers Want to Know

What happened at Anak Krakatau? BMKG dated the first eruption of this episode to September 4, 2026. A continuous explosive eruption lasting more than 24 hours followed in early September, sending ash and gas high into the atmosphere. That phase subsided on September 6, and the volcano returned to intermittent Strombolian activity.

Is the volcano still on alert? Yes. The most recent published weekly summary, covering August 27 through September 2, lists Anak Krakatau at Alert Level 3 on a scale of 1 to 4, the second-highest level. NASA reported that the volcano remained at that level after airports reopened. PVMBG raised the aviation color code to Orange on July 2.

How high did the ash go? BMKG reported ash reaching about 20,000 feet east of the volcano and about 50,000 feet to the west by September 6.

Are the airports open? Yes. BMKG said all eight airports that closed on Java and Sumatra reopened on September 8 after tests showed no ash. Individual flights can still be affected by new ash warnings.

Who faces the greatest risk? People downwind of the volcano, particularly to the east toward Jakarta and West Java, face ashfall exposure. Aviation faces the widest operational risk because ash reached cruising altitude.

Does volcanic ash require the same precautions as wildfire smoke? No. The Indonesian Humanitarian Coordination Platform noted that ash differs from peatland fire smoke in particle characteristics, dispersal and protective measures. BMKG advised limiting outdoor activity and wearing masks and protective eyewear outdoors.

When is the next official update expected? The Smithsonian and USGS Weekly Volcanic Activity Report updates every Thursday by 2300 UTC. PVMBG posts daily observations and issues aviation notices whenever ash is detected.