Mexico’s Popocatepetl, which threatens 22 million people, is a much-watched volcano

MEXICO CITY — Mexico’s Popocatepetl volcano erupted again this week, spewing towering clouds of ash, forcing 11 villages to cancel school classes.

Residents weren’t the only ones keeping a close eye on the towering peak. In Popocatepetl, every time you detect a sigh, a rattle, or a ripple, dozens of scientists, a network of sensors and cameras, and a roomful of high-powered equipment are watching your every move.

Affectionately known as “El Popo,” the 17,797-foot (5,426-meter) volcano has persistently spewed toxic fumes, ash, and glowing chunks of rock for nearly 30 years since it awoke from a long slumber in 1994.

The volcano is 45 miles (72 kilometers) southeast of Mexico City, but much closer to the eastern edge of the metropolis of 22 million people. The city also faces threats from earthquakes and subsidence, but the volcano is the most visible potential threat—and the most closely watched. A major eruption could shut down air traffic or engulf a city in suffocating clouds of ash.

Around the summit are six cameras, a thermal camera and 12 seismological monitoring stations, operating 24 hours a day, all reporting to the equipment-filled command center in Mexico City.

A total of 13 scientists from a multidisciplinary team work in shifts around the clock in the command center. Warning of an impending ash cloud is crucial because people can take precautions. Unlike earthquakes, the warning time can be longer for a volcano, and the peak is usually more predictable.

Recently, researcher Paulino Alonso toured the measured values ​​at the command center of Mexico’s National Disaster Prevention Center (known as Cenupred). It’s a complex task that involves seismographs that measure the volcano’s internal shaking, which can indicate hot rock and gas moving up through vents at the summit.

Observations of gases in nearby sources and at the summit—and wind patterns that help determine where ash might blow—also play a role.

The forces inside are so great that they can temporarily deform the summit, so cameras and sensors must monitor the shape of the volcano.

How do you explain all this to the 25 million non-experts living within a 62-mile (100-kilometer) radius who are so used to living near the volcano?

The authorities came up with a simple idea of ​​a volcano “brake light” with three colors: green for safety, yellow for warning and red for danger.

The brake light has turned “yellow” for most of the years since its introduction. Sometimes the mountain gets quiet, but not for long. It rarely spews its molten lava: instead, it rains “explosive”-type hot rocks that crash down its sides, emitting bursts of gas and ash.

The center also has monitors in other states; Mexico is a country that is all too familiar with natural disasters.

For example, the early warning system for earthquakes in Mexico also operates in the command center. Because the city’s soil is so soft — it’s built on a former lake bed — an earthquake hundreds of miles away on the Pacific coast can wreak havoc on the capital, as it did in 1985 and 2017.

Coastal seismic monitors send messages that travel faster than earthquake shock waves. Once the sirens start blaring, it can give Mexico City residents half a minute to get to safety, usually on the streets outside.