Popocatépetl volcano: Alert level raised as ash shutters Mexico schools, airports

The stoplight warning system for the Popocatépetl volcano in Mexico rose to yellow phase 3 on Sunday, according to officials. 

In a tweet, the government shared a chart showing that alarm was just below the "red" level. 

The phase includes large domes developing and exploding with increasing intensity, sending rock into the air and pyroclastic material down the sides.

For most of the years since the stoplight was introduced, it has been stuck at some stage of "yellow." 

On Tuesday morning, the government tweeted that there is no visibility toward the stratovolcano, which is situated about 40 miles southeast of Mexico City, but that it "has been possible to observe an emission with a low ash content." 

'EL POPO,' MEXICO'S SECOND-TALLEST VOLC...