Paso de Cortes

A cloud of ash belches out of Mexico's P...A cloud of ash belches out of Mexico's Popocatepetl volcano, some 55 km from Mexico City, as seen from Paso de Cortes, in the Mexican central state of Puebla, on May 20, 2013. Authorities have raised the alert level to "Yellow Phase Three," the fifth of a seven-stage warning system, restricting access to an area of 12 km around the volcano while preparing evacuation routes and shelters. AFP PHOTO/Arturo AndradeARTURO ANDRADE/AFP/Getty Images

Arturo Andrade/AFP

A cloud of ash belches out of Mexico’s Popocatepetl volcano, about 55km from Mexico City, as seen from Paso de Cortes, in the Mexican central state of Puebla. Authorities have raised the alert level to “yellow phase three”, the fifth of a seven-stage warning system, restricting access to an area of 12km around the volcano while preparing evacuation routes and shelters.