These ghostly hoops have been spotted above volcanic events around the world. But how they form has long been a mystery.

A calm sailboat floats below Mount Stromboli, Aeolian Islands, Italy, where volcanic ash meets the Mediterranean Sea.

If the 1.5 degrees Celsius temperature rise limit set at the Paris climate summit is breached, extreme rain deemed a once-a-decade rainfall event will occur 1.5 times every 10 years and be over 10% ...