.ON APRIL 2ND a contraption resembling a snowfall equipment on the deck of the USS Hornet, a defunct aircraft-carrier anchored in San Francisco Gulf, started making a haze of salty spray particles. The medical partnership responsible for the job, the Marine Cloud Brightening (MCB) program, led by the College of Washington, is investigating whether such sprays may assist to slow weather adjustment through a kind of "geoengineering". What performs this phrase method-- and also why is it thus controversial?