.ON APRIL 2ND a gizmo being similar to a snowfall machine on the deck of the USS Hornet, an invalid aircraft-carrier moored in San Francisco Bay, started making a haze of salty aerosol particles. The clinical cooperation responsible for the project, the Marine Cloud Brightening (MCB) program, led due to the College of Washington, is examining whether such sprays could aid to slow weather improvement through a type of "geoengineering". What does this condition mean-- and also why is it thus controversial?