.ON APRIL 2ND a contraption resembling a snowfall equipment on the deck of the USS Hornet, an obsolete aircraft-carrier moored in San Francisco Bay, began creating a mist of salted aerosol fragments. The medical partnership behind the task, the Marine Cloud Brightening (MCB) programme, led by the University of Washington, is actually checking out whether such sprays might assist to slow weather modification via a form of “geoengineering”. What performs this condition mean– and also why is it therefore controversial?