.ON APRIL 2ND a contraption looking like a snow machine on the deck of the USS Hornet, a nonexistent aircraft-carrier anchored in San Francisco Gulf, started generating a mist of salted spray particles. The medical cooperation behind the venture, the Marine Cloud Whitening (MCB) programme, led due to the College of Washington, is actually exploring whether such aerosols might assist to slow weather modification by means of a form of "geoengineering". What performs this condition mean-- and why is it thus disputable?