Ice can form on airplanes when temperatures are near freezing and there is visible moisture, such as clouds or rain.

WSJ: NTSB: Pilot action, icing led to NJ plane crash

Springwater is trapped by the terraces, and then evaporates, forms clouds and falls again as rain higher up the mountain.

BBC: The perfect trip: China

This left Pakistan with too much to do and as rain clouds gathered in the East, they slipped further and further behind on Duckworth-Lewis.

BBC: Jonathan Agnew column

In China, they use artillery shells, rockets and airplanes to spray existing cloud systems with silver iodide, a compound that triggers moisture in the clouds to form ice crystals, which then fall to earth as snow or rain.

WSJ: Blizzard Renews Storm Over China Making Snow