It had been raining in the valley bottom at night. The very precise snowline around Langdale last week brought home to me that that same rain had been falling as snow half a mile away, and that up the hillside, movement of a few metres must have meant moving from rain into snow. So the conclusion is that all precipitation starts as snow and then melts as it falls.
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/prcp/rs.rxml Now I'm thinking about the thermodynamics. How quickly can the thermal energy from the warmer air flow into a snowflake and cause a change of state? It clearly isn't an adiabatic process.
