Camping is only prohibited on the Balmoral Estate when the Royals are there, and then it still doesn't include Lochnagar, just the area in about a 2-mile radius around the castle, and south of the river Dee. As in almost all of Scotland, you can pitch up your tent on Lochnagar wherever you want, and whenever you want. There are plenty of SAS members in the area though, generally staying in derelict buildings like Gelder Shiel and Bovaglie.
is out of date.
for local weather
http://www.sais.gov.uk/ .... for avelanch risc