There is something of a micro-climate effect in Loch Ness, so this region often has notably different weather from the rest of the Scottish Highlands. Whereas Fort William, at the western end of Great Glen (a series of valleys along southern Scotland), often gets upwards of 80 inches of rain (about 200cm) per year, the lake itself averages around 24 inches (60cm) a year.

Despite the northern latitude of the lake (about 56°N), the climate of Loch Ness is greatly moderated by the Gulf Stream winds that blow from the southwest, so the weather remains pleasant for most of the year. Winters, especially, are not severe, and snowfall is rarely heavy enough to cause roadblocks or traffic problems. The lake itself, which contains the largest volume of fresh water of any lake in Scotland, also remains at 42°F (5°C) year-round and moderates the amount of snowfall and ice in the surrounding villages. If you visit during the winter, however, it is still recommended that you bring a heavy coat and be prepared for temperatures below freezing. A raincoat or umbrella is a good idea no matter what season it is, but Scotland is not a land known for extreme weather, despite the often unpredictable day-to-day variation.

 Until recently, it used to be said that the best times to visit the Highlands were in May and September, when there was usually plentiful sunshine, and a dearth of midges; however due to climate change, this is no longer reliable.  These days there can be terrific weather from early March. The warmer drier summers seem to be reducing the number of midges so June, if dry tends to be relatively midge free, but they can still be encountered after the end of September. A reliable chemical to stop you getting bitten is Avon Skin So Soft. It doesn't stop the midges landing on you, for example it is not a repellant, but it is very effective at stopping them biting.

BBC's Inverness & Region Weather  - 5 Day Forecast