Loch Linnhe is the sea loch that runs for 30 miles from the Scottish sea port of Oban, down to Fort William, where it changes its name to Loch Eil and carries on for a further 10 miles to Kinlocheil (this translates to ‘End of Loch Eil’). This sea loch combined with the mountains creates a weather funnel aimed straight at Ben Nevis. Add Ben Nevi’s height, well over a kilometer above the loch and it magnifies the weather. If it is windy at loch level (sea level) it will be much stronger on the summit. My house is about 40m above sea level, so if we have a frost I know it will be a very cold day on top of Ben Nevis summit.
The Ben Nevis weather rules are very simple. What ever is happening at sea level, more of it will be happening at the summit!
Depending on how flexible you are with your route to the top of Ben Nevis you can plan for this. If the wind is coming in from the west going up the scramble called Ledge Route on the North Face of Ben Nevis may keep you out of the wind until you get to Carn Dearg at 1221m, a subsidiary summit of Ben Nevis.
It is really the weather, not the time of year that you need to be aware of.