What is the best time to visit Dumbarton?

For long daylight hours and your best chance of getting at least a few sunny days, May through August is the best time to visit. However, the area can get very crowded with tourists, especially around Loch Lomond, and on the West Highland Way, in July and August. September and October both offer a good alternative as the large crowds will have gone and the landscape is covered in rich reds and blooming purple heather.