The abundance of water vapor over Lake Superior promotes heavy snow in the winter, rain squalls in summer, and dense fog in spring and fall. The immense thermal capacity of the lake moderates temperatures, cooling warm air masses in the summer and warming cold air masses in the winter before ice covers the lake. The relatively deep waters outside of the Isle Royale Archipelago rarely warm past 45 degrees F at the surface during the summer, with the surface of relatively shallow bays, coves, and harbors rarely exceeding 50 degrees F. Subsurface temperatures remain below 40 degrees F year-round.

The effect on visitors is:

1. Paddlers should prepare for very cold trips in any waters directly connected to Lake Superior.
2. Divers are advised to use a drysuit.
3. All visitors should prepare for fog and frequent squalls.
4. The dense foliage surrounding many trails and portages will be covered in heavy dew in the morning.

The rugged terrain of Isle Royale controls its microclimates, as the steep northern sides of the ridges block prevailing cool northwesterly winds, with the more gently sloped southern sides of the ridges retaining more heat from the sun, making them warmer and drier.

The effect on visitors is:

1. Winds are stronger on the north side of the island, reducing the number of biting insects at campgrounds such as Todd Harbor, while creating rougher seas for boaters and paddlers.
2. Trails on southward facing slopes such as the trail from Rock Harbor to Mount Franklin will be much hotter in the summer than ones on exposed ridges on northward slopes, such as the trail on the Minong Ridge.