Northern lights in Green Bay: tonight’s chance

The bay as a horizon

Green Bay grew up where the Fox River empties into the southern tip of a long arm of Lake Michigan, and that arm, the bay of Green Bay, runs off toward the northeast. It gives the city something Milwaukee and Chicago lack: open water in a northerly direction. From the river mouth and the bay shore just north of town, the dark surface of the bay fills the north-northeast. To the true north the western shore is lit by small communities, but the bay itself carries the eye far toward the Upper Peninsula.

The Door Peninsula, on the far side of the bay, is the darker option. Its western shore faces across the bay to the northwest, and its towns are small and spaced out. On a storm night, a pullout on that shore looking over the water is about as good a view as eastern Wisconsin offers.

At the meeting point of two storm levels

With a threshold of Kp 6.1, Green Bay sits just beyond the line where the NOAA scale moves from G1 (Minor) to G2 (Moderate). A G1 night, however strong, usually falls short, and even the first G2 blocks only just reach the threshold. That makes the decimal in the three-hour forecast worth reading closely: a block a third of a Kp below the line is a likely no here, a third above it a real maybe. Because the threshold and the overhead Kp are the same, a night that does reach Green Bay's level tends to light up a broad band of the northern sky. The storm scale guide lists what each level means in practice.

Spring fog, autumn clarity

The bay stays cold well into spring, and humid air moving over it tends to produce low cloud and fog. That is a likely reason NASA POWER averages put May, at 75%, ahead of the winter months for cloud amount. By September the picture has turned around, with 59% and nights that already last 8.2 hours of full darkness. January follows, thanks to the cold and dry air of Arctic highs. The Wisconsin page gives every town's threshold, and the Marquette page covers the Lake Superior shore farther north.