Lowitz Arcs - Portugal

Classic Lowitz Arcs over Porto, Portugal. One of 70 images by Jonathan Shock (site) 26th Nov '08. All three rare Lowitz arcs are visible. Upper and lower arcs curve away from the parhelion (sundog) to touch the 22° halo. The almost never seen Lowitz middle arc slopes downward through the sundog. Images ©Jonathan Shock, shown with permission.

Lowitz arcs were first recorded in 1790 over St Petersburg but their existence was long doubted until photographs were obtained comparatively recently. The way they are formed from ice crystals still remains controversial.

The traditional model assumes regular and thin hexagonal plate crystals taking all rotational positions about a horizontal axis passing through opposite prism edges and producing three arcs - see diagram below. It predicts the arc positions but rarely their extent or relative intensities.

Jonathan Shock took photographs over 22 minutes. Initially there was an upper Lowitz arc at the 2 o'clock position from the sun and above it but not near the sundog. There was a weak upper tangent arc, a hint of a Parry arc, a bright parhelic circle and very bright 120° parhelion. The 22° halo was probably absent. Later on, a suspicion of a Lowitz middle arc projected downward from the sundog - see the lower image at right. Upper and lower Lowitz arcs near the parhelion were still not apparent. The top image is one of the final ones taken when all three arcs had developed near the parhelion. As in many displays, cloud streaks can be mistaken for Lowitz arcs, Jonathan's many images helped avoid that.

The montage shows successive enhancements of the near parhelion arcs. The HaloSim simulation used a mix of traditionally oriented crystals and regular hexagonal plates with limited rotation about the Lowitz axis. Rather thick plates were needed.

The same crystals simulated the lower image taken earlier in the display. A better fit might have resulted from non-regular hexagonal plates to emphasise the middle arc without producing so much intensity in the upper and lower arcs near the parhelion.

There is unease about having to invoke such special requirements. Attractive explanations are ones that predict reality with minimum assumptions. Do we have the explanation for Lowitz displays wrong? Not impossible. But there is another factor, the appearance of Lowitz arcs is much more sensitive to crystal shapes and their particular orientations than most other halos. A tangent arc looks much the same regardless of the variety of hexagonal crystal shapes and lengths that make it - Lowitz arcs do not. Professor Lowitz certainly started something over two hundred years ago.