In winter, halos can also form directly at ground level when moisture-saturated air crystallizes. This typically occurs at very low temperatures below –10 °C, for example in river valleys. Additional condensation nuclei can strongly influence the temperature at which ice crystals form, as can orographic lifting processes over mountain ridges and passes. Consequently, ice fog halos are frequently observed in ski resorts, where snow cannons provide abundant condensation nuclei, but they can also form on mountain passes where fog enriched with dust from the valleys flows over the highest points, crystallizes into ice particles, and subsequently dissipates.
Similar conditions are found along the crest of the ridge of Ore Mountains (Erzgebirge). This tilted fault-block mountain range rises gradually from the Saxon side and then “drops” abruptly at the ridge into the Eger Valley, which lies up to 1,000 meters lower on the Bohemian side. In the Bohemian Basin, persistent fog often develops during high-pressure weather patterns, sometimes lasting for days or even weeks. Numerous power plants enrich this fog with condensation nuclei. The very cold, moisture-saturated air over the basin is denser and has higher air pressure than the often warmer air mass along the ridge. When this fog flows over the Ore Mountain crest, orographic lifting and pressure equalization cause water vapor within the fog to freeze around the abundant condensation nuclei, forming minute ice crystals. The large number of nuclei promotes ice crystal formation even at comparatively high temperatures, because in this supersaturated air water droplets freeze readily and in significant numbers.
Thus, under south to southeasterly winds, it is sometimes possible to observe spectacular diamond dust halos between the Bohemian Klínovec (Keilberg, 1,244 m) and the Saxon Fichtelberg (1,215 m) at temperatures from –2 °C. This diversity of halo types is unique in Central Europe. Snow cannons play a minimal role here, as the prevailing Bohemian winds carry ice crystals from natural fog across the summit. In the adjacent valleys, however, additional crystals may form from the fine droplets produced by snow cannons when winds are calm. Nevertheless, diamond dust halos have been documented in the Fichtelberg–Klínovec region long before the advent of snowmaking equipment.
Beginning on 11 December 2024, a large high-pressure system in the Bohemian Basin produced dense fog, which in lower elevations of the central and eastern Ore Mountains resulted in freezing drizzle and glaze ice. Webcams occasionally showed brief halo displays, but the fog’s upper boundary was high, allowing little sunlight to penetrate.
Conditions changed on 13 December, when the fog top descended to roughly 1,000 meters. Shortly after sunrise, both webcams and photographs taken by observers on-site captured impressive halo formations. In addition to the 22° halo and sundogs, the upper tangent arc and the Parry arc were visible, the latter appearing just before sunset as a distinctive “double V” shape nestled against the upper tangent arc.
In the early afternoon, we set out for the Ore Mountains’ “diamond dust halo paradise” and descended into the lower fog within the Czech Neklid ski area on Klínovec. What awaited us there was breathtaking—a glittering, three-dimensional halo display featuring 18 distinct halo types and 25 single halos. Ice crystals sparkled in every direction, and near the zenith and antisolar point it was especially difficult to match the dazzling glints to their corresponding halo forms. Only later, upon reviewing the photographs—of which a small selection is presented here—were the arcs clearly identifiable. Altogether, the following halo types were visible to the naked eye:
- 22° halo
- Left and right parhelia (sundogs)
- Upper and lower tangent arcs
- Upper and lower light pillars
- Circumzenithal arc
- 46° halo
- Parhelic circle
- Right 120° parhelion
- Supralateral arc
- Infralateral arc
- Concave and convex Parry arcs
- Subsun
- Wegener’s anthelic arc
- Tricker’s anthelic arc
- Tape arcs
- Heliac arc
- Anthelic arc
- Moilanen arc
For us, this was the second-largest halo display we have ever witnessed, surpassed only by an event in 2014, when 20 halo types and 23 single halos were seen simultaneously. However, the 2014 display was short-lived, whereas this time the halos persisted for more than 30 minutes.
Text and photos: Claudia and Wolfgang Hinz
