Freezing Rain in Hungary’s Pilis Hills: Ice-Coated Forest, December 2014

At the start of December 2014, the wooded hills around Budapest woke up encased in ice. Branches wore coats of glaze centimetres thick, trees bowed to the ground and whole forests turned glassy and silent. Photographs shared by the Hungarian Meteorological Service (OMSZ) show the scene in the Pilis region.

The freezing-rain event of 1 December 2014

According to OMSZ’s analysis, rain began in the early hours of Monday 1 December and fell almost without a break until late evening. Between noon on Sunday and noon on Tuesday, 30 to 40 mm fell widely, and more than 50 mm along a line from Komárom through Budapest to Szolnok.

The heaviest icing hit the higher ground of Pest County: the Gödöllő Hills, the Buda Hills, the Visegrád Mountains and the Börzsöny. Ice also built up in the Mátra and Bükk ranges further east. In the Buda and Visegrád hills, above roughly 300 to 400 m, the air sat at around −1 °C, and the János-hegy station in Budapest stayed just below freezing throughout. The day before, on 30 November, the Pilis had already been dusted with rime.

How freezing rain forms: warm air over cold

The weather pattern behind the event was a classic recipe. A persistent high-pressure system north-east of Hungary fed cold air from beyond the Carpathians across the Great Plain, forming a shallow freezing layer near the ground. At the same time, a strong Mediterranean low pushed a warm front north, and its mild, moist air rode up over the cold layer at about 1 km altitude.

Snow falling from the clouds melted in that warm layer, then dropped as rain into the sub-zero air below. The drops became supercooled, staying liquid below 0 °C, and froze the instant they touched a branch, wire or road. The result is glaze: clear, heavy ice that follows the exact shape of whatever it coats.

Glaze versus rime ice

• Glaze comes from freezing rain. It is smooth, transparent and dense, and can build up to several centimetres.
• Rime forms when supercooled fog or cloud droplets freeze on contact. It is white, brittle and feathery, and grows towards the wind.
• Ice pellets (sleet) freeze in mid-air before landing, so they bounce off instead of coating surfaces.

The close-ups here show both clear and milky ice around the same twigs, a reminder that different kinds of icing can stack up during a long cold spell.

Ice-storm damage to forests

Ice is beautiful and brutally heavy. Once glaze passes about 6 mm thick, loaded limbs start to snap, and dormant trees are especially vulnerable. OMSZ reported significant forest damage in the Hungarian hills, and the wide views here show bent trunks and broken branches lying across the paths. Stay out of iced woodland until the thaw: falling limbs are the main danger.

The Pilis, the Transdanubian range north-west of Budapest whose summit, Pilis-tető, reaches 756 m, is normally a favourite hiking area for the capital. Its forests were once royal hunting grounds of the medieval kings of Hungary.

For more icy forests, see Marko Korošec’s Javornik rime photographs, the companion Javornik photo essay and Beijing’s 2014 frozen-garden photographs.

Related reading

• Jan Bainar’s Frosted Trees in the Beskydy Mountains
• Daniel Barreto’s Woodhouses: Digital Homes Inside Imaginary Forests

Sources