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Some characteristics of ice and snow caves in Croatia

Most of the caves and pits with ice and snow in Croatia are situated in Dinaric Karst Mountains (Gorski kotar area, Kapela Mt., Velebit Mt., Dinara Mt, Biokovo Mt.). Generally, the highest mountains parts (higher than 1500 m a.s.l.) have a humid boreal climate (Köppen's type Df) and the lower parts have a temperate humid climate (Cfb) highly modified by the relief (distribution of ridges and depressions suitable for retention of colder air even in lower altitudes below 1000 m a.s.l.). Mean annual temperature in the area 1000 m a.s.l. is about 5.5°C and in the highest parts around 3.5°C. The coldest months are January and February (mean annual temperature between -2 and -5°C) and the warmest one is July (12-16°C). Due to the position between Adriatic Sea and the Pannonian Basin there are important climate modifications. The most important ones are very high amounts of precipitation due to the vicinity of the Adriatic Sea, general atmospheric circulation, and a high relief. Annual precipitation varies from 2000 to 3900 mm/year. According to these data there are good conditions for the accumulation of ice and snow in caves and pits. In our presentation we will use data from Risnjak and Velebit Mt. area. The Risnjak Mt. area is located in Gorski kotar in NW Croatia. There are 80 speleological features (mostly pits) developed in carbonate rocks of mostly Jurassic age. Most of them have occasional accumulation of snow fallen down from the surface and ice formed by freezing of drip water. According to meteorological data collected in the 105 m deep Nestasna jama (1335 m a.s.l.) air temperature in the summer at the bottom of the vertical passage filled with permanent ice was 0°C to 1°C with a very small amplitude during a one week measurement period. The northern Velebit karst area is conducive to the formation of deep pits, three of these are deeper than 1 km: Lukina Jama - Trojama pit system (-1392 m), Slovačka Jama (-1320 m) and the Velebita Cave system (-1026 m). So far, more than 280 pits have been discovered in an area of 25 km2 . The area is composed of carbonate lithostratigraphic units ranging from the Middle Triassic to the Paleogene. Lukina jama is almost vertical from the entrance (1475 m a.s.l.) up to the depth of 550 m with one inclined snow shelf at a depth of 320 m. From 50 to 320 m, the deposits of snow and ice are on the walls, so the pit temperature in this part is 0°C. In deeper parts there is a positive temperature gradient up to 4°C at the bottom. Patkov Gušt pit is 553 m deep. The entrance is at 1450 m a.s.l. From the depth of 50 to 135 m there are thick deposits of snow and ice on the walls. At the depth of 105 m there is the narrowest passage part (2×1.5 m) which is occasionally completely filled with snow and ice. Below the depth of 130 m the pit widens. The walls are completely or partially covered by ice coating up to 300 m depth. Ice breaking and collapse was noticed in summer. Ledena jama in Lomska duliba is 536 m deep. It has a 42 m thick layer of ice starting at the depth of 62 m. Paž pit is 400 m deep. There is a negative temperature gradient in this pit in the summer from about 20°C near the entrance down to 0°C at the depth of 200 m. Down to the bottom the walls are partially covered by ice which is falling down to the hall where the temperature is 0°C. Most of the pits on Mt. Velebit have vertical entrances with temperatures close to 0°C down to about 100 m, so in many other pits we found ice and snow. On the other hand, in some of deepest pits in the same area – Slovačka jama, Velebita, Lubuška jama – there is no ice and snow, most likely because the entrance is not vertical. In the upper parts of these pits there is also a negative temperature gradient, but the minimum temperature is a few degrees above 0°C.

ice caves; microclimate; Velebit; Risnjak

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