Original scientific paper
THE CLIMATIC CHARACTERISTICS OF VELEBIT AND THEIR INFLUENCE ON THE RELIEF FORMATION
DRAŽEN PERICA
DANIJEL OREŠIĆ
Abstract
The average annual insolation becomes lesser from the coastline toward the top of the mountain and inland Lika's region. In the colder half of the year, the insolation is higher on Zavižan (top of the mountain) than in Gospić (situated in Lika) because of the frequent fog and low overcast. Quite contrary is in the warmer half of the year because of the orographic clouding. The slope aspect and inclination are important for the amount of the sun radiation received and consequently for the thermomechanical destruction of the bare rocky surface. The thermal influence of the sea is limited to the coastline and lower parts of the SW side of Mt.Velebit.In the warm part of the year the warming of the barren carbonate rock surface is responsible for a greater number of warm and hot days on the seaward mountain side. The intense warming influences the thermomechanical destruction at the same time increasing evapotranspiration, which leads to the drought on the lower parts of the SW mountain side. In such conditions the intensity of corrosion and biocorrosion is diminished. During intense droughts it is especially low in the shallow pedological cover prevailing on the coastal and lower parts of SW mountain side. This is confirmed by the new measurements of the corrosion intensity. Corrosion intensity is greatest on the central parts of the SW mountain side, while it is lower towards lower parts and especially lower towards mountain top. Lower and central parts of SW mountain side are characterised by a far greater corrosion intensity in the soil than on the surface. With the increase of the altitude, the temperature on the SW mountain side declines. It, however,
declines slower on the lower parts and quicker on the upper parts of the SW mountain side. Mt. Velebit has been also the south and southwest border of the Lika and Gračac plains ("polje"). In winter Lika and Gračac plains become filled up with the cold air, which moves from the inland towards the sea and is stopped by Mt.Velebit. This is often accompanied by clear and calm weather which is the cause of temperature inversions. Thus on the lower parts of the NE mountain side, due to the relief of plains' rim, the decline of the temperature with the increase in altitude in winter time is negligible and sometimes there is an inversion. As regards the annual average, temperature declines slower on the lower parts and faster on the upper parts of the NE mountain side. Temperature differences on the Mt.Velebit can be significant on relatively short distances. This can be observed in vegetation distribution. Criogenic processes as well as mechanical weathering conditioned by low temperatures and great temperature amplitudes are predominant processes in the upper parts of Velebit mountain range. The reason for extremely low corrosion intensity in the soil in the upper parts of Velebit is in physiologica drought conditioned by the high number of cold days, when due to the freezing of water the corrosion stops and the biocorrosion is low (short vegetation period). The whole area of Mt. Velebit has a mediterranean precipitation regime. All the precipitation gauges show a maximum of precipitation in fall, and a secondary maximum in spring. Great fluctuations of monthly precipitation ammounts are characteristic for the investigated area. The difference in the maximum and minimum ammounts of monthly precipitations
becomes greater as we move from northern to southern Velebit. With the increase in altitude the difference is diminishing. With the increase of altitude, the total amount of precipitation becomes greater, however not at the same rate everywhere. The slowest increase rate is on the northern Velebit, where the isohyet of 2,000 mm passes approx. at 1,400 m. On the area of Rožanski kukovi and Hajdučki kukovi, there is over 2,000 mm of precipitation. On the central Velebit the annual amount of precipitation is greater and the isohyet of 2,000 mm lowers gradually from 1,500 m on the central part of northern Velebit to 900 m on the SW mountain side, and on the NE side to the bottom of Lika plain ("polje"). The largest amount of precipitation is registered on southern Mt.Velebit tops (Vaganski vrh and Sveto Brdo), where there is approximately 3,500 mm of precipitation yearly. On the NE mountain side, the amount of precipitation is declining as we move away from the mountain top. In the base of NE mountain side there is also some gaining of the amount of precipitation as we move alongside it from NW to SE. Relatively large amount of precipitations as well as their monthly distribution is having a significant influence to the intensity of the corrosion process. The precipitation maximum in autumn and the organic decay of plants have had a significant contribution to the corrosion proces due to the greater concentration of C02 in the soil. However, towards the mountain top the corrosion process diminishes and the criogenic process strenghtens due to the low temperatures and freezing of water. Great importance for the corrosion process has a large amount of additional precipitation, not measwred by normal gauging stations but by totalisators in the highest mountain parts. In such conditions, the corrosion intensity on the surface has been even greater than in the soil. The errosion processes are influenced by the fact that most of the precipitations are usually in the form of short but intensive showers and by the generally great slope inclination favouring surface drainage. The number of days with snow cover as well as its height increase with altitude. The duration of snow cover is considerably longer in the forest areas than ori the open fields, on northern aspected slopes and specially long (often to the second half of July) on the bottom of deep valleys ("dolinas") and on the bases of avalanche fields. In the areas where the slope inclination is over 30 degrees the avalanche are quite often, while on the slopes over 60 degrees snow does not hold but is evacuated downhill immediately. The absence of snow cover on the parts of the mountain top has been the cause of intense mechanical weathering and appearing of periglacial forms. Frost weathering is especially visible on the gravitationaly accumulated rock fragments. On the other hand, snow resting in depressions and rock fissures prolongs the corrosion process. This is notable in widening and deepening of valleys ("dolinas") and depressions ("uvalas") bottoms in the upper mountain parts. Among the prevailing continental winds, the main wind is bora, well known on the SW mountain side (in Senj blows from NE and E, in Karlobag from N, NE and E). While on the seaward mountain side there is bora, in Lika blows norther wind which is often changing direction along the NE side of the mountain and frequently blows as western wind at the foot of the mountain. At the same time, along the top of the mountain, due to relief influence, eastern, northeastern (Baške Oštarije) or northwestern (Zavižan) winds very often blow very strong. The most important maritime winds are scirocco (SE wind) on the coastal part of the SW mountain side and western, southwestern, southern and southeaster winds on the top of the mountain, NE mountain side and in Lika. During the summer frequently blow the sea breezes (in Croatia called maestral), mostly as northwestern and western winds and its direction is modified by the Velebit channel form. Sometimes it is difficult to differ maestral from the daily wind circulation. During the calm eves and nights, especially in the warmer half of the year, often blows strong wind from NE to Sw mountain side because of the temperature differences between them. In winter, similar to bora, it blows the snow away from the mountain top and wind exposed sites on the slopes and builds up snow on leeward sides. On the lower parts of the NE mountain side, winds are less strong and the share of calms is greater. Towards mountain top, the winds are stronger and the number of days with strong and very strong winds is greater. With the prevailing bora, SW mountain side is most windy (especially Senj). In the geologic history, the climatic changes conditioned the change of the paleokarstic relief and the formation of other relief types. The glacial period (Wiirm III) is characterised by the development of glacionival karstic forms in the area above 1,300 m of altitude, while in the lower parts they are mainly stipulated by thermic inversion. The 12 degrees C lower temperatures on the top parts of Velebit favoured the development of glaciers. As the glaciers are developed in the existing valleys ("dolinas") and depressions ("uvalas"), they have reshaped their sides and bottoms.
Keywords
Hrčak ID:
41550
URI
Publication date:
30.4.1999.
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