Cold air lake

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Kaltluftsee in the Alp Hintergräppelen in Switzerland. The lower temperature in the cold air lake is reflected in the formation of frost
Frost formation in a cold air lake formed overnight in a paleodoline
During the night, lakes of cold air are formed by the inflow of cold air, which has been cooled by contact with the ground and radiation . During the day, the inversion mostly disintegrates . In cold temperatures and snow cover, inversions in cold air lakes can persist for several days.

A cold air lake is an accumulation of cold air in a depression and is common in mountains . Cold air lakes form mainly at night when the earth's surface and thus the air directly above it cools down . This cold air flows down a mountain slope and can build up in depressions or other obstacles. A temperature inversion occurs there when the cold air layer flows directly under a warm one without mixing. The temperature difference can be, in extreme cases 30 ° C. In a cold air lake, therefore, frost and fog can form within a narrow range . They are often found in karst landscapes and occur mainly during high pressure weather in autumn and winter.

Due to the locally limited microclimate , they can be colonized by other plant species than the surrounding area. They are therefore relevant for plant sociology and can offer a micro refuge . Since the end of the 19th century, cold air lakes have become objects of demonstration of the formation of meteorological inversions, as well as long-term refuges for heat-sensitive biotic communities in the face of climate change .

Definition and etymology

" A cold air lake is an accumulation of locally formed cold air near the ground in closed, concave terrain or on obstacles due to cold air congestion " . The terms cold lake, cold island, frost hole, cold air puddle or hollow frost are also used synonymously. A distinction is made between cold air lakes and cold holes . The latter cold air outlets are found, among other things, especially downhill on block heaps of montane and alpine locations in weather-bearing cavity systems (wind tunnels) in solid and unconsolidated rock. In principle, however, cold holes can also occur in cold air lakes, but then they are not the cause of the stable cold air stratification. In plant sociology , alpine plant populations in cold-air lakes of the limestone Alps are sharply separated as limestone snow soils from those of the snow valleys in the central Alps that occur on silicate substrate .

The expression cold air lake or cold lake is derived from the observation that cold air behaves in a similar way to water, which always approaches the lowest point. We therefore speak of a flow of cold air. Therefore, concave terrains are always cold islands at night.

Requirements and processes

An essential prerequisite for severe frosts is a high sky-view factor (= low elevation of the horizon). The sky-view factor is defined as the proportion of the visible sky (Ω, gray area) above a certain observation point. Here in a two-dimensional representation based on a cold lake of the South Dinarides
The deepest frosts in Germany are measured in the closed depression of the Funtensee. A relatively high sky view factor is essential for this, as it significantly determines the intensity of the long-wave nocturnal radiation.

In general, the microclimatic conditions of the air layer close to the ground are largely determined by the relief of the soil surface. Radiation and heat turnover play a decisive role here. The energy balance at the interface between the ground and the atmosphere determines the properties of the micro or site climate or the laminar boundary layer. The ground cover has a significant influence on the radiation conditions. The presence of a snow cover is dominant. In addition, bare soil or vegetation modify the radiation conditions and the thermal properties depending on the density, structure and composition of the plant cover as well as pore volume, thermal and thermal conductivity or moisture and water conductivity of the soil.

The formation of a cold-air lake takes place in orographic depressions, which can develop in many sizes due to geology, geomorphology and climate, but often in level-deepening closed karst forms of karst areas.

The following meteorological and topographical conditions favor radiation and, in combination, lead to extremely low temperature minima:

  • A cold starting air mass with low humidity
  • Calm
  • Little or no cloud cover
  • Freshly fallen snow
  • A slight increase in the horizon
  • Great altitude

If one takes the strength of the temperature inversion and thus also the measured lowest temperatures as a measure for the power of a cold air lake, the most important topographical prerequisite is the lowest possible horizon elevation (and thus a high sky view factor). Comparative observations in the green hole and other, neighboring depressions have shown that the depth and size of the depression (or the area of ​​the cold air catchment area) are of secondary importance compared to the sky view factor. The Glattalp , where the lowest temperature recorded so far in Switzerland was recorded at -52.5 ° C, has an overflow height (difference between the lowest point of the depression and the lowest saddle or overflow point) of just 14 in the area of ​​the measuring station m on.

Basically, the cooling in a cold air lake in the mountains is very abrupt; in the first hours after sunset the thermometer drops suddenly. In the lowlands, on the other hand, it takes longer to cool down. Cold air lakes are exposed to a regular daily change in radiant weather conditions : While the cold air lake fills up at night, it dissolves again during the day due to thermal circulation. With ideal radiation conditions (few clouds, no wind), the temperature drops to the dew point in the course of the first half of the night and the excess water is deposited on the surface in the form of dew or frost . If there is an increased supply of moisture (water, previous humidification of the atmosphere by precipitation), fog or high fog can also form, the vertical extent of which is limited by the upper inversion limit.

The temperatures fall particularly low after fresh snow has fallen. Loose fresh snow has very good insulation properties due to the high proportion of air, which effectively prevents the geothermal heat flow. The lowest temperature usually occurs in the first windless and cloudless night after the flow of very cold air masses. If there is a continuous cloud cover and wind, no cold air lake will form.

In addition, it could be shown that in alpine landscapes with a large number of sinkholes sinkholes as cold air collection points are also favorable collecting basins for recent airborne dust entry.

Types

From the meteorological point of view, a distinction is made between two forms:

  • Cold air lakes of short duration - nocturnal formation and destruction of the inversion layer during the day
  • Persistent cold air lakes - over several days

The short-shaped appearances of cold air lakes are dominated by the radiation and are mostly determined by a temperature inversion near the ground. In them, low-lying cold air accumulates over the night, while it is dissolved during the day by lifting the convective boundary layer.

Persistent cold air lakes form under complex atmospheric conditions. They arise through interlocking atmospheric processes. To a greater extent, differential temperature advancements and subsidence modulate their strength and duration, while mesoscale air movements and radiative, turbulent and cloud formation processes influence their formation.

Temperature profile in the cold air lake of Hintergräppelen and a background station above the cold air lake from Dec. 4 to 11, 2017 (00 UTC each time). The formed inversion lasted 72 hours and reached a maximum strength of 26 Kelvin.

Manifestations

Cold air lakes can be observed directly in the area due to certain phenomena. Rime , hoar frost or blankets of snow allow the microclimatic changes from the ground to be followed up the slopes. In winters with little snow or late fall Raureifbedeckung is at Kahl Frost an unmistakable feature in troughs. Hoar frost can persist for several days in places that are not exposed to direct sunlight.

Botanists first identified inverse stratification of vegetation zones in deep sinkholes and uvals in karst areas of Slovenia and Croatia at the end of the 19th century. The deepest regional and subcontinental frosts outside the (sub-) polar zone were also recorded in Uvalen . While the tree line has only an upper limit in the temperate latitudes, following the general climate, the mountain forest in cold air lakes also has a lower limit. As soon as trees grow out of the protective winter cover of snow, the shoots and buds are damaged by frost, which means that under the harsh conditions, forest trees only appear in crippled form.

In the first half of the 20th century, the formation of cold air lakes led to significant health impairments in industrial areas with inversion weather conditions . In 1930, in the narrow Meuse valley near Liège , an inversion weather pattern in the first days of December led to thick fog forming. As a result, the fluorine-containing exhaust gases from the zinc and superphosphate factories located there accumulated in extraordinary quantities. Hundreds of people became ill and over 60 lost their lives.

A risk to hikers arises from choosing the wrong campsite. If the tent is set up on the bottom of cold lakes, freezing temperatures can still occur there even in midsummer, although tropical days were recorded in the nearest large cities . The meteorologist Manfred Dorninger reported that on a student excursion in the Grünloch in Austria at an altitude of 1270 m on a June day in the morning of −7 ° C and the temperatures later on the same day in Vienna were 35 ° C. In the Alps, cattle therefore avoid the bottom of cold lakes and prefer higher slopes.

Ecological importance

Cold-air lakes (including local ones) are unsuitable for growing most types of fruit, as they are extremely susceptible to late frost. In mountains with a strong relief, cold-air lakes can be developed into relict locations as locations for cold-loving plant species, which otherwise no longer occur there due to unfavorable macro-climatic conditions. In extreme cases, an inversive arrangement of vegetation levels may develop in which the most cold-adapted biocenoses occupy the deepest places. Classic examples of inverse gradation were described from the Dinarides . Here, some sinkholes show the sequence of steps snow valleys - curved pine - spruce forest - beech forest, which is from the bottom (coldest) to the top (warmest). While in natural tundras of cold-air lakes strong frost favors alpine vegetation even in lower elevations and preserves it extrazonally for thousands of years, crops in hollow locations suffer so severely that they cannot be cultivated economically without protective structures. Walls or hedges can provide some relief as they reduce the long-wave radiation at night. However, species that emerge early are never permanent in frost holes. Snow cover offers a certain amount of protection, however, in large pan-shaped hollows, frost may occur even in midsummer when there is no more snow cover. In the Alps and other European high mountains, unmistakable signs of regular frost damage are stunted growth and extremely slow growth and dwarfism. Actually large trees, such as beech or spruce, show stunted growth forms in cold air lakes, although normally growing large trees can be observed on the slopes above.

Plant species that occur diagnostically in cold-air lakes are among other mountain pines ( Pinus mugo ), which are often dominant here with high annual humidity. Also Avens ( Dryas octopetala ), as the most important relic of the Pleistocene ice ages, could be picked up in some southern pools of cold air in the Velebit relatively close to the Mediterranean. These indications speak in favor of a long-term mitigation of temperature changes, as cold air lakes for glacial relics of flora and vegetation offer micro-locations (favored spaces) far from the main area. The very irregularly distributed arctic-alpine species of the spring pasque flower ( Pulsatilla vernalis ) is also known to spread from mountain depressions, where it persists for a long time due to diurnal temperature inversions. Such cold-air lake occurrences have been described from the Julian Alps in Slovenia , where the cold-loving ( psychrophilic ) species is associated with the silver arum. The Dinaric Hornwort ( Cerastium dinaricum ), which otherwise only occurs on the north-facing hillside debris locations near the peaks of the higher mountains of the Dinarides, is described in Slovenia from lower-lying cold-air lakes.

In the northwestern Dinarides of Slovenia and Croatia, the small radiated seed ( Heliosperma pusillum ) is also a species of snow valley societies of the karst sinkholes, where it occurs in the Drepanoclado-Heliospermetum association . In addition to the deciduous moss Sanionia uncinata, it is also associated with the stump-leaved willow ( Salix retusa ) and the Ice Age relic of the silver arum. During vegetation-ecological studies in snow valleys in Snežnik and Velebit , a preference of the small ray seeds in mountain locations for cool and humid locations with particularly long-lasting snow cover, late aperitif, low solar radiation and low temperatures was observed. Such microclimatic locations in the Liburnian Karst represent frost sinkholes of the high elevations, which are preferred locations of the association with the accumulation of frost days. The locations in Slovenia are at altitudes from 1100 to 1300 meters and in northwestern Croatia from 1400 to 1500 meters. There are azonal locations of the subalpine beech and spruce forests of closed frost karst dolines, which are attributed to the subalpine vegetation zone due to the temperature conditions.

In sub-Mediterranean and Mediterranean climates in southern Europe, cold-air lakes form microrefugia , as they sometimes contain glacial relics of the flora or arctic-alpine plant stocks within forest zones. In particularly vivid cases, the bottom of such cold air lakes is covered by snow soils from the plant sociological societies Arabidion caeruleae and Salicion retusae , while there is forest all around. Glacial relics such as silver arum ( Dryas octopetala ) are promoted at such locations through feedback between the microclimate and periglacial processes. Alternation of frost, frost blasting of the rock, soil protected from evaporation by rock layers, as well as the resulting phenomena of solifluction and polygon soil formation, the locations are clearly different from the surroundings. Since the sequence of stages is reversed in cold-air lakes, inverse vegetation arrangements are a clear ecological distinguishing feature. The vertical distribution of plants is therefore conditioned by frost tolerance. The most frost-insensitive species thrive in the deepest and most frost-exposed micro-locations of the cold-air lakes.

Well-known cold air lakes

The cold air lake in Opuvani do in Montenegro is home to the southernmost and Mediterranean populations of alpine salamanders and silver arens in Europe

Germany
Well-known cold air lakes in Germany are the Funtensee (lowest temperature measured so far in Germany: −45.9 ° C on December 24, 2001 at the Meteomedia station (today MeteoGroup ) and −45.8 ° C on January 25, 2000 the DWD station ) and the Albstadt-Degerfeld sinkhole (lowest temperature measured so far in Baden-Württemberg: -36.1 ° C).

The Funtensee- Uvala (1601 m above sea level) is located in the Berchtesgaden National Park. It has an area of ​​almost 0.75 km² and was created from the corrosion of karstifying carbonates as early as the Young Tertiary. Like all comparable alpine depressions, the Uvala is of polygenetic origin, as it was further formed by glacial erosion and, to a lesser extent, fluvial or fluvio-glacial erosion. The alpine cold air catchment area extends to the Großer Hundstod (2594 m) in the Steinerne Meer.

Austria
In Austria, the green hole in the period between 19 February and 4 March -52.6 ° C, the lowest known temperature in Central Europe measured 1,932th At the Scheichenspitzkar in the Dachstein Mountains, -48.4 ° C was measured on January 2, 2008.

Switzerland
In Switzerland, the lowest temperature ever recorded of −52.5 ° C was registered on February 7, 1991 on the Glattalp . A minimum temperature of −46 ° C is known from the Combe des Amburnex in the Vaudois Jura. In La Brévine on January 12, 1987, the lowest temperature recorded in any place in Switzerland was recorded at −41.8 ° C.

Other stations from the MeteoSwiss monitoring network that are located in open cold-air lakes in high valleys and where very low temperatures can occur are Samedan , Ulrichen and Andermatt .

Italy
On February 10, 2013, in the sinkhole Busa Nord di Fradusta at 2607 m. ü. M. measured a temperature of −49.6 ° C.

Slovenia
In the at 1592 m. ü. On January 9th, 2009 a temperature of −49.1 ° C was measured.

USA
On February 1, 1985, a temperature of −56.3 ° C (−69.3 ° F) was measured in Peter Sinks.

Montenegro
Reliable meteorological measurements from cold-air lakes in Montenegro are available via the weather station in Grahovo . According to the Godišnjak SHMZ (1951–1990), it recorded an absolute minimum of −28.8 ° C. In contrast, the Crkvice weather station, 230 m higher and about 5 km away , had reached an absolute temperature minimum of only 22.3 ° C in the same period; here, too, there is a depression. In the physiogeographically part of the Hochkarst region, glacial relics are also observed in a palaeodoline . The Opuvani do below Velika Jastrebica at an altitude of 1570 m in the Orjen Mountains forms silver arum ( Dryas octopetala ) and chives ( Allium schoenoprasum ) as well as representatives of the snow valley communities with blunt-leaved willow ( Salix retusa ), long-spur violets ( Viola calcarata subsp. zoysii ) and mountain plantain ( Plantago atrata ) a microclimatic glacial relict site next to the Mediterranean Sea. The alpine salamander is also a surprising discovery here . It is the southernmost and Mediterranean location of the cold-loving species.

literature

  • Cold air lake. In: Lexicon of Geosciences. Volume 3: Instr to Nor . Spektrum Akad. Verlag, Heidelberg 2001 ( Spektrum.de )
  • M. Dorninger (2016) Topographical and meteorological factors for extremely low temperature minima in cold-air lakes. Promet 98: 43-58
  • Whiteman, CD; Haiden T; Pospichal B; Eisenbach S; Steinacker R (2004) Minimum temperatures, diurnal temperature ranges, and temperature inversions in limestone sinkholes of different sizes and shapes. J Applied Meteorology 43: 1224-1236
  • R. Wagner (1970) Cold air lakes in the sinkholes. Acta Climatologica 9: 23-32 Szeged

Individual evidence

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  2. ↑ CD Whiteman, T. Haiden, B. Pospichal, S. Eisenbach, R. Steinacker: Minimum temperatures, diurnal temperature ranges, and temperature inversions in limestone sinkholes of different sizes and shapes. In: J. Appl. Meteor. Volume 43, 2004, pp. 1224-1236.
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  54. ↑ Selected US City and State Extremes. (No longer available online.) National Climatic Data Center, February 19, 2002, archived from the original on September 2, 2013 ; Retrieved December 20, 2016 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.ncdc.noaa.gov
  55. ↑ Pavle Cikovac: Sociology and site-related distribution of fir-rich forests in the Orjen Mountains - Montenegro . Diploma thesis at the LMU, Faculty of Geography, Munich (2003). (PDF)
  56. ↑ Pavle Cikovac, Ingo Hölzle 2018: GLACIAL RELICTS IN THE MEDITERRANEAN DINARIDES - A PHENOMENON OF COLD-AIR POOL MICROCLIMATES? Abstract, Conference: 7th Balkan Botanical Congress - 7BBC 2018 At: Novi Sad, Serbia researchgate.net
  57. ↑ Pavle Cikovac & Katarina Ljubisavljević 2020: Another isolated relic population of the Alpine Salamander ( Salamandra atra Laurenti, 1768) (Amphibia: Caudata: Salamandridae) in the Balkans. Russian Journal of Herpetology, Vol. 27/2: 109-112, April 25, 2020 (PDF)
  58. ↑ Pavle Cikovac & Ingo Hölzle 2018: On glacial microrefugia Opuvani do - Mt. Orjen. 7th Balkan Botanical Congress, University of Novi Sad 10-14 September 2018. (PDF)

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