Padavine v Sloveniji

Abstract

This article gives us an account of the problems of the distribution
of precipitation in Slovenia. The basis for the elucidation of these
problems are carthographic presentations. They are at the beginning
of every chapter, the rest of the chapters being the explanatory text.
The factual material is arranged in the usual manner. There are
the following chapters: preface, yearly and monthly distribution of
precipitation, annual march of precipitation, wet and dry months,
number of days with precipitation, occurence of dry and wet periods,
fluctuation of precipitation, maximum daily amounts, amounts in short
periods and final conclusions.
The annual areal distribution of precipitation shows how strong
is the influence of the Alpine-Dinaric mountain barrier on the amount
of precipitation. Those areas, rising most steeply from their immediate
surroundings (Snežnik, Trnovski gozd and higher parts of the Julian
Alps), receive even more than 3000 mm of yearly precipitation and
thus rank high among the wettest parts of Europe. The cause for
such an amount are the damming processes in front of the AlpineDinaric barrier. This lies in NW-SE direction and is hit at right angle
by the southwestern winds which are the main suppliers of moisture
in sputhwestern part of Jugoslavia. Because the sea is near, the air
has not yet lost its moisture. This is the second reason for the great
amount of precipitation along the Alpine-Dinaric barrier. The coast
is only about 40 km from Trnovski gozd, but receives only a third of
precipitations (about 1000 mm) of the high plateaus in the interior.
The amount of precipitation decreases farther towards the interior,
though not so markedly than towards the sea. Prekmurje, about 200 km
east of the mountain barrier, receives about 800 mm of precipitation,
i. e. only about 200 mm less than the coast which is five times nearer
to the belt of maximum precipitation. Ali these facts have already
been discussed in the older literature on this subject. The annual distribution of precipitation in the period 1952—1940 reflects only an
insignificant influence of the relief on the inner side of the AlpineDinarc mountain barrier. According to author's opinion this is due to
precipitation in the unstable atmosphere, to the flooding of the area
by cold air, above which the moisture is being discharged by the overlying warmer air, and finally, to the proximity of the cyclonic centers, where the air rises intensely even if there are no orographic barriers.
The distribution of precipitation by months is not much different
from the yearly distribution, as the Alpine-Dinaric barrier is the area
of maxiraura precipitation throughout the year and the lower parts
of Slovenia (the coastal belt in the west and the lowland of Prekmurje in the east) are also the driest parts. During the winter months, as
well as during the spring and autumn months, the coastal belt receives
more precipitation than its Continental counterpart in Prekmurje. During the summer months, the belt of maximum precipitation is less
marked because the greatest amount occurs in the mountains in the
north (Julian Alps, Karavanke, Pohorje). Nevertheless, the western
mountain barrier stili stands out. Only in the southern end of the
mountain barrier, in Snežnik, the belt of abundant precipitation
disappears during the summer. It is also only during the summer that
Prekmurje receives more precipitation than the coastal belt. The proportion is approximately 7 :4.
An essentially other picture is revealed if monthly amounts of
precipitation are expressed in percents of the total annual amount.
Maps drawn on this basis show that the greatest amounts of precipitation in the south occur in November, December, January, February
and March, while in other months this happens in the north. The areas
of the highest plateaus stand out particulary. The main area of
prevalent summer precipitation is the low-lying region in the northeast
of Slovenia, and not the mountainous northwestern region. The distribution of precipitation by seasons (in percents of the annual total)
shows a very uniform regional pattern in spring. At that time the
difference between the area of the high mountains, which is best
provided with precipitation, and the driest area in Ravensko (Prekmurje), is less than 6 % of the annual amount of precipitation. In
summer, the difference is the greatest — 20 °/o. Prekmurje receives
then nearly 35 °/o of the annual amount of precipitation, while Snežnik
receives in the same season merely 15
4/o. In autumn and winter, the
difference is in the fact, that in autumn the amount of precipitation
exceeds the seasonal average ali over Slovenia, while in winter it is
everywhere below the seasonal average. Even Snežnik, which is best
provided with precipitation during the winter-time, does not receive
then more than 22 °/o of the annual amount.
A map of isohygromens, drawn on the basis of pluviometric coefficients, shows the unexpected fact, that — apart from the low coastal
belt — the number of dry monts is greatest in the western mountain
barrier. There are 8 such months, nearly as much as in Prekmurje. This
is the consequence of the extraordinary abundant orographic precipitation in October and November, when the average monthly amount
is very much above that one for the other months. Because of the
high values for October and November, the ideal monthly average
(M 16 :12), derived from the annual average, is very great. Thus the
quotient from the actual monthly average and the ideal monthly average is less than 1.00 for most months, which consequently appear as
dry. These results are in accordance with those attained by means of
the pluviometric regime.
The maps showing the number of days with precipitatiton also
reveal some unexpected facts. They demonstrate that October and November are not the months with the greatest number of days with a measurable precipitation — even if they receive the greatest amount. That
month is actually May. February, on the other hand, is the month
with the smallest number of days with precipitation. The total number
of such days in a year is rather equal both in Prekmurje and in Primorje (the coastal belt), that is just over 100 in some places. .This is
about 70 days less than in the Julian Alps, where the number is the
highest in ali Slovenia. On the whole the distribution of the number
of days with precipitation is on the line with the monthly distribution
of precipitation. The difference is mainly reflected in the less dense
iso-lines on maps showing the number of days with precipitation. The
proportion of the numbers of days with precipitation for stations having the greatest and the smallest number is 2:1 , whereas the proportion is 4: 1 (3200 mm : 800 mm) if the amount of precipitation is
being considered.
If only days with 10 mm or more of precipitation are taken into
account, the picture is different. There are places with 75 and such
with only 26 such days, which gives a proportion of 3:1 . Therefore
maps drawn on this basis for succesive months show great differences
between the western mountain barrier and Prekmurje, that has the
least days with abundant precipitation. The number of the days
with 10 mm or more of precipitation is about 40 in Primorje and about
30 in Prekmurje. If the number of such days is considered by months,
one can see that only in July and August the number is higher in
Prekmurje rather than in Primorje. The western mountain barrier
stands out especially in October and November.
May is the month with the greatest number of days with small,
hardly measurable amount of precipitation, while October (rarely November), on the other hand, is the month with the greatest number
of days with abundant precipitation. This discrepancy is explained by
the author in this way: there is a great difference between the warm
sea in the mediterranean basin and the cold air which overflows the
sea from the north during the autumn and causes the unstability of
the air and thus abundant precipitation. In the spring months, April
and May, the sea is rather cool, but the heating of the continent is progressing rather quickly. When the indrafts of the air from higher
latitudes occur, thermal differences are not so great and consequently
the barometric differences are also not great. Therefore the precipitation is less abundant and intensive. This is the answer to the question
why the number of days with abundant precipitation is greater in
October than in May. The socond question is, why there are more days
with slight precipitation in May rather than in October. It is probable
that lesser humidity of the atmosphere is responsible for that. Further,
the heating of the mainland is already quite strong in May, and the
air above it is quite stable, nearly just as much as the air is stable
above the sea in autumn. Because of the lesser humidity, the frequency,
rather than the amount of precipitation, is more marked. The amount, however, is not small, because the spring maximum in a great part of
Slovenia occurs just in May.
The proportion of 2 :1 appears also if the number of dry days is
examined. It is the same proportion which characterises the regional
pattern of most dry and most wet parts of Slovenia. This is almost
neeessary because a dry period is limited by two days (one at the
beginning, the other at the end of a wet period) with precititation of at
least 0,1 mm, i. e. by the same value as in the former chapter. The proportion is W2:l, but in the opposite sense that if the number of days
with precipitation was concerned. The number of days included into
dry periods is 125 in Primorje, while in the Julian Alps it is just over
70. There is a marked difference between Primorje and Prekmurje.
There is a balance as far as the days with precipitation are concerned;
but the number of days included into the dry periods of 10 days or
more is in Primorje by 25 °/o greater than in Prekmurje. The maps
show that this difference is mainly due to the conditions during the
summer, which is the main season of precipitation in Prekmurje. On
the maps representing the number of dry days, it is shown that the isoIines run paralel to the Dinaric plateaus in ali seasons except during
the summer when the areal distribution is zonal. Thus the iso-lines on
the map showing the annual distribution also run parallel to the western mountain barrier. The greatest number of dry days is in February.
Then only on the edge of the Plain of Friuli (station Lig) and along
the Kolpa river (station Sinji vrh) the number of days included into
dry periods is less than 10. May shows the smallest number of such
days; only Haloze hills in the east have more than five dry days. An
investigation of maximum duration of dry periods shows that there
are no great differences as far as areal distribution is concerned. The
maximum duration is 30—40 days and occurs during the late winter.
The frequency of days included into wet periods also shows the
dependance on the Dinaric—Alpine barrier, where there are over 20
such days. This is rather surprising as Slovenia is one of the areas which
rank high among the most wet parts of the world. Ali the eastern and
southeastern part of Slovenia has on the average less than five days
of precipitations included into periods of 10 or more days with at least
0,1 mm of precipitation. Maximum duration of a wet period can be as
much as 20 days in the northwest and may occur at any month except
January and February, both well known for the small amount of precipitation.
Fluctuation of precipitation is an important fact in characterisation
of precipitation, and consequently, of the climatic conditions in general.
The greater the fluctuations are, the more transitional climate has the
area. The map of the annual variability shows that a belt of relatively
small variability (up to 14 %) extends from the sea to Kozjak hills
(NW of Maribor). The rest of Slovenia to the northwest and southeast
has the variability between 14 and 16 %, in Haloze and in the Julian
Alps even over 18 %. For consideration by months February and June have been analysed more in detail because of the implemented economic importance. The lines of equal variability on the map show a zonal
pattern during the summer. In the north variability is under 40 °/o and
in the south it is under 50 °/o. During the winter the lines of equal variability run paralel to the western mountain barrier. There the variability
is over 60 °/o, while in the southeast it is under 50 °/o for the respective
month.
The map of the maximum annual departures from the normal
shows, that they are greatest in the areas just mentioned, i. e. in the
Julian Alps, in the extreme South of Slovenia and in the part lying to
the east of river Drava. Moreover, the maps showing the departures
from the normal precipitation, oompiled for each month, reveal the fact
that Slovenia is very heterogenous as far as the regional distribution
is concerned, inspite of the small size of the country. In course of the
16-years period there were only 4 years, when the regional distribution
of precipitation showed the same trend. Three of these years had precipitation above the average and only in one year it was everywhere
below the average. This is a proof that Slovenia is a transitional area
also af far as climate is concerned.
The western mounthain barrier. is again very marked on the map,
showing maximum daily precipitations. The ascent of the air, which
occurs along the barrier, causes processes of convection, and amounts
to a stabilisation of the atmosphere. Along the barrier as much as
300 mm of precipitation can fall in one day, but the maximum daily
amounts tend to decrease towards the east. They can, however, stili
be quite high even far from the western mountain barrier.
The analysis of separate days shows that the belt of maximum
precipitation alogn the Alpine—Dinaric barrier is caused by the damming processes. It is a physical necessity that the amount of precipitation in such cases is the greater, the greater the velocity of the air
carrying water vapour is. As the velocity of the air is a function of the
differences of temperature, it is the greatest in the area of the polar
front. The author's conclusion is, therefore, that changes at the time,
when the maximum precipitations occur in given sectors of the Alpine
and Dinaric mountains, are a result of the annual shift of the polar front.
The polar front is in its most southern position in Europe during the
winter half of the year, i. e. at the time when southern and middle
latitudes of the Mediterranean have the maximum precipitation. In
March the maximum is in the Dinaric mountains and in April it occurs
in the southern Alps. The day to day analysis shows', that the polar front
extends mainly in SW-NE direction, and that it represents actually
a front side of valleys or troughs of the cold air, penetrating either
across Western Europe or across the eastern part of the Atlantic. It is
well known from the literature — apart from author's own findingsthat the depth and the position of these intrusions depend on the size and
the situation of the Azores anticyclone, differing with the seasons.
During the spring the depth of the intrusions towards the south is
lesser and lesser because of the growing strength of the Azores anticyclone. Accordingly the area of maximum precipitation moves towards
the northeast, as it is a direct consequence of the shift of the polar
front, especially of the front parts of the troughs. In April and March
the intrusions are stili pronounced; they penetrate far towards the
south. A strong southwest wind is then blowing over Slovenia which
causes much orographic precipitation.
With approaching summer the Azores maximum moving towards
the north begins to be effective. Consequently the intruding
troughs of cold air are pushed towards the northeast, so that the Alps
are not on the front-side but rather on the back-side of each trough.
Northern Alps and the northern parts of the Dinaric and Rodope
mountains lie no more in the leeward side, but on the windward side.
Maximum summer precipitation, characteristic of Central Europe
(including the Pannonian plain and its southern rim) is the direct result
of the changed direetion of the winds. In the light of this scheme, May
and September can be considered as the transitional months. In the
second half of the year the processes should take the opposite course.
According to the scheme just presented, the maximum summer
precipitation in northwest Slovenia is also due to the passing fronts.
The great influence of the heating of the noar-the-surface layers of
the air on the diseharge of moisture and its amount cannot be overlooked. An analysis of thunderstorms has shown, that most abundant precipitation occurs often before the cold front passes by, mostly
during the morning hours. Taking into account the prevailing southwestern direetion of the fronts and the winds, one can conclude, that during the colder half of the year the frontal and orographic precipitation
is characteristic of the greater southwestern part of Slovenia. During
the summer months, May and September included, the frontal precipitation is essentialy inereased by the processes of convection, caused by the overheating of the lower layers of the air. Thus, the differences
of the conditions during the summer, and those during the rest oft the
year, lie basicaly in the direetion of the inflowing polar air masses.
During the summer, the polar air comes from the northwest and the
north, and consequently maximum diseharge of moisture occurs in the
northern part of Slovenia. During the rest of the year, however, the
polar air comes — intermediately — from the southwest, and therefore
the southwestern part of Slovenia is the area of maximum precipitation. In contrast to most of Slovenia, which has maximum precipitation in autumn, the northeastern part is characterised by maximum precipitation in summer. According to older statements the boundary between both areas follows river Savinja down to Celje, and continues
towards Macelj hills in the east. More recent data, for the period
1925—1940, show that the boundary in its western seetor follows the
courses of rivers Mislinja and Paka, i. e. it lies more towards the east.
However, no elear-eut lines exist in Nature. Because an autumn maximum occurs in the high Pohorje and Haloze hills (with Macelj), and
a summer maximum occurs in the near-by lowland of Ptujsko polje
just to the northeast (and even in the northern part of the basin of
Celje!), the whole belt lying between Mislinja, Savinja and Voglajna
rivers on one side and river Drava on the other, can be considered as
a transitional belt, which separates the two areas of either pronounced
summer precipitation or pronounced autumn precipitation. This belt is,
moreover, the only climatic "boundary" which can be drawn on the
basis of ali maps reproduced in this article. If Koppen's or Trewartha's
classification is applied, ali Slovenia falls into one climatic province
with a rather uniform distribution of precipitation throughout the
year, because the summer maximum precipitations in the northeastern
part are not enough prominent (less than 36 % of the annual amount).
In the concluding chapter the author again stresses, that the period
of 16 years is relatively short for consideration of such an element as
precipitation. Therefore the conclusions cannot have the same value
as if they would derive from the analysis of a longer, normal period.
This is true even more, because this very period has been excessively
wet, at least in the southern part of the basin of Ljubljana, if compared
with the average values for the last 100 years. This fact should not be
forgotten by anyone interested in the distribution of precipitation,
either from a theoretical or from a practical point of view.