Spatial distribution of precipitation in the Federation of Bosnia and Herzegovina

Precipitation includes all forms of steam condensed or sublimated in the air, that precipitate to the ground in fluid or solid form. Precipitation has the greatest spatial and temporal variability of all climatic elements. There are great differences in the amount of precipitation between some years, especially between the same months of different years. Depending on where precipitation is formed, it can be high/cloud precipitation or low/ground precipitation; depending on the type of formation, precipitation can be frontal, convective and orographic.

High precipitation includes: rain, sleet, snow, hail, graupel and ice pellets, while ground precipitation includes: dew, rime, frost and freezing rain.

The precipitation regimen depends on geographical position and general atmospheric circulation, but also on local factors: terrain orography, altitude, distance from sea (continentality), type of land and vegetation, water surfaces, etc.

Precipitation and air humidity are closely related. Major precipitation increases humidity of air, while a lack of precipitation usually causes dry air.

Precipitation is measured using rain gauge, pluviograph and totalizer. That is how daily, monthly, and annual precipitation volumes are determined. Daily precipitation volume is a volume of precipitation in 24 hours (from 7:00 AM local time on the first day until 7:00 AM on the following day). Precipitation volume is expressed in mm or l/m2.

Precipitation form can be solid, liquid or mixed. Solid and mixed precipitation must be melted before measuring. Snow is the most common form of solid precipitation. Snow depth is also measured where we distinguish measuring of total snow cover depth, and measuring of the new snow that accumulated over the last 24 hours. Snow depth (recent and total) is expressed in cm, and is measured as well as the precipitation volume at 7:00 AM. Snowfall is not necessarily related to its preservance on the ground. Whether it will stay on the ground or melt, depends on its volume, air and soil temperature, wind, and sunshine. Snow can stay on the ground for quite some time after it stops falling. It depends on the temperature, since it has to be below 0°C. Depth of snow cover and its duration on the ground is variable in both space and time.

In addition to volumes, regime, distribution and frequency of precipitation are the most important characteristics of this climate element. Precipitation regime is temporal distribution of precipitation over a year, while distribution of precipitation refers to spatial distribution of precipitation.

Precipitation regime with the majority of precipitation occurring in the warm part of the year (April – September) is called continental, and it is used to determine continentality of an area.

In the maritime regime, majority of precipitation occurs in the colder part of the year (October - March).

There are two pluviometric regimes occurring in our country. To define the border separating the maritime from the continental pluviometric regime, we used the Vemić’s continentality evaluation criterion. According to this criterion, if during the period from October to February more precipitation occurs than in the period from March to September, than q is less than 50%, and the regime is maritime, and vice versa. The border between these two types of climate is defined by the 50% isoline, which is called the continentality line (Šegota, 1986), and it’s orientation is NW–SE. It enters Bosnia and Herzegovina south of Bihać, and goes deeper inland via Srnetica, Vitorog, Vranica, Bitovnja, Bjelašnica, Treskavica and Maglić (see map below).

The area north of the 50% continentality line is characterised by a continental pluviometric regime which means frequent and abundant precipitation, main maximum in May and June, and secondary in November. In the winter period, precipitation is lower, and the absolute minimum occurs in February. Precipitation during winter is usually snow. In the area of continental pluviometric regime, precipitation is evenly distributed over the year, which is evident from relative annual fluctuation between 2.9 and 8.9%.

Spatial distribution of mean annual sums of precipitation of the continental pluviometric regime shows increase going southwards from the north, and westwards from the east. The highest levels of average annual precipitation are recorded in the north-west (900 to 1300 mm), while the lowest precipitation levels are recorded in the area around Odžak, Orašje and northern slopes of Majevica (below 800 mm), making it the driest area of the Federation of Bosnia and Herzegovina.

The main maximum of precipitation during summer months is a result of a significant number of cyclones moving from the Gulf of Genova across the Gulf of Trieste further east, across the valleys of the Sava and the Danube to the Black Sea, while the secondary maximum is a result of the maritime influence. However, there are some anomalies in certain parts of the year, when secondary maximum occurs sooner than expected (October).

The north-west of the country is characterised by continental precipitation regime with some maritime influences from the south. This area is peculiar for being located between two neighbouring climates (Mediterranean on the west, and mountainous on the south), and they both strongly influence climatological indicators, which is especially obvious in case of sudden income of warm and cold waves. The north-west of the country is open to continental air from the north and north-east. River valleys spread these influences deeper inland into the mountain-basin area. Annual average of precipitation volumes decreases from south to north, ranging between 1300 mm in Bihać and approx. 900 mm in Cerik.

The north-west of the country (Velika Kladuša, Bihać) records the main maximum precipitation volume in the colder part of the year (November), and the secondary in May. Minimum precipitation is recorded in January.

Average annual number of precipitation days ≥ 0.1mm ranges from 113 (Cazin) to 157 days (Bihać). Snow is typical of the winter period. Maximum snow depth goes up to 100 cm (Bihać 105 cm), depending on the terrain orography; snow drifts may be much higher. Annual average of days with a snow cover ≥ 1 cm is 50 to 70, and at higher altitudes, up to 110 days.

Low mountains, uplands and lowlands of the north-east of the Federation of BiH have prominent features of the continental pluviometric regime, with May and June fighting for the title of the rainiest month. The main maximum of precipitation in Tuzla occurs in June (111 mm). Minimum precipitation may occur in January, February, March or October; the minimum precipitation in Tuzla is recorded in February (55 mm).

The area covering the uplands and highlands of the southern part of the central Bosnia has a pluviometric regime characterised by a mixture features of continental and maritime type. The main maximum occurring in the colder part of the year, shows that the Mediterranean influence is still strong, but in other aspects, this area belongs to the continental type, since in the warmer part of the year, (March - September) the share of precipitation increases. Mean annual total precipitation ranges between 780 mm (Goražde) and 1114 mm (Bjelašnica). Spatial distribution of the annual precipitation is rather uneven due to the complex relief. Windy sides of high mountains have annual levels of precipitation ranging between 1500 and 2000 mm, while sheltered river valleys and basins record precipitation at merely 700-800 mm (Bugojno).

Regarding the number of days with precipitation ≥ 0.1 mm in this area, Bjelašnica leads with 170, while Goražde has only 120 of such days. Frequency of precipitation days peaks between November and June, ranging between 14 days in November and 17 in June.

Snow has a significant share in the annual precipitation volume, since it is a regular phenomenon in the winter half of the year. Maximum depth of snow cover ranges between 51 cm in Zenica and 100 cm in Butmir, which was recorded in January, 1967.

The maximum snow depth on Bjelašnica (2067 m) was recorded in March 1986, and it was 303 cm. It should be noted that these values were obtained from the stations that allow for such measuring - local snow drifts may reach even greater heights. Evidently, stations at higher altitudes usually record deeper snow, as well as frequency of days with snow cover ≥1.0 cm. Annual average of days with snow cover of ≥ 1.0 cm is 50-60 days in the lowlands, while at higher altitudes, up to 200 days (Bjelašnica).

Meteorological stations south of the continentality line (Bosansko Grahovo, Bosanski Petrovac, Drvar, Kazanci, Kulen-Vakuf) are under evident maritime influence due to which this part of the country is dominated by the maritime precipitation regime with the main maximum in November (Bosansko Grahovo - 159 mm) and minimum in July (Bosansko Grahovo 74 mm).

Precipitation frequency is presented as a number of days with precipitation ≥ 0.1 mm, so-called precipitation days, and it shows that the annual average in this area is 110 to 150. It peaks in the period April - June, with monthly average of 11-15 days, and records minimum values in September and October with 7-9 days a month.

Karst fields of the south-west Bosnia are influenced by the maritime pluviometric regime, and some continental influences from the north. Maximum precipitation occurs in November and December, and minimum in July, which is a result of maritime influence on one side, and continental/altitude influence on the other. These maritime influences are considerably weakened by Mount Dinara as the natural obstacle.

Some of the valleys and surrounding plateaus show certain differences in the precipitation volumes. The maximum annual total ranges between 1100 mm (Livno) and 1400 mm (Glamoč).

Area south of the continentality line (Mostar,Čapljina Neum) belongs to the prevailing maritime pluviometric regime characterised by high frequency and volume of precipitation in the colder part of the year, with dry summers and precipitation minimum occurring in July. The main maximum precipitation occurs in the colder part of the year (November - December), and the secondary in the beginning of spring (March). Depending on the latitude, relief, and local conditions, maritime type is modified deeper inland sometimes causing premature maximum precipitation volumes (November). The main minimum occurs in July. Summers are dry with precipitation less than 50 mm. Mean annual total precipitation in this area ranges between 1100 mm (Čapljina) and 1800 mm (Trebimlja).

Leeward zones of Čabulja, Čvrsnica, Velež and Prenj record maximum annual precipitation volumes in the Federation of BiH, with average annual precipitation of more than 2600 mm (Bogodol, 2680 mm). This area is exposed to humid air masses coming from the south that produce abundant precipitation due to upward movement along windward slopes. Since this entire area shows major differences in precipitation volumes over the year, annual fluctuations are obviously significant, ranging between 8.4 and 12.6%. Months with maximum precipitation volumes are November and December with values between 141 mm in Čapljina and 406 mm at the Bogodol station. July and August record lowest levels of precipitation with 34 mm in Neum, and 68 mm in Bogodol.

The number of days with precipitation ≥ 0.1mm is also much greater during autumn and winter months compared to summer months. It ranges between 11 and 13 days in November and December, and 4 and 7 in July and August. Typically, precipitation in form of snow is rather rare so it occurs in isolated cases. The maximum snow depth in Mostar was 37 cm, and it was recorded in March 1971; in Čapljini 34 cm, as recorded in January 1985. Average annual number of days with snow cover ≥1.0 cm is 2-6 days in the south, and it grows with altitude and latitude.