Air temperature

Air temperature, along with precipitation, is the most important parameter in studying a climate.

As air warms up and cools down, its temperature follows the same mechanism. There are several ways for the surface to warm up or cool down the air: conduction (conducting of heat), absorption of long-wave radiation from the surface and counter-radiation from the atmosphere, convection, advection, and evaporation (Spahić 2002).

Daily and annual fluctuations in air temperature are caused by two factors: insolation and Earth radiation. During the day, when insolation dominates, temperature in the ground layer increases, while in periods of dominant radiation, the temperature decreases.

There are other factors that also affect daily and annual air temperature fluctuations, namely: latitude, altitude, day length, relief, season, cloudiness, air circulation, type of land, vegetation.

Air temperature is measured according to standards of the World Meteorological Organisation, in meteorological shelters, two metres above the ground. Meteorological shelter is installed above a natural surface, doors facing north. Temperature is measured according to the climatological schedule (07, 14 and 21 hours). Mean daily temperature is calculated as follows:

$$t=\frac{t07+t14+2*t21}{4}$$

Under normal conditions, air temperature goes down as the altitude goes up, unevenly, in terms of both space and time. Decrease in air temperature caused by increasing altitude can be caused in two ways: as a result of altitude increasing above a flat area (in free atmosphere) and due to increasing altitude in mountainous areas. Decreases in air temperature with higher altitudes show some annual fluctuations - the largest decrease is during summer 0.75°C /100 m, and the smallest in winter with about 0.3°C/100 m. In the central mountainous area of the Federation of Bosnia and Herzegovina, and up in the north, these values range between 0.51°C/100 m during summer and 0.38°C/100 m in the winter period.

Distribution chart for the mean annual air temperatures shows influences of the most important climate modifiers (effects of the Adriatic Sea, the Dinaric system, openness towards the continental Europe, and jagged relief) on the temperature attributes of the Federation of Bosnia and Herzegovina. The lowest mean annual temperatures are recorded in the central mountainous areas. From this mountain massif northwards, mean annual air temperatures increase and even out in the plains with values at about 11.0°C. Temperatures show greater increases in the area from the mountains towards the south and the south-east, with the values up to 15.0°C.

According to the spatial distribution of the mean annual air temperatures, the north-west of the country shows values between 9.6°C in Cazin and up to 10.6°C in Bihać. Towards the east, these values increase and range from 10.0°C in Tuzla to 11.4°C in Orašje. In the central mountain-basin area, mean annual air temperatures range from 6.1°C in Ponikve to 11.6°C in Jablanica. Mountain tops of this area have mean annual minimum as low as 1.2°C (Bjelašnica, 2067 metres). In the south of the country, mean annual air temperatures are the highest, with 14.1°C in Široki Brijeg, and 15.2°C in Neum. South-west of the Federation of Bosnia and Herzegovina, in the area separated by mountains from the sea and the continental area, mean annual air temperature ranges from 5.8°C in Kupres to 9.3°C in Drvar.

Analysis of annual fluctuations of the air temperature in the Federation of Bosnia and Herzegovina shows that the coldest month is January, and July is the warmest. Average annual temperature in the mountain-basin central region is negative, ranging from -6.5°C on Bjelašnica to -0.5°C in Maglaj. Positive values of the mean January temperature are recorded in Konjic (0.8°C) and in Jablanica (1.6°C) where both are towns affected by maritime influences from the Neretva valley. Up in the north, mean January temperatures are negative reaching up to -1.0°C in Cazin and Gradačac. Meteorological station Bihać is an exception, where this value is positive at 0.3°C. In the south, mean January temperature peaks at 4.8°C in Mostar and 6.6°C in Neum.

In the central region, where mountain climate and mountain-basin climate dominate, mean July values range between 9.5°C on Bjelašnica and 20.7°C in Zenica. Due to significant orographic differences in the area, the difference in mean July temperature is more prominent here than in other parts of the country. In the south, these temperatures range between 23.7°C in Čapljina and 24.7 °C in Mostar. In the north, mean July temperatures reach 21.5°C (Orašje). The south-west of the Federation of Bosnia and Herzegovina records mean July temperatures ranging from 15.0°C in Kupres to 18.8°C in Drvar.

Mean temperature for climatological winter season (December - February) is higher in the south of the Federation compared to the continental part, which is due to the proximity of the sea, latitude, altitude and orographic features. Mean winter temperature in Ljubuški and Čapljina is 6.0°C, and 7.4°C in Neum. Locations with lowest values of mean winter temperatures are Glamoč -2.1°C, Makljen -2.2°C, and the coldest is Kupres with mean winter temperature of -2.9°C.

Bjelašnica is a good example of extreme weather. In that area, mean winter temperature is -6.0°C. There is also the lowest mean seasonal temperature amplitude, if we consider both winter and summer (14.8°C).

Autumn in the Federation (September – November) is on average warmer than the spring (March – May). These differences range from 0.1°C to 2.2°C, and are greater in the south compared to the continental part. Only the plains in the north have springs warmer than autumns by 0.3°C. The difference between mean temperatures of the coldest and the warmest month (January and July) is the greater up north in Orašje (21.8°C). The smallest annual amplitudes are recorded in Neum (18.0°C) and on Bjelašnica (16.0°C).

Absolute maximum air temperatures

The highest absolute maximums (41.0°C), on the territory of the Federation, are recorded on the south, in Mostar, Čapljina, Stolac, and Široki Brijeg. Above 2000 m, (Bjelašnica) the absolute maximum is 23.2°C. The amplitude of absolute temperature maximums reaches 18.0°C, which is less than the amplitude of the absolute temperature minimums which is 23.8°C. Absolute temperature maximums usually occur in July, sometimes August.

Absolute minimum air temperatures

Extreme minimum values of air temperature usually occur in the coldest month, January, and sometimes, although rarely, in December or February, as a result of winter anti-cyclonic conditions. In the south of the country, due to maritime influences, absolute minimum air temperatures are higher compared to the inlands, ranging from -8.2°C in Neum to -20.0°C in Stolac. In the mountain-basin part of the country, these values range from -21°C on Komar to -32.0°C in Glamoč and Visoko.

It is practical to know the number of days with extreme temperatures in a month or year, as well as their geographical distribution (number of frost days and summer days), since they paint a clearer picture of temperature conditions in a place or a region.

Frost day is a day on which the minimum air temperature is ≤ 0.0 °C, and summer day is a day on which the maximum temperature is ≥ 25°C.

Frost days, according to their origin, can be radiation frost or advection frost days. Radiation frost days, or static frost days, result from cooling of land surface during the night, which is a microclimatic local phenomenon. Advection or dynamic frost covers large areas and comes as a result of cold air intrusion. On the territory of the Federation of Bosnia and Herzegovina, frost usually occurs in the colder part of the year, from the beginning of October to the end of April. It is often the case that frost days occur from September to May, depending on the altitude. The only season without frost days is summer, exception being Mount Bjelašnica. The largest annual average of frost days on Bjelašnica is 195, and the smallest in Neum, 12 days. Large number of frost days in January is not only typical of mountainous areas (Bjelašnica 31, Kupres, Makljen and Nišići, 28 on average) but also of basins (Tuzla and Zenica 25, Sarajevo 24 days), as well as of northern parts of the country (Orašje 22 days). In the south, air temperatures below 0.0°C occur regularly during winter and their number is also the largest in January. Annual average of frost days ranges between 22 in Mostar and 56 in Stolac. Frost begins in the second half of November, or early December, and ends in March, very rarely occurring in the beginning of April. It is evident that the number of frost days is the lowest in the area under maritime influence, and that it increases as we look further inland.

Unlike frost days, summer days occur mainly in the warmer part of the year. Average number of days with maximum air temperature above ≥ 25.0°C, in its annual fluctuations, peaks in July and August. The only station that has never recorded such event is the Bjelašnica station. From Herzegovina northwards and from Posavina southwards, the number of summer days decreases, in line with the level of warming up. (Milosavljević). Altitude increases, so areas above 1000 m have less than 10 summer days (Makljen 4, Nišići 9 days).

If we look at the seasons, the average number of days with maximum air temperature of ≥ 25,0 °C is greater in autumn than in spring, throughout the Federation, that is, such distribution is very similar to the annual distribution of mean monthly temperature values. On average, October has 6 of these days in the south, and 2 to 3 in the continental part. In November, the possibility of such temperature values is almost non-existent