Water vapour content and air saturation with water vapour are of great importance since these values indicate plenty about vapour condensation, i.e. about creation of fog, clouds, rain, snow, etc. Water vapour content in the air is determined using the following values: water vapour pressure, relative humidity, absolute humidity, and saturation deficit. (Milosavljević, Klimatologija 1988).
Relative air humidity is the level of saturation of air with water, i.e. the ratio between the water vapour quantity contained in the air at a given moment, and maximum quantity the air could contain at a given temperature to become saturated. It is expressed in percent (%).
Air temperature affects the quantity of water evaporating from the ground surface. Evaporation process is continuous, and the warmer the air, the more water evaporates. As altitude increases, temperature decreases, as well as the quantity of water vapour. Evaporation is more extensive during windy weather than the still. In summer months, relative humidity increases with altitude, and the winter months, it decreases.
Daily fluctuations of relative air humidity are mostly the opposite of the daily fluctuations in air temperature. Maximum values are reached in morning hours, the minimum in the afternoon when the temperature reaches its maximum. In the mountains, air humidity increases during the day, and decreases during the night. This phenomenon is affected by upward movements of humid air up the slopes during the day, and the opposite during the night.
The annual distribution shows that relative air humidity peaks in late autumn (November) and in the first half of the winter period (December and January); it reaches its minimum values during summer, usually July, sometimes August. Such distribution may be somewhat different in spring months due to sudden warming, the end of winter, or major precipitation resulting from barometric depression transiting over the area. Relative humidity does not evenly change with altitude. Horizontally, it does not significantly change, especially on a monotonous terrain, and it depends largely on local conditions, primarily orography.
Due to the above stated, it is hard to distinguish maritime from the continental relative humidity flow in the horizontal distribution of annual air humidity values. Minimum values on the territory of the Federation of BiH are recorded in the southern parts of the country, (Mostar 62%, Neum 67 %, Livno 69%, Domanovići 70%). According to Milosavljević, annual relative humidity below 70% belongs to the “low humidity”, while values above that belong to “moderate” and “high humidity”. Therefore, the maritime type of relative humidity distribution (less than or equal to 70%) is present in the limited area parallel to the Adriatic coast, although maritime influences can also be felt deeper inland.
The rest of the FBiH territory shows continental distribution of relative humidity (more than 70%) with some local peculiarities. Sarajevo, Bihać, Posušje, Prozor, Glamoč, are some of the places with mean annual relative humidity of air is ranging between 71 and 75%, and is considered “moderate” by Milosavljević. With an exception of Konjic and Tomislavgrad, almost all locations with mean annual relative humidity between 75 and 80% (“increased humidity”) are located deeper in the continent. Mean annual relative humidity above 80%, categorised as “high humidity”, is recorded in some areas in Krajina, in the central mountainous area, in east Bosnia, and in the north of the Federation of BiH. Bjelašnica records a maximum of mean annual air humidity of 85%.
As per seasons, minimum average values on the territory of the Federation of BiH are recorded in Herzegovina, in the wider area around Mostar, and they range between 56% in summer and 66% in winter. Livanjsko field also has relatively low air humidity values ranging between 64% in summer and 74% in winter. With an exception of Bjelašnica with the average annual value of 85%, maximum annual air humidity values are recorded along rivers, in Zavidovići, Maglaj.and Goražde 84%, Orašje 83%. Compared to these stations located in river valleys, relative humidity on Bjelašinica is somewhat lower during the coldest winter months, but higher in summer, due to significant difference in altitude. Although the relative air humidity does not make sudden horizontal changes, these values usually gradually increase from the south to the north of the Federation. Average monthly values during summer can only be found in several places (Mostar, Ljubuški), in the warmest summer months of July and August, at a bit under 55%, which is the limit below which air is defined as very dry. The majority of stations in the Federation show the annual average of ranging between 55 and 74% (dry air) and between 75 and 90% (moderately humid air), depending on whether it is a cooler or a warmer period.
As per seasons, the south of the Federation, especially Mostar, records the lowest relative air humidity in all seasons. It should be mentioned that in spring, besides Mostar, the city with the lowest value of this parameter is Sarajevo, which is, although located in the mountains of central Bosnia, records average humidity of 65%. The highest values of relative air humidity per season are recorded on Bjelašnica (87% in spring and 82% in summer), 87% is recorded in autumn in Gorazde and Maglaj, and in the winter season, the highest mean relative air humidity is 90%, recorded in Zavidovići.
Mean monthly values of relative air humidity for the territory of the Federation of BiH range from 53% in July in Mostar to 90% in the coldest winter months (Gračanica, Goražde, and Zavidovići). High average values are due to many factors, but also due to long periods of fog in this part of year, resulting from stable anti-cyclonic conditions in the atmosphere. In general, maximum mean monthly values in most stations are registered in December, very rarely in January (Makljen 86%, Kazanci 82%). At the stations in Herzegovina, the maximum is usually recorded in late autumn, in November, due to maritime precipitation regime and the fact that winter period in this area is characterised by frequent weather changes where winds are one of the main factors affecting the weather and, therefore, the relative humidity. Changing southerly winds that bring heavy rain and humid, warm air, and bora, northern wind that brings dry and cool air, are among the main reasons for moderate humidity during winters in this area.
Minimum mean values at these stations are recorded mostly in July, when Herzegovina and the narrow area below Dinara record mean annual humidity below 65%. (Mostar 53%, Livno 61%) This is due to good weather that dominates the summers, and due to the karst terrain with scarce vegetation, and perhaps due to the anticyclonic southerly wind bringing warm and dry air. Although the summer period is of similar weather throughout the Federation of Bosnia and Herzegovina, monthly amplitudes between northern and southern locations are evident - the peak is recorded in July at 29%.
Absolute minimum value of relative air humidity on Bjelašnica is 2% in December and 21% in July. In Tuzla, the absolute minimum relative humidity is 34% in December, 13% in July, and 6% in September, at the end of summer.
Maximum average number of days with relative humidity below 30% is recorded at the majority of stations during summer and in early autumn. On average, Livno has 6 in July, 9 in August; Mostar has 5 in July and 7 in August; relatively high values compared to other stations are recorded in Sarajevo with 5 in July and 6 in August. In addition, during spring, Sarajevo has the largest mean number of days with relative humidity below 30% (7 in April and 6 in May). In December and January, such days occur only in Livno and on Bjelašnica, two for each in every month; Mostar records one of such days in January.
In vegetation seasons (01.04 - 30.09) average values of relative air humidity vary from min. 53% in July in the Mostar area, up to 85% in Maglaj, in September.
Water vapour pressure
Vapour pressure of water is partial pressure of water vapour in the air expressed in hectopascal (hPa). Annual fluctuations of vapour pressure values, unlike those of relative humidity, are similar to the variations of the air temperature, where daily and annual fluctuations of water vapour pressure are not as prominent as in air temperature, so we cannot talk about them being parallel. As altitude increases and air temperature decreases, water vapour pressure decreases as well.
In horizontal distribution, vapour pressure values decrease as we move from the south of the country north. Mean annual value of vapour pressure is at its minimum value on Bjelašnica (6.1 hPa). Maximum annual mean of 10.9 hPa was recorded in Mostar.
If we look at the seasons, vapour pressure is higher in summer, peaking in July, rarely August. On the territory of the Federation of Bosnia and Herzegovina, maximum mean monthly values in the reference period 1961 – 1990, were recorded at stations in Sanski Most (17.4 hPa), Tuzla (16.6 hPa), and Mostar (16.2 hPa). August maximum is recorded in Bihać at the value of 16.0 hPa, while in Zenica, July and August maximums are identical at 15.7 hPa. Minimum water vapour pressure is recorded in January. Minimum mean monthly values during winter months range between 3.5 on Bjelašnica and 6.0 hPa in Mostar. With such distribution of maximum and minimum values, annual amplitudes of water vapour pressure range between 6.2 hPa on Bjelašnica and 12.2 hPa in Sanski Most. On Bjelašnica, mean monthly values of water vapour pressure during the year are significantly lower than in other stations, ranging between 3.5 hPa (January and February) and 9.7 hPa (July).