Sunshine duration - insolation

Sunshine is of great importance for the life on Earth, and it is a very important climatic element. Sunshine duration is called insolation. Sunshine duration is the time during which the Earth surface receives direct solar radiation. It is measured by a heliograph, and expressed in units of time.

For measuring insolation, we use the Campbell - Stokes heliographs. The main part of this heliograph is a glass sphere located in the middle of a metal base that can be adjusted to the latitude. Paper tape is placed behind the sphere in the east-west direction, so solar rays focus in the sphere leaving a burnt mark on the paper tape. Since only some of the meteorological stations in the Federation of Bosnia and Herzegovina have a heliograph, other stations and areas measure the daily sunshine by estimating the ratio between clouds and sunshine at the nearest station that has a heliograph.

There is actual and possible (astronomical) insolation. Actual insolation depends on the latitude, level of cloudiness, duration of daylight, and openness of the horizon. Possible (astronomical) insolation is derived from heliographic altitude of the location and Sun declination, assuming the sky is clear of clouds.

Solar energy, as the source of life, is of great importance for human health, agriculture, forestry, energy and tourism.

Characteristics of distribution of total annual sunshine duration are mostly determined by geographic position, relief, and distribution of cloudiness. The annual fluctuation (Graph 1) shows that insolation is longest in summer (July, August), and shortest in December or November, which is related to daytime length, and cloud cover, or prolonged duration of certain weather types above our country.

In the annual distribution, sunshine duration increases from south to north. Southern part of the Federation of BiH records maximum hours of insolation. In the far east of Herzegovina, in the narrow area reaching out to the sea (Neum), and some of the continental parts of Herzegovina, annual insolation is well over 2000 hours, ranging from 2287 in Mostar (6.1 hours a day) to 2508 hours in Neum (8.1 hours a day).

As we move towards the central part of the country, the number of hours of sunshine reduces, ranging between 1517 in Zenica (4.2 hours a day), 1678 in Bugojno (4.6 hours a day), and 1770 hours a year in Sarajevo (4.8 hours a day). The lowest values of sunshine duration were recorded along the River Bosna (Zenica), and the reason can be found in the overly clouded atmosphere, high annual cloudiness (62%), and, possibly, in the unfavourable location of the station, etc. 

Northern parts of the Federation of Bosnia and Herzegovina have a different climate than the central highlands, so insolation is different there, lasting longer than in the mountains, but still shorter than in Herzegovina. Far northeast is sunnier than the northwest, especially in late winter/early spring, and in autumn. The average annual sunshine duration in the far northeast (Gradačac) is approx. 1930 hours (4.8 hours a day), and in Bihać, in the far northwest of the country, approx. 1740 hours (4.7 hours a day).

The sunniest month is July, with an average of 10 hours of sunshine. (Mostar - 10.4 hours a day). Longest duration of insolation occurs in July in Herzegovina, with an average of more than 300 hours, and shortest is recorded in central and basin areas of Bosnia, with an average of about 220 hours (Zenica, Ivan-Sedlo).

In regular annual fluctuations of insolation, spring increase is usually smaller than the autumn decrease. June increase is particularly small, which is related to cloudiness and rain typical of this month.

Major decrease in insolation occurs in September, and in the western parts of the country, in October; the most prominent decrease in insolation duration occurs in November, which is related to changes in air circulation, increased cloudiness, and main autumn maximum of precipitation.

Horizontal distribution of winter insolation shows that increase in latitude leads to shorter insolation. In winter, northern parts are more exposed to influences of stable anticyclonic conditions, which results in fogs that can linger for most of the day and shorten the insolation. 

In winter season, the Sun shines 146 hours on average in the continental part of the country, and up to 360 hours in Herzegovina. The annual fluctuation shows that December is the month with least insolation, when the Sun shines less than two hours a day on average. (From 1.0 hour in Zenica to 3.3 hours in Mostar). December record the longest insolation in the south of the country, with an average of 100 to 110 hours (Neum 110 hours, Mostar 103 hours). During December, the highest mountains of Bosnia record an average of insolation between 100 and 80 hours, while lowlands record between 50 and 30 hours. The shortest December insolation was recorded in Zenica (31 hour).

Summer distribution of insolation also decreases with increasing latitude, but to a much lesser extent. Despite the fact that stable anticyclone dominates this area at this time of year, bringing nice weather, there is a noticeable difference in summer insolation duration between southern and northern parts of the country. The reason can be found in convective clouds formation in the areas of more prominent relief, which results in cumulus clouds that reduce insolation. In summer season, the average number of hours of sunshine ranges between 600 and 700 hours in the continental part of the country; the south records between 800 and 900 hours.

In spring, in the south of the country, sunshine lasts 540 to 640 hours on average, and in the central mountain - basin part of the country, it ranges between 400 and 470 hours; in Krajina and in the northeast of the country, maximum sunshine duration is 500 hours.

In autumn, the average number of hours of sunshine is 326 hours in the continent, up to 550 hours in the Neretva valley, and the far south. In winter season, the Sun shines 146 hours on average in the continental part of the country, and up to 360 hours in Herzegovina.

In the vegetation period, average duration of sunshine of the Federation of Bosnia and Herzegovina ranges between 1100 and 1700 hours.