### Determining mean dates of beginning and end of periods with air temperature above temperature thresholds (5°c, 10°c, 15°c)

Daily/annual fluctuation of air temperature depends on climate characteristics of the given area. Thus we have early or late crossing of air temperature thresholds.

Temperature thresholds are very important for active life of plants: 0°C, 5°C, 10°C and 15°C. At 0°C, the active life of plants shuts down, above 5°C grasses and true grasses of moderate zone begin vegetating, sub-tropical plants begin vegetating above 10°C, and tropical plants above 20°C.

Mean air temperature above 5°C represents a biological zero for the majority of plants - it is the temperature at which they begin and end their vegetation cycle. Early or late ending of their vegetation cycle is strongly affected by zonality and altitudinal distribution, and with increased latitude and altitude - the quantity of heat reduces.

For the purpose of this climate atlas, we have processed and analysed the mean beginning date, end date, and mean duration of the period with mean daytime temperature ≥ 5°C, 10°C, and 15°C taken at weather stations in the Federation of Bosnia and Herzegovina in the period 1961-1990.

The mean beginning date, end date, and mean duration of the period of these typical threshold temperatures, were determined using methods based on graphs and calculations.

To determine the beginning/end date using such methods, one must have an overview of the annual temperature changes for the given location, and use it to calculate typical temperatures.

Calculation method is based on the fact that the slope of the upward and downward sections of the annual temperature curve is rather linear.

The upward section of the annual temperature curve is calculated using the following equation:

$$d_+=D\frac{P-t_b}{t_a-t_b}$$

For the downward section of the curve:

$$d_-=D\frac{t_a-P}{t_a-t_b}$$

Meaning of the symbols:

• P temperature threshold (5°C, 10°C, 15°C)
• d+  difference in days between the mean date of the month above the threshold temperature and the date on which the threshold temperature P for the upward section of the curve occurs
• d-  difference in days between the mean date of the month below the threshold temperature and the date on which the threshold temperature P for the downward section of the curve occurs
• D difference in days between the mean dates of the month and the mean monthly temperatures or
• ta mean monthly temperature above the threshold P
• tb mean monthly temperatures below the threshold P

Analysis of the years of data from the period 1961 -1990 has shown an evident rule concerning horizontal and vertical distribution of stations. The beginning of the vegetation period is the date on which the mean daytime air temperature exceeds 5°C, when the majority of plants enter their growth and development stage. Areas with the earliest beginning date for this period match the areas of the latest end dates for the period and vice versa.

 Station Threshold 5°C Threshold 10°C Threshold 15°C Beginning of the period End of the period Duration of the period (in days) Beginning of the period End of the period Duration of the period (in days) Beginning of the period End of the period Duration of the period (in days) BIHAĆ 7 March 24 November 262 10 April 23 October 196 14 May 21 September 130 BJELAšNICA 26 May 28 September 125 19 July 15 August 27 19 July 15 August 27 BUGOJNO 19 March 12 November 238 22 April 12 October 173 1 June 8 September 99 ČAPLJINA 16 January 16 December 334 20 March 14 November 239 28 April 11 October 166 GORAŽDE 10 March 15 November 250 14 April 17 October 186 20 May 17 September 120 GRADAČAC 6 March 23 November 262 7 April 24 October 200 9 May 26 September 140 IVAN-SEDLO 4 April 5 November 215 6 May 4 October 151 25 June 25 August 61 JAJCE 12 March 19 November 252 14 April 20 October 189 20 May 18 September 121 LIVNO 22 March 14 November 237 26 April 12 October 169 4 June 8 September 96 MOSTAR 18 January 16 December 332 19 March 16 November 242 26 April 17 October 174 SANSKI MOST 10 March 20 November 255 12 April 19 October 190 17 May 18 September 124 SARAJEVO 15 March 17 November 247 19 April 17 October 181 25 May 16 September 114 TUZLA 11 March 19 November 253 12 April 19 October 190 16 May 17 September 124 ZENICA 9 March 17 November 253 11 April 20 October 192 15 May 19 September 127

Period with temperatures above 5.0°C in lowlands of the northern Bosnia begins, on average, in the first decade of March, and in the far north-west, during the first and second decade of March. As the altitude increases, dates on which the temperature exceeds 5°C are increasingly postponed in the central parts of the country as well, and occur during the second and third decade of March, and in the mountains, in the first decade of April. (Table 1)

In the south of the country, vegetation begins in late January/early February; in some coastal locations, it lasts all year long.

Vegetation period in lower parts of northern Bosnia usually ends in the second and third decade of November, and in the southern parts of the country, in the second decade of December. In the mountains, vegetation ends much earlier, on average - in the third decade of October.

The average duration of vegetation period in the lowlands is about 259 days, in the central basin area about 247, and in the south of the country, about 326 days. As the altitude increases, duration of vegetation is shorter; above 1000 meters, its average duration is about 195 days, while at heights above 2000 meters, average duration of vegetation is merely 125 days.

Distribution of beginning dates and end dates with temperatures above 10°C in horizontal and vertical direction is quite similar as for the 5.0°C threshold. In the northern part of the country, this period, on average, begins in the first or the second decade of April, in central parts of the country in the third decade, and in the highlands - in the first decade of May. The end of this period in northern parts of the country commences in the second decade of October, while in the mountains, in late September or early October.

In the southern parts of the country, this period starts earlier, in the third decade of March and first decade of April. The period with temperatures above 10°C ends in the first and second decade of November. Periods with mean daytime temperature above 15°C in the south of the country on average begin in late April and early May, and end in the second decade of October. In the lower parts of the country, this period begins in the first decade of May, and ends in the third decade of September. As the altitude increases, the date of beginning of this period is significantly delayed - it occurs in the second or third decade of June. At altitudes above 1300

m, due to a short period with air temperatures above 15°C, determining of this threshold is not of any major practical use.