The Orontes basin receives highly variable rainfall related to its topographic features. Rainfall decreases in Lebanon from south to north, from 400 mm in Baalbek to less than 200 mm in Qaa, and then increases in the Syrian section of the basin. The natural passage in the coastal mountain between Tripoli and Homs lets through maritime influences far inward (466.9 mm / year in Homs, 409.7 mm / year at Qattinah). The Syrian coastal mountain is lower than that in Lebanon, which favors valley stations. An average of 368.0 mm / year was recorded in Ar Rastan, 383.9 mm in Muhradah and335.3 mm in Hama. Salamiyah located further east, receives 300.9 mm. North of the valley, the Nahr al Kabir pass also allows the penetration of maritime influences. Northern stations are better endowed, with rainfall at 502.5 mm, where 679.1 mm can be recorded in Idlib and 679.1 mm in Jisr ash Shughur.
The rainy season runs from October to May (Fig. 1) with a concentration of rainfall in the three winter months (December, January and February) that accounts for more than 50% of the annual total (61.7% in Qaa, 58.1% at Hermel, 57.4% in Al Qusayr, 58.9% in Homs, 59.7% in Muhradah and 55.5% in Jisr ash Shughur). This reflects the high frequency of disturbed weather patterns generating rainfall during this period (Traboulsi, 1981, Blanchet, 1993; Blanchet and Traboulsi, 1993, Traboulsi, 2004). The average monthly maximum is to be found in January as regards all stations and it is between 20 and 22% of the annual total. Spring rains take second place, accounting for between 19% and 23% of the annual total, while autumn rains account for only 15 to 20%. Summer rains are merely the expression of a few thunderstorms restricted in time and space (0 to 1.9%). The dry season is due to the presence of subtropical high pressures in altitude (500 hPa level).
Figure 1: Average rainfall regime in the Orontes valley.
Given the apparent movement of the sun, the rainy season, combined with advections of cold polar air, corresponds to low temperatures, while the dry season corresponds to high temperatures.
The average annual temperature regime is Mediterranean (Fig. 2). The lowest average is in January. The Syrian-Lebanese mountain barrier prevents softening maritime influences from getting through. This average is 6.6 ° C in Qusayr, 6.7 ° C in Qattinah, 6.3 ° C in Homs and 7 ° C in Hama. Jisr ash Shughur, higher in latitude and altitude, recorded a higher average - 7.6 ° C - thanks to the Nahr al
Kabir passage . The highest monthly average is in August (24.5 ° C in Qattinah, 25 ° C in Homs, 28.3 in Hama and 29.4 ° C in Jisr ash Shughur). For 6 months (4 months in Homs and Qattinah), it remains above 20 ° C and for 4 months (2 months in Homs and 1 month at Qattinah), it is above 25 ° C in Hama and Jisr ash Shughur (June, July, August and September).
Figure 2: Average thermal regime in the Orontes valley
The average minimum temperatures follow the same rhythm. They record the lowest values in January (1.6 ° C in Hama, 2.6° C in Homs, 3.0° C in Qattinah and 3.6° C in Jisr ash Shughur. They remain below 5° C during the three winter months (December, January and February), and below 10 ° C for 6 months of the year (5 months in Jisr ash Shughur). The absolute minimum is -10 ° C in Homs, -9.0 in Qattinah and -7.8 in Hama at the same period, while it is -2.4° C in Jisr ash Shughur for the period between 1957 and 1999 (-9.0° C between 1955 and 1969).
As for maximum temperatures, the monthly average exceeds 20° C as early on as in the month of April and reached 30° C for 4 months (in Qattinah, the average daily maximum does not reach 30° C). The absolute maximum, occurring in August (in July in Hama), reached 47.5 ° C in Jisr ash Shughur, 44.6° C in Hama, 42.5° C in Homs and 41.8° C in Qattinah.
The dry season extends from 5 months in Jisr ash Shughur to 8 months in Hermel.
Figure 3: Umbro-thermal diagrams showing the lenght of the dry season in the Orontes valley
Blanchet G., 1993 : Circulation atmosphérique et précipitations au Liban. Publications de l’Association Internationale de Climatologie 6 : 325-332.
Blanchet G. et Traboulsi M., 1993 : Froid, pluie, neige et tempête au Proche-Orient durant l’hiver 1991-92. Publications de l’Association Internationale de Climatologie 6 : 433-441.
Traboulsi M., 1981 : Le climat de la Syrie : exemple d’une dégradation vers l’aride du climat méditerranéen. Thèse de 3e cycle, Université Lyon II, 477 p.
Traboulsi M., 2004 : Les précipitations au Proche-Orient, variabilité spatio-temporelle et relation avec la dynamique de l'atmosphère (1960-61/1989-90), Atelier de Reproduction de thèse, Lille 3, 233p.