The Sultanate is characterised by hot and dry summers and mild winters, except for the Dhofar region in the south of the country, which is affected by monsoon climate from June to September each year. Summer starts in May and ends by October. Usually, a thermal low is established over the region with the core over north-western India, Pakistan and Oman. This thermal low contributes to the dynamics of the south-westerly circulation over the Indian Ocean and the Arabian Sea, such that, once it is established, it generates the south-westerly monsoon. Having travelled a long way over the Indian Ocean, the south-westerly winds are warmer, moist and unstable. During the summer season the average temperatures range between 27-37°C at As Seeb, 17-23°C at Saiq on Al Jabal Al Akhdar, and 24 and 30°C at Salalah. During this season, the southern region of Oman experiences monsoonal weather. This is because of its location in the fringes of the strong southwesterly monsoon winds which originate far out in the Indian Ocean. More extensive rains are also experienced around the mountains and the nearby area of northern Oman in the form of thunderstorm activity triggered by the moist monsoon current entering this region due to the westward movement of the seasonal trough across Oman.
The winter season extends from November to April. During this season the Arabian Peninsula is dominated by a high pressure ridge extending from Siberia over central Asia. The main air flow associated with this system is not itself rain-bearing. The prevailing condition is generally relatively cool with comfortable dry continental air.
The trajectory of this air flow is however modified by two other factors before reaching Oman. The first is the Zagros mountains in the south of Iran which act as an obstacle to the air flow, and the second is the warm water of the Sea of Oman. That is why by the time this air reaches Oman, it will already be modified to a warm temperature which is usually prevailing in the northern coastal areas.
Although the predominating weather is dry during the winter, rainfall sometimes occurs in association with migratory low pressure systems from the west. The average temperatures range between 20–28°C at As Seeb, 19–26°C over the interior, and 20–27°C at Salalah. Lower temperatures occur at higher altitudes. Temperatures on Al Jabal Al Akhdar range between 8 and 14°C, depending upon the elevation above sea level. Skies over the Sultanate are mostly clear: Muscat averages nearly ten hours of sunshine a day. Both to a native resident and a visitor alike, it is clearly visible that the Sun is a dominant force of nature.
In the Sultanate, with high temperature and low humidity common, conditions for evaporation are very favorable. Potential evaporation is estimated between 3,000mm in the Interior, through 2,100mm on Al Batinah coast, to some 1,700mm on the Salalah plain. These high potential evaporation rates apply to open water surfaces, which in the Sultanate are few. Actual evaporation is dependent on the available supply of water. Thus, if an area receives 100mm of rain per year, some 95mm of this rain may evaporate, and 5mm may be transferred from the runoff producing zones to runoff absorbing zones.