Map from Google - Oman

In Oman, the climate is tropical desert almost everywhere, with some summer rains in both the northern and southern mountain areas, and clouds brought by the summer monsoon along the eastern coast. The monsoon, from mid-June to mid-September, is due to wet currents blowing towards the Asian continent, and brings limited effects in terms of rain, but it's nonetheless able to influence the climate of the country.
The map below shows the climate zones of Oman.

Climates in Oman

Along the coast of the Gulf of Oman (zone 1 in the map) the climate is desert and sunny all year-round, with a few winter rains, more likely in the far north, ie the Musandam Peninsula, an exclave of Oman in the United Arab Emirates, where 15/20 millimetres of rain per month fall from December to March. This area is very hot in summer, with highs around 37/38 °C on average, and the humidity of the sea that makes the heat oppressive. When the wind blows from inland the humidity drops, but the temperature can approach 50 °C. The winter temperature is pleasant: the daily average is around 20 °C in the far north, and 21 °C in Muscat, the capital. In contrast to the northern tip, in summer, along the northern coast (called Al Batinah) where Muscat is located, the hottest month is June, because in July and August the clouds brought by the southwest monsoon can lower a bit the daytime temperature, which passes from 40 °C in June to 38 °C in July and 36 °C in August.
Here are the average temperatures of Muscat.
Average temperatures - Muscat
Min (°C)171820242930302827242118
Max (°C)252630354040383636353027

In Muscat the rains are very scarce, given that they amount to 100 millimetres per year, and in summer the rains are virtually nonexistent. Here is the average precipitation.
Average precipitation - Muscat
Prec. (mm)22271511512500510103

The Gulf of Oman is warm enough for swimming all year round, and it gets very warm in summer, as we can see in the following table.
Sea temperature - Muscat
Sea (°C)242424272931313029292725


In the Musandam Peninsula, there are mountains which reach their highest point in Jebel Al Harim, 2,087 metres high, but they are desert, because in summer they do not receive rainfall.
In the mountains located south of the northern coast (zone 2 on the map), called Al Hajar, and culminating in Jebel Shams, 3,009 metres high, the south-west monsoon brings a bit of rain, which added to the modest rains (and sometimes even snowfalls) of winter, raises the total of the year in some areas above 200/300 mm, still not much, but sufficient for the growth of some shrubs or drought-resistant plants. In Saiq, at almost 2,000 metres above sea level, the average temperature in January is 10 °C, so it is quite low, with possible frosts at night. In June and July, highs are around 29/30 °C. On the contrary, at lower altitude the summer heat is scorching.
Average temperatures - Saiq
Min (°C)57101418202120171396
Max (°C)151820242931313028242017

In Saiq, 310 millimetres of rain per year fall, with a relative maximum in summer. Here is the average precipitation.
Average precipitation - Saiq
Prec. (mm)203035302015455030151010310

Along the coast of the Arabian Sea (zone 3) the climate is desert; moving south, winter gets progressively warmer, while summer becomes less hot, but also more humid and cloudy, due to the greater influence of south-west monsoon. A moderate wind blows often, making the sea choppy. However, there can be periods that typically last two or three days, in which the wind blows from inland, causing sudden increases in temperature, especially from April to July, with peaks above 40 °C, often accompanied by sandstorms; these events are more likely in the northern part of the coast, while in the south the coast of Dhofar is more sheltered from these hot winds.
In the island of Masirah, the temperature is high even in winter; in this season there is little rain and the sun shines. The temperature gradually increases in April, May and June, when it reaches 35 °C, then it moves back to 31/32 °C in July and August, when humidity and cloudiness increase, even though the rains are still scarce. In a typical year, just 40 mm of rain fall.
Average temperatures - Masirah
Min (°C)191921242626242324232321
Max (°C)272830343635323131323028

The effects of the monsoon are more evident in the southern part of the coast. In Salalah, the capital of Dhofar, the daytime temperature reaches 32/33 °C in May and June, but then it goes down to 27/29 °C in July and August, when even a bit of rain falls, just 20/25 mm per month, under the form of frequent drizzle. In addition, in July and August a permanent cloud cover is formed on this part of the coast; for this reason the daily temperature range is narrow.
Average temperatures - Salalah
Min (°C)192022242727252424232220
Max (°C)282830323332292729313129

Here is the average precipitation in Salalah. Although in July and August there are "officially" only 7/8 days of rain per month, days with light drizzle occur more often.
Average precipitation - Salalah
Prec. (mm)10668822251050596

The Arabian Sea is warm enough for swimming all year round, and it is warmer in spring and autumn, while in summer the water temperature drops a bit, because the wind increases evaporation and mixes the water layers.
Sea temperature - Salalah
Sea (°C)252526282928252425282726

In the southern coastal range (zone 4) of the Dhofar region, north of Salalah, the climate is cooled by the altitude, and the summer monsoon succeeds in discharging a bit of rain, so that the landscape is green, at least along the southern slopes exposed and during the rainy season. At 700/1,000 metres above sea level, the temperatures are mild in winter, around 21/22 °C during the day, and similar in summer, while in spring and autumn they are higher, around 28/30 °C. In summer, the rains are usually light but they occur almost daily.
Here are the average temperatures of Qairoon Hairitti, at 850 metres above sea level.
Average temperatures - Qairoon
Min (°C)131417202121201919191714
Max (°C)212325293029232126282622

Dhofar mountains

In the vast inland desert area (zone 5) which covers most of the country, the climate is desert, mild in winter (with cool nights and warm days), and hot in summer, with average highs regularly above 40 °C, but with peaks of 50 °C. Even here, however, in summer some cloud banks can pass, and there's a slight decrease in maximum temperatures in July and August, but generally it doesn't rain.
Average temperatures - Haima
Min (°C)121417222527282524211815
Max (°C)273033394344424240373329

Every now and then, from May to November, Oman can be reached by tropical cyclones, which bring gale force winds and torrential rainfall, as happened with cyclone Keila on early November 2011.

When to go

The best time to visit Oman, making excursions in the desert, in cities and in mountains, is winter, from December to February, which is sunny, pleasantly warm on the coast, and quite warm in the desert, during the day (but cool at night), and sometimes cold in the mountains.
For a beach holiday, as we have seen the sea is warm all year round; the sea temperature is good even in the period from November to March, but in November there is still some risk of cyclones on the coast of the Arabian Sea, while in January the temperature can sometimes be a bit cool in the coast of the Gulf of Oman, at least for those who are sensitive to cold.

What to pack

In winter: in Muscat and the north, light clothes for the day, sweater and jacket for the evening and cool days, scarf for the wind. For the high mountains, spring/autumn clothes, sweater and jacket for the evening. For the desert, light clothes for the day, scarf or turban, desert boots or sandals, sweater or jacket for the evening and early morning. For the Arabian Sea coast (see Salalah and Masirah), light clothing, a sweatshirt for the evening. For the southern coastal range, light clothes for the day, sweater and jacket for the evening. For the reef, snorkeling equipment, water shoes or rubber soled shoes.

In summer: in Muscat, the north and the desert, loose fitting, lightweight clothes, made of natural fabric (cotton or linen), desert turban, sweater and scarf for air conditioned places. For the high mountains, light clothes, sun hat, a sweatshirt for the evening. For the Arabian Sea coast (see Salalah and Masirah), light clothes, light sweatshirt and scarf for the breeze, light raincoat. For the southern coastal range, spring/autumn clothes, a light jacket and a sweatshirt, raincoat.
For women, it is better to avoid shorts and miniskirts outside tourist resorts.