Map from Google - Saudi_Arabia

In Saudi Arabia, the climate is generally desert, and is very hot in summer all over the country; the rains are very scarce almost everywhere, and concentrated from November to April. However, there are differences among the various areas that are worth considering.

Saudi Arabia, climate of the coasts
Along both the coasts of western and eastern Saudi Arabia, the climate is desert, and it's particularly mild in winter, even warm in the central and southern part of the Red Sea coast, while in summer it's hot everywhere, but it's sultry on the shores of the Red Sea, while it's a bit drier, and therefore torrid, on the shores of the Persian Gulf.
On the coast of the Red Sea (called Tihamah), the average temperature in January goes from 14 °C (57 °F) in the northernmost part, on the Gulf of Aqaba, where sometimes it can get cold at night, to 25 °C (77 °F) of the southern part, where, therefore, winter does not exist, and it's hot all year round. Summer is very hot everywhere.
Here are the average temperatures of Al Wajh, in the northern part of the coast.
Average temperatures - Al Wajh
Al WajhJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)131416202224252524221815
Max (°C)242527303234343533322926
Min (°F)555761687275777775726459
Max (°F)757781869093939591908479

In Al Wajh, the sun shines all year round.
Sunshine - Al Wajh
Al WajhJanFebMarAprMayJunJulAugSepOctNovDec
Sun (hours)9101011121414131211109

The Red Sea is warm enough for swimming all year round, even in the northern part, although it goes down to 23 °C (73 °F) in February and March, while it gets very warm in summer. In the northernmost part, in the Gulf of Aqaba, it is a bit cool in winter, since it goes down to 21 °C (70 °F) in February and March.
Sea temperature - Al Wajh
Al WajhJanFebMarAprMayJunJulAugSepOctNovDec
Sea (°C)242323242627293029282725
Sea (°F)757373757981848684828177

Already in Jeddah, in the central part of the west coast, the average in January is 24.5 °C (76 °F). Summer is very hot and humid, with highs around 38/39 °C (100/102 °F) between June and August, and the heat is sweltering because of the humidity coming from the sea. By contrast, in the days when the wind blows from the interior, the temperature can reach as high as 46/47 °C (115/117 °F). Rainfall is definitely scarce: the rainiest month is November, with only 20 millimetres (0.8 inches) of rain.
Average temperatures - Jeddah
Min (°C)202021222425272826242221
Max (°C)293032353738393938373431
Min (°F)686870727577818279757270
Max (°F)8486909599100102102100999388

Jeddah is a very sunny city, but in summer some cloud banks can pass. However, this phenomenon is even more evident in the southern part of the Red Sea coast, which in summer, from June to September, is affected by the passage of clouds from Africa, due to the southwest monsoon (see below), which on the mountains can also bring some rains.
Sunshine - Jeddah
Sun (hours)79101112111011111198

At the latitude of Jeddah, in winter the Red Sea is a bit warmer than the northern part of the coast.
Sea temperature - Jeddah
Sea (°C)262525262829313131302927
Sea (°F)797777798284888888868481

Mecca is not right on the coast, however, it lies on the plains and west of the mountains, therefore it's still hot throughout the year, in fact it's even hotter than Jeddah, especially in summer, when highs stay normally around 43/44 °C (109/111 °F). The humidity is lower than on the coast, but the combination of humidity and temperature makes the heat oppressive. However, Mecca is really a very hot city, since during the day the temperature normally exceeds 40 °C (104 °F) for many months, from May to September; there are about 300 days with more than 32 °C (90 °F) per year, and the daily average of the whole year is as high as 30.5 °C (87 °F). Mecca is therefore one of the hottest large cities in the world.
Average temperatures - Mecca
Min (°C)191921252829293029262320
Max (°C)313235394244434343403532
Min (°F)666670778284848684797368
Max (°F)8890951021081111091091091049590

In Mecca, only 110 mm (4.3 in) of rain per year fall. Here is the average precipitation.
Average precipitation - Mecca
Prec. (mm)21371010155142220110

Also in Mecca the sun shines very often, however in summer there can be some cloud banks coming from Africa. Anyway, there are as many as 3,400 hours of sunshine per year.
Sunshine - Mecca
Sun (hours)89991011101091098

Mecca, Sacred Mosque
In Jizan, in the southernmost part of the coast, the winter is even warmer, while the summer gets a bit less hot, because of the south-west winds that in this season blow in a more evident way. On the other hand, the high humidity makes the heat sweltering, while average high temperatures are still of 38 °C (100 °F) in the summer months.
Average temperatures - Jizan
Min (°C)222324262830303029262523
Max (°C)313133353738383838373432
Min (°F)727375798286868684797773
Max (°F)8888919599100100100100999390

Because of the southwest monsoon, in summer it is possible to see some rain, especially in August. Here is the average rainfall.
Average precipitation - Jizan
Prec. (mm)133614819249181216133

The sun at Jizan shines all year round, but in summer there is a certain decrease in the hours of sunshine, again because of the southwest monsoon.
Sunshine - Jizan
Sun (hours)889101197891098

In the east, on the coast of the Persian Gulf, the climate is sunny and desert. Winter is mild, but it's cooler than in the Red Sea: the average in January is between 12 and 17 °C (54 and 63 °F); sometimes at night it can get cold, especially in the northern part, near the border with Kuwait, with lows around freezing (0 °C or 32 °F). Summer is very hot and sunny, a bit less humid than in the coast of the Red Sea, because the prevailing wind comes from the northwest, originally from the Iraqi desert, and therefore it does not come directly from the sea.
In Dammam, the average goes from 15 °C (59 °F) in January, to 37 °C (99 °F) in July and August. Throughout the year, only 90 mm (3.5 in) of rain fall, concentrated from November to April. During winter, there can be very cool days, or even a bit cold at night. Summer is scorchingly hot, with peaks of 50 °C (122 °F).
Average temperatures - Dammam
Min (°C)101115202528292925211611
Max (°C)212429344144454542372923
Min (°F)505259687782848477706152
Max (°F)70758493106111113113108998473

In Dammam the sun shines all year round, sometimes however some cloud banks can pass even in summer.
Sunshine - Dammam
Sun (hours)888910121111111098

The Persian Gulf, in the central part where Dammam is located, is a bit cool from January to March, while it becomes very warm from May to November. In summer, the Persian Gulf becomes the warmest sea in the world, even more so than the Red Sea.
Sea temperature - Dammam
Sea (°C)212020232730323333312724
Sea (°F)706868738186909191888175

In the northernmost part, near the border with Kuwait, the sea in winter is a bit colder, so that the temperature drops to 18 °C (64 °F) in February and March. In the southernmost part, in the small coastal stretch between Qatar and the Emirates, the sea in winter is instead a bit warmer, so much so that it does not go below 23 °C (73 °F).

Saudi Arabia, climate of the Syrian Desert
In the far north, there is an offshoot of the Syrian desert, which has a desert continental climate, with relatively cold winters, also due to the higher altitude, and hot summers. In Turaif, at 850 metres (2,800 ft) above sea level, the average temperature of January is 7 °C (44.5 °F), that of July is 28.5 °C (83 °F), with highs around 37 °C (99 °F). During winter, cold waves can occur, in which the temperature can drop to -10 °C (14 °F), and more rarely (given the low precipitation) it can even snow. On the contrary, in summer, despite the altitude the temperature can reach 45 °C (113 °F).
Average temperatures - Turaif
Min (°C)1361115182020181373
Max (°C)131620263135373735282115
Min (°F)343743525964686864554537
Max (°F)556168798895999995827059

Precipitation occurs from November to April and is scarce, around 80 mm (3.1 in), while from June to September basically it never rains.
Average precipitation - Turaif
Prec. (mm)1210131250000711979

In Turaif the sun shines all year round, but especially in summer, when the sky is almost always unclouded.
Sunshine - Turaif
Sun (hours)7891012141414131087

Saudi Arabia, climate of the plateau
Most of the Arabian Peninsula is occupied by a plateau, called Najd, whose altitude goes between 400 and 1,200 metres (1,300 and 4,000 feet). In addition to the rocky deserts, there is a series of sandy deserts connected to one another, called An Nafud, ad-Dahna, and Rub 'al Khali; the latter is the southernmost and largest, a huge expanse of sand. The plateau, desert like almost all of Saudi Arabia, has a fairly uniform climate: almost always sunny, mild in winter, but with very cool or cold nights, and very hot in summer, but with low relative humidity. The main differences occur in winter, when the northern part is much colder than the southern.

Rub' al Khali

In Ha'il, located in the north, at 1,000 metres (3,300 feet) above sea level, the average temperature of January is 10.5 °C (51 °F), while that of July it's 31 °C (88 °F). In winter, days are usually mild, while nights are often cold. Precipitation amounts to 150 mm (6 in) per year, with a maximum of 40 mm (1.6 in) in November, and no rains from June to September.
Average temperatures - Hail
Min (°C)45915202223242116105
Max (°C)172024293438394038322419
Min (°F)394148596872737570615041
Max (°F)6368758493100102104100907566

The capital Riyadh is located in the centre of the country, at 600 metres (2,000 feet) above sea level. Winter, from December to February, is mild: the average in January is 14.5 °C (58 °F). Sometimes at night the temperature can approach freezing, but there are also warm days, in which the temperature gets close to 30 °C (86 °F), even in winter. In summer, from June to September, it never rains, and the temperatures are very high, around 44 °C (111 °F) in July and August, but with peaks of 47 °C (117 °F). However, relative humidity is very low.
Average temperatures - Ryadh
Min (°C)91115202628292926211611
Max (°C)202328333942444440352822
Min (°F)485259687982848479706152
Max (°F)68738291102108111111104958272

Throughout the year, only 100 mm (4 in) of rain fall, almost all from November to April, with a maximum of 20/25 mm (0.9/1.0 in) per month in March and April.
Average precipitation - Ryadh
Prec. (mm)17922266000011317111

In Riyadh, the sun shines all year round, however, here also some cloud banks can sometimes pass, even in summer.
Sunshine - Ryadh
Sun (hours)7878911111091097

Medina is located in the west, at 600 metres (2,000 feet) above sea level like Riyadh, but in winter it's milder than the capital, since it has a January average of 18 °C (64 °F), with highs around 24 °C (76 °F). Sometimes it can get cold at night, but more rarely than in Riyadh, actually even in winter the temperature often touches 28/30 °C (82/86 °F). Summer is similar to that of Riyadh, scorchingly hot and sunny.
Average temperatures - Medina
Min (°C)121317212628293028231814
Max (°C)242731354043434442373126
Min (°F)545563707982848682736457
Max (°F)75818895104109109111108998879

In the south, the desert of Rub 'al Khali, uninhabited and mysterious, is quite warm even in winter, with highs in January around 25/26 °C (77/79 °F). At night it can get pretty cool, but temperatures do not drop below freezing. In summer, the southernmost part of this desert is touched by the south-west monsoon, so that some cloud banks may pass, and sometimes even a rain shower may occur.
Very rarely, the Rub 'al Khali desert may be affected by the remains of tropical cyclones coming from the Arabian Sea, which can bring storms of sand raised by the wind, and unusual rains in the middle of the desert. Cyclones form from May to December, and are more frequent at the beginning of the period (May-June), and a little less at the end (October-December).

In Saudi Arabia, there are practically no rivers of any importance, but as happens in desert countries, there are many dry riverbeds, called wadis, which can sometimes turn into ephemeral streams after brief but violent showers.

Saudi Arabia, climate of the western mountains
The western part of Saudi Arabia, known as Hejaz, is crossed by a mountain range parallel to the coast of the Red Sea. In the north (the area circled in green) the climate is desert, with no summer rainfall, though some rains are possible in winter, and even snowfalls at the highest altitudes, as happens in Jebel al Lawz, 2,580 metres (8,465 feet) high, in the north-west.
In the southern part of the mountains, in the Asir region (burgundy area), there are several peaks above 2,000 metres (6,500 feet), among which is the highest mountain of Saudi Arabia, Jabal Sawda, 2,990 metres (9,810 feet) high. Here, the climate is obviously tempered by the altitude, in addition, in summer, especially in the southern part, it is affected by the southwest monsoon, a wet current affecting southern Arabia, which brings a bit of rain from June to September, as well as widespread cloudiness. Some rain may fall also in spring, between March and May. In some areas, precipitation exceeds even 250 mm (10 in) per year, so the climate becomes semi-arid; on the south-west facing slopes, there are verdant landscapes and terraces, in which water springs are used for irrigation.
In Abha, at 2,000 metres (6,500 feet) above sea level, the average temperature of January is 14 °C (57 °F), that of July is 24 °C (75 °F).
Average temperatures - Abha
Min (°C)89111315171717151197
Max (°C)202123252831313029262321
Min (°F)464852555963636359524845
Max (°F)687073778288888684797370

In Abha the rainiest season is spring, but then some rains occur even in summer. Here is the average precipitation.
Average precipitation - Abha
Prec. (mm)1814474824817245355218

In Abha the sun often shines all year round, but the least sunny months are July and August.
Sunshine - Abha
Sun (hours)9910999889999

Asir Mountains, Arabia

When to go

In most of Saudi Arabia, where the winter is mild (northern Red Sea coast, central plateau where Riyadh and Medina are located, Persian Gulf coast) and in the warmest areas (Mecca, Jeddah, south-central Red Sea coast and Rub 'al Khali), the best time is from December to February, to avoid the heat that can be intense in the rest of the year (but especially in summer).
However, in the northern areas, north of Riyadh (see Ha'il, Rafha, Tabuk) you can avoid winter, which can be cold, choosing the months of March and November. In the far north, in the Syrian desert (see Turayf, Al Qurayyat) where the winter is longer, you can choose April and October.
For swimming and sunbathing, the best month is November in the Persian Gulf (in March the sea is still a bit cool, while in April the weather starts to be hot), and March and November in the north-central part of the Red Sea, while you can go from November to March in the south-central part of the Red Sea, from Jeddah down to the border with Yemen, because in these parts the sea and the weather are warm even in winter.
In summer, it's too hot for sunbathing.

What to pack

In winter: in the Syrian desert, warm clothes, down jacket, hat, raincoat or umbrella, scarf. In the Persian Gulf coast, spring/autumn clothes, t-shirt for warm days, sweater and jacket for the central-southern part (see Dammam, Al Khobar), heavier jacket and hat for the northern part (Khafji), scarf for the wind, possibly an umbrella. For the Red Sea coast, north of Jeddah, light clothes for the day, sweater and light jacket for the evening, which sometimes can also be used in Jeddah and Mecca; in the southern part (see Jizan), in addition to light clothes, just bring a scarf for the breeze and possibly a light sweatshirt for the evening. For the plateau, Riyadh and Medina, spring/autumn clothes, warm jacket and hat for the evening, especially in the north; for the Rub 'al Khali, light clothes for the day, scarf or turban, desert boots or sandals, sweater or jacket for the evening and early morning. For the western mountains, in the north, warm clothes, down jacket and sweater; in the south, spring/autumn clothes, light for the day, sun hat, sweater and jacket for the evening.

In summer: for the Syrian desert and the plateau above 700 metres (2,300 feet) (see Ha'il), light clothes, sun hat, sweatshirt or sweater for outdoor evenings, scarf; for Mecca, the plateau and Riyadh, the Rub 'al Khali and the coasts of the Persian Gulf and the Red Sea, lightweight clothes, long and made of natural fabric (cotton or linen), desert turban, sweatshirt and scarf for air conditioned places. For the western mountains, light clothes for the day, sun hat or desert turban, sweatshirt or sweater for the evening, hiking shoes; for the southern part, raincoat or umbrella.

Women should cover their entire body.