Map from Google - United_Arab_Emirates

The UAE have a desert climate, with very mild winters and hot and sunny summers, when the humidity of the Persian Gulf makes the heat unbearable. The annual precipitation is almost everywhere below 100 millimetres (4 inches), and is concentrated in the winter months. The rains are rare but occur in the form of showers or downpours, which sometimes can be intense.
The United Arab Emirates are seven, almost all overlooking the Persian Gulf (Abu Dhabi, Ajman, Dubai, Ras al-Khaimah, Sharjah and Umm al-Quwain), and one overlooking the Gulf of Oman (Fujairah). The biggest emirate, which also includes the capital, is Abu Dhabi, even though the most famous, because of its futuristic buildings and modern tourist resorts, is Dubai.
In Dubai, the average temperature goes from 18.5 degrees Celsius (65.5 °F) in January (so we are at the boundary between the subtropical and tropical climate), to around 35 °C (95 °F) in July and August.
Here are the average temperatures of Dubai.
Average temperatures - Dubai
Min (°C)141517202426283027231916
Max (°C)232427323738404138353126
Min (°F)575963687579828681736661
Max (°F)7375819099100104106100958879

In a typical year, only 98 mm (3.8 in) of rain fall, most of which between December and March. In Abu Dhabi, still on the coast but more to the west, it rains even less, around 50 mm (2 in) per year.
Here is the average precipitation in Dubai.
Average precipitation - Dubai
Prec. (mm)1035251010000021598

The sea is warm enough for swimming all year round, although from January to March it drops to 23 °C (73 °F); in summer it gets very warm, so that it exceeds 30 °C (86 °F) from June to October, so as to allow long baths. On the Gulf of Oman, in winter the sea is a bit warmer than in the Persian Gulf.
Sea temperature - Dubai
Sea (°C)232323252931323332312825
Sea (°F)737373778488909190888277

Winter in the Emirates, from December to February, is very mild, or pleasantly warm, the maximum temperatures ranging from 22 to 25 °C (72 to 77 °F). Sometimes, especially in January, a bit of cold air from the north can lower the night temperatures to around 7/8 °C (45/46 °F), and the daytime temperatures to around 15 °C (59 °F).
Summer is very hot and sunny, with daytime temperatures ranging from 40 °C to 45 °C (104 to 113 °F) between June and September, and with the wind from the Persian Gulf that brings moist air from the sea, making the heat sweltering. Even the months of May and October are hot, with highs around 35/37 °C (95/99 °F). The highest recorded temperatures are around 50 °C (122 °F) on the coast, and even more in the interior.
However, luckily the buildings are air-conditioned.

Abu Dhabi

In the northeast, the Al Hajar Mountains, which separate the coast of the Persian Gulf from the Gulf of Oman, the climate is still desert, although in the winter months it rains a bit more often than in plain. The temperatures do not drop much compared with the plain, if not above 1,000 metres (3,300 feet), where it can get cold in winter, while in summer the heat is more bearable.
The south of the Emirates, in the emirate of Abu Dhabi, is occupied by the vast sand expanse of the Rub al-Khali, and is almost uninhabited except in the oases, such as that of Liwa, and near oil and gas extraction facilities. Here the climate is similar to that of the coast, and even hotter in summer, but the air is drier because of the greater distance from the sea.

Emirates, desert

All year round, but especially in spring, the United Arab Emirates may be affected by strong winds, which can bring dust and sand storms.

When to go

The best time to visit the United Arab Emirates is winter, from December to February, which is a pleasant and sunny season. Thus you will avoid the oppressive heat of the long summer months.
For a beach holiday, the best months are March and November (with a preference for November, because the sea is warmer), because when it gets too hot, even sunbathing can be a challenge.

What to pack

In winter, light clothes for the day, sweater and jacket for the evening and for cool days, scarf for the wind.
In summer: lightweight clothes, loose-fitting and made of natural fabric (cotton or linen), desert turban, sweatshirt and scarf for air conditioned places. For the reef, equipment for snorkeling, water shoes or rubber soled shoes. For women, it is best to avoid shorts and miniskirts outside tourist resorts.