The climate of Qatar is desert, with very mild winters, and very hot and sunny summers. Being a small and flat country, it has a uniform climate throughout the territory.
Basically, there are two main seasons: a cooler season from December to February, and a hot season from April to October, within which we can distinguish a very hot period from May to mid-October. March and November are transitional months, warm but without excesses.
In Doha, the capital, the daily average temperature ranges from 18.5 degrees Celsius (65 °F) in January to 37 °C (99 °F) in July. Here are the average temperatures of Doha.
Doha - Average temperatures
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)141618232830323129262217
Max (°C)222429344042434239363025
Min (°F)576164738286908884797263
Max (°F)72758493104108109108102978677

The prevailing wind is moist and blows from the north-west, more frequently in the summer months. Another wind, hot and dry, can blow throughout the year, but preferably in spring; it blows from the south and it's able to raise the temperature to about 30 °C (86 °F) in winter and to about 40 °C (104 °F) and above from April to October, while drastically lowering relative humidity; it is also able to raise dust and sand storms.
The scarce rainfall occurs during the winter months, and it amounts to less than 100 millimeters (4 inches) per year. Every now and then, some rains may also occur in spring, in March, April and even May, while in summer, it never rains. The rains in Qatar occur in the form of brief (but sometimes heavy) showers, which can sometimes fill the wadis, the river beds which remain dry most of the year, and even cause flooding.
Here is the average precipitation in Doha.
Doha - Average precipitation
MonthJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec.(mm)131716940000131275
Prec.(in)0.50.70.60.40.2000000.10.53
Days2221000000019

In winter, from December to February, temperatures are mild and the sky is often clear, so, it's a spring-like season. Lows are around 14/17 °C (57/63 °F) and highs around 22/25 °C (72/77 °F). However, there can be some cool and windy days, especially in January, with temperatures below 10 °C (50 °F) and night and around 15/16 °C (59/61 °F) at mid-day.
Summer is very hot: from June to August, highs are around 42/43 °C (108/109 °F) and lows around 30/32 °C (86/89 °F). There are torrid days (that is, very hot and dry), when the wind blows from the Arabian desert, and the daytime temperatures rise above 40 °C (104 °F), and sometimes up to 45/47 °C (113/117 °F), while other periods are less hot but more humid, with highs around 37/38 °C (99/100 °F) and sweltering heat. Over the course of the summer, relative humidity gradually increases, so much so that the second part of the season, in August and September, is particularly stifling. The combination of moisture and temperature that is recorded in summer in the Persian Gulf countries is actually one of the worst in the world.

Doha

The sun in Qatar shines all year round, sometimes, however, some cloud banks can reach the country, even in summer. Here are the average sunshine hours per day.
Doha - Sunshine
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Hours888910111011101098

The sea in Qatar is a bit cool from January to March, particularly in February, when it reaches the lowest temperature, about 20 °C (68 °F), while it's still quite warm in December, when it's about 24 °C (75 °F). In summer, the waters of the Persian Gulf become very warm, exceeding 30 °C (86 °F) from June to October.
Doha - Sea temperature
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Temp (°C)222021232730323333312824
Temp (°F)726870738186909191888275

Best Time



The best time to visit Qatar runs from mid-November to mid-March: it is the only time of the year that is not hot. In general, the weather is mild and sunny from December to February, and warm but not too much in November and March. Some cloudy days are possible, sometimes with rain, especially from December to February.

What to pack



In winter: bring light clothes for the day, a sweater and a jacket for the evening and for cool days, and a scarf for the wind.
In summer: bring lightweight clothes, made of natural fabric (cotton or linen), a desert turban, and a sweatshirt and a scarf for air conditioned places. It's better for women to avoid shorts and miniskirts outside of tourist resorts.
When going to the reef, you can bring snorkeling equipment, including water shoes or rubber-soled shoes.