Map from Google - Fuerteventura

In the island of Fuerteventura, the climate is very mild and almost desert. Along the coast the annual rainfall amounts to less than 200 millimetres (8 inches) per year, most of which occur in autumn and winter. Unlike other islands of the Canaries, in the interior of Fuerteventura the altitude reaches only 600/700 metres (2,000/2,300 feet), too low to cause a significant increase in precipitation in the slopes exposed to the wind.
Here is the average precipitation of Puerto del Rosario.
Average precipitation - Fuerteventura
FuerteventuraJanFebMarAprMayJunJulAugSepOctNovDecYear
FuerteventuraJFMAMJJASONDY
Prec. (mm)161413411004102133117
Prec. (in)0.60.60.50.200000.20.40.81.34.6
Days32210000122316

The temperature in Fuerteventura is very mild, spring-like much of the year: during the day it goes from 20 °C (68 °F) in January, to 27 °C (81 °F) in the period from July to September. Moreover, temperatures generally tend to stay around the average, except when the wind blows from Africa, which can raise the temperature by several degrees.
Here are the average temperatures.
Average temperatures - Fuerteventura
FuerteventuraJanFebMarAprMayJunJulAugSepOctNovDec
FuerteventuraJFMAMJJASOND
Min (°C)141415151618202120191715
Max (°C)202122222425272727252421
Min (°F)575759596164687068666359
Max (°F)687072727577818181777570

The island is particularly exposed to the northeast trade winds, which blow steadily throughout the year, especially in the afternoon. The waves that are generated by the wind are particularly appreciated by surfers. Sometimes, however, there can be a hot and dry wind from Africa, which can reduce visibility, carrying dust or sand from the desert, and can cause an abnormal heat, which is felt especially in the interior of the island, and typically lasts one or two days.
The sun in Fuerteventura shines all year round, but especially from May to September, when blue skies are the norm.
The sea is cool, because of the cool sea current called Canary Current, which flows to the west of Morocco: the sea temperature ranges from 18 °C (64 °F) in February, March and April, to 22 °C (72 °F) in the period from August to October. This means that you can try to swim even in winter, at least if you are not sensitive to cold, while in summer the sea is still cool.
Here are the average sea temperatures.
Sea temperature - Fuerteventura
FuerteventuraJanFebMarAprMayJunJulAugSepOctNovDec
FuerteventuraJFMAMJJASOND
Sea (°C)191818181920212222222120
Sea (°F)666464646668707272727068

Fuerteventura, beach

When to go

Typically, Fuerteventura is appreciated for beach tourism and surfing, so on the understanding that due to its mild climate it can be visited all year round, the best time to visit it goes from May to mid-October. For the low rainfall, however, after all the island lends itself to this type of activity also in the second half of October and in November, when the sea is still relatively warm, although the first Atlantic disturbances can bring some cloudiness and some rain, which can happen also in winter. On the other hand, in autumn and winter, Atlantic depressions can bring high waves, generated by westerly or southerly winds, which are different from the typical trade winds, and can be appreciated by surfers.

In suitcase - what to bring

In winter: light clothes for the day, sweater and jacket for the evening, possibly a raincoat or umbrella.
In summer: summer clothes, but also a scarf for the wind, a light sweatshirt, a light jacket for the evening and for windy afternoons; comfortable shoes for hiking.