Map from Google - Canary_Islands

The Canary Islands enjoy a remarkably mild climate, being located on the Atlantic Ocean, just north of the Tropic, a short distance from the coast of Morocco and Western Sahara, in a stretch of sea where a cool current flows; they are also subject to the trade winds, which blow constantly from the north east. The Canary Islands belong to Spain; there are seven major islands (Tenerife, Fuerteventura, Gran Canaria, Lanzarote, La Palma, La Gomera, El Hierro), plus some smaller islands and islets.

Along the coasts, the daily average temperatures in January and February are around 17/18 °C (63/64 °F), while those of July, August and September are around 23/24 °C (73/75 °F).
The following temperatures, the averages of Las Palmas de Gran Canaria, give us a good indication of all the coasts of the Canary Islands.
Average temperatures - Las Palmas
Las PalmasJanFebMarAprMayJunJulAugSepOctNovDec
Las PalmasJFMAMJJASOND
Min (°C)151515161719202121201816
Max (°C)212122222325272727262422
Min (°F)595959616366687070686461
Max (°F)707072727377818181797572

Temperature fluctuations are small: in winter, temperatures in the lowlands almost never drop below 10 °C (50 °F). On the other hand, the highest temperatures are about 37/38 °C (99/100 °F), and even 40 °C (104 °F) in Santa Cruz, and are recorded with the hot and dry wind blowing from the Sahara desert (which, however, in recent years have become more frequent). In these situations of intense heat (called Calima), which are usually brief and are more frequent in July and August, in the easternmost islands the air can be filled with dust and sand, while in the western islands, an unusual (for these islands) and annoying calm dominates: these periods are recommended for trekking and hiking, because at hill altitudes (called medianías) the African heat is even more intense than on the coast. Apart from some rare rainy and windy periods in late autumn and winter, those of Calima are the only unpleasant moments of this climate, which is almost always enjoyable.

Tenerife

Although they have in general an excellent climate, the Canary Islands are not all equal.
In the islands equipped with mountains in the interior, such as Tenerife and El Hierro, you can find microclimates depending on slope exposure and altitude, so that the areas exposed to the northeast trade winds, which are forced to rise on the slopes (so they cool down and condense into clouds, rain or mists which can also cause drizzle) are more cool and cloudy, while the southern slopes (being in the leeward side) are more dry and sunny. In the airport of San Cristobal de La Laguna (aka Tenerife North), the average temperature varies from 13 °C (55 °F) in January to 21 °C (70 °F) in August, so it's definitely cooler than the warm and sheltered Santa Cruz.
At 2,400 metres (7,900 ft), in the Teide observatory of Izaña, the daily average goes from 4/5 °C (39/41 °F) in January (with night temperatures around freezing), to 18 °C (64 °F) in July. The volcano Teide, however, is 3,700 metres (12,100 ft) high, so it's the highest peak of Spain and of all the Atlantic Ocean; during winter its top is covered with snow. Other volcanoes, though lower, are found on the islands of La Palma and Gran Canaria.
The eastern islands of Lanzarote and Fuerteventura, which have only hills in the interior and are windswept, are a paradise for surfers, while they are not particularly appreciated by those who seek, in addition to the sea and the beaches, cities and interesting places to visit.

The north-east trade winds, typical of subtropical climates, are the prevailing winds, but they follow a diurnal pattern similar to that of the sea breeze, so they are most intense during the warmest hours and in the afternoon, and often blow at around 50 kph (30 mph). Due to the cool and breezy weather conditions, outdoor activities and sunbathing are pleasant, but it must be remembered that the tropical sun's rays (we are just north of the Tropic of Cancer) are very strong, therefore there's a risk of sunburn without appropriate precautions. The period when the sun is strongest, runs from late May to late July.

The rainfall pattern in the Canary Islands is typically Mediterranean, with a maximum in winter (but also in November in many areas) and a minimum in summer, with virtually no rain in the warmest months. However, the amount and frequency of rainfall greatly varies depending on slope exposure, as well as altitude, so there are semi-desert areas, usually those facing south and west, and almost all the lowlands and the coasts, where the average rainfall does not exceed 250 millimetres (10 inches) per year, so that the landscape is often arid, and areas with moderate rainfall, similar to that of many Mediterranean islands, typically, the east and north facing slopes, especially at an altitude between 1,000 and 1,800 metres (3,300 and 5,900 ft).
Here is the average precipitation in Las Palmas de Gran Canaria, on the coast: as you can see, the rains are very scarce.
Average precipitation - Las Palmas
Las PalmasJanFebMarAprMayJunJulAugSepOctNovDecYear
Las PalmasJFMAMJJASONDY
Prec. (mm)1824147200010131827133
Prec.(in)0.70.90.60.30.10000.40.50.71.15.2
Days33310000123420

In the Canaries, the amount of sunshine is high from May to September, while in the other seasons the sunny days are still frequent, but every now and then some weather fronts can pass over the islands, bringing clouds and rains. In the north-facing slopes, however, cloud banks and mists can form.

The temperature of the sea is not very high, due to the cold current, which makes the climate so mild: in the central islands (Tenerife, Gran Canaria), it ranges from 19 °C (66 °F) in winter and early spring, to 23 °C (73 °F) from August to October, which therefore is the best period for swimming; however, with a little courage you can try even in winter. In the western islands (La Palma, El Hierro), where the Saharan wind blows more rarely, the sea is slightly warmer, because the cold current flows near the coast of Morocco, therefore in the east. For the same reason, in the easternmost islands (Lanzarote, Fuerteventura), the sea is slightly cooler (about one degree Celsius less than in Tenerife).
Here are the average sea temperatures in Tenerife.
Sea temperature - Tenerife
TenerifeJanFebMarAprMayJunJulAugSepOctNovDec
TenerifeJFMAMJJASOND
Sea (°C)191919192021222323232220
Sea (°F)666666666870727373737268

When to go

Having a very mild climate, the Canary Islands can be visited throughout the year. However, on late autumn and in winter, temperatures are spring-like, and some cloudy days are be possible, as well as a bit of wind and rain. While northern European tourists often arrive at this time of year to escape the Nordic winter, finding many sunny and pleasant days, for swimming and sunbathing the period from May to mid-October will be preferable, although the sea is still cool in May, and will warm up gradually in the following months.

What to pack

In winter: light clothes for the day, sweater and jacket for the evening, possibly a raincoat or umbrella. To climb mountains, and in particular Mount Teide, down jacket, hat, gloves, hiking shoes.

In summer: summer clothes, but also a light sweatshirt, a light jacket for the evening and for windy afternoons; comfortable shoes for hiking. For Lanzarote and Fuerteventura, a light scarf for the wind and the wind-borne desert sand and dust. To climb mountains, and in particular Mount Teide, hiking shoes, sweatshirt and jacket for higher elevations.