Map from Google - Tenerife

In Tenerife, the largest of the Canary Islands, the climate is subtropical oceanic on the coast, that is, very mild and sunny most of the year, with little rainfall concentrated in the period from October to March. In inland areas, however, the climate varies with altitude and slope exposure. In fact, the slopes exposed to the northeast trade winds receive a fair amount of rain and are green, while in the rest of the island the rainfall is scarce, usually below 250 millimetres (10 inches) per year on the coasts, but often even below 150 mm (6 in), which explains the aridity of the landscape on coasts and southern slopes.
The prevailing winds, the trade winds, make the northern side more humid, while tempering the heat along the coast during the summer; the trade winds blow with greater intensity in the middle of the day and in the afternoon, and are often moderate or even quite strong.
The days of Calima, characterized by abnormal heat and light wind, dominated by warm air masses from Africa, are relatively rare (although in recent years they have become more frequent), and are most common in spring and summer, in practice these are the only days in which the temperature exceeds 30 °C (86 °F), and in which the heat is unpleasant. The stifling heat is felt also on the hills, and often even more than in the plains, because warm air is lighter and tends to rise, while the coasts may remain tempered by the coolness coming from the sea.
The amount of sunshine in Tenerife is very good throughout the year, however, in addition to the low pressure systems that may pass during the October to March period, in summer cloud banks can form on the Atlantic Ocean and pass over the island, while on the northern coast, and especially on the mountain slopes exposed to the north, local clouds and fogs can form. The south coast is therefore the sunniest.
Here are the average sunshine hours per day in Tenerife Sur, on the south coast.
Sunshine - Tenerife Sur
Tenerife SurJanFebMarAprMayJunJulAugSepOctNovDec
Tenerife SurJFMAMJJASOND
Sun (hours)6777891097766

Tenerife, sea and Teide in the background

In the main town, Santa Cruz de Tenerife, daytime temperatures are very mild in winter, with highs around 21 °C (70 °F) in January and February, and warm in summer, with highs around 28/29 °C (82/84 °F) from July in September. Here are the average temperatures.
Average temperatures - Santa Cruz
Santa CruzJanFebMarAprMayJunJulAugSepOctNovDec
Santa CruzJFMAMJJASOND
Min (°C)151516161819212121201816
Max (°C)212122222426282928262422
Min (°F)595961616466707070686461
Max (°F)707072727579828482797572

The rain pattern is Mediterranean, which means that the bulk of the rain falls in autumn and winter, because these are the seasons in which the Canaries can be reached by some Atlantic depressions, while in summer it practically never rains. The total quantity of the rain that falls in Santa Cruz is low: just 270 mm (10.5 in) per year, therefore at a semi-desert level. In autumn and winter, weeks can pass without a raindrop falling, but sometimes there may be heavier rains, associated with flooding, as happened in March 2002, December 2013 and October 2014. Here is the average precipitation.
Average precipitation - Santa Cruz
Santa CruzJanFebMarAprMayJunJulAugSepOctNovDecYear
Santa CruzJFMAMJJASONDY
Prec. (mm)4035301551003305060269
Prec. (in)1.61.41.20.60.20000.11.222.410.6
Days54321100136632

Santa Cruz overlooks the sea, but there are neighbourhoods that climb the inland slopes, and along the road to San Cristobal de La Laguna you find a progressively cooler and humid weather. When looking for meteorological data of Tenerife, you often find those of the airport of La Laguna, which is the airport of the northern part of the island (called Tenerife Norte), and which is sometimes presented generically as "Tenerife", but being located at 600 metres (2,000 feet), it has significantly cooler temperatures: highs are on average around 16 °C (61 °F) in winter, and 24/25 °C (75/77 °F) in summer. Here are the average temperatures.
Average temperatures - La Laguna
La LagunaJanFebMarAprMayJunJulAugSepOctNovDec
La LagunaJFMAMJJASOND
Min (°C)101011111214161716151312
Max (°C)161718182022252625222017
Min (°F)505052525457616361595554
Max (°F)616364646872777977726863

Lying on the northern slope, La Laguna is also more humid and rainy: rainfall amounts to 605 mm (24 in) per year, with a maximum from November to January, of 90/110 mm (3.5/4.3 in) per month, distributed in 8/9 days per month, but also in March and October it's about 60/65 mm (2.4/2.6 in) per month: it is a very cool and moist microclimate, for a town of the Canaries. In summer, however, here too there is little rain. Here is the average precipitation.
Average precipitation - La Laguna
La LagunaJanFebMarAprMayJunJulAugSepOctNovDecYear
La LagunaJFMAMJJASONDY
Prec. (mm)8070604020105515458080515
Prec. (in)3.12.82.41.60.80.40.20.20.61.83.13.120.3
Days87764221368964

La Laguna, moist area of Tenerife

To the south of La Laguna, the northern slopes of Tenerife above 1,000 metres (3,300 feet) are even wetter and are covered by forests. Green areas and covered by forests are also found in the municipality of La Orotava, on the north side of Mount Teide, between 1,000 and 1,800 metres (3,300 and 5,900 ft), where the trade winds are felt most. Above this altitude, however, the rainfall becomes scarcer and the vegetation becomes sparser.
The Teide observatory of Izaña is located at 2,400 metres (7,900 ft) above sea level: at this altitude the winter is cold, in fact the average temperatures from December to February are about 4/5 °C (39/41 °F); already at this altitude sometimes it can snow. Summer is mild, with temperatures not so high, but the (almost) tropical sun, at high altitudes is very strong. Unlike on the coasts, here in September the temperature begins to decrease. Here are the average temperatures.
Average temperatures - Teide observ.
Teide observ.JanFebMarAprMayJunJulAugSepOctNovDec
Teide observ.JFMAMJJASOND
Min (°C)1134610141410742
Max (°C)881012141923231914119
Min (°F)343437394350575750453936
Max (°F)464650545766737366575248

In the observatory, 435 mm (17 in) of rain or snow fall, concentrated from November to March, while in summer the rains are sporadic.
Here is the average precipitation.
Average precipitation - Teide observ.
Teide observ.JanFebMarAprMayJunJulAugSepOctNovDecYear
Teide observ.JFMAMJJASONDY
Prec. (mm)856565251510215355075435
Prec. (in)3.32.62.610.6000.10.61.42317.1
Days54431000234632

At higher altitudes, we find a montane dry vegetation, as well as lunar landscapes and rocky deserts. Mount Teide is a volcano, 3,718 metres (12,198 feet) high. There is a cable car, the Mt. Teide Cable Car, which leads up to 3,550 metres (11,600 ft): to get there you must book in advance.

Teide Observatory of Izana

On the southern coast, where there are towns like Playa de Los Cristianos, Playa de las Américas, Costa del Silencio, there is the other airport of the island, Tenerife Sur. Here the temperatures are similar to those of Santa Cruz and the rest of the coasts of the Canary Islands.
Here are the average temperatures.
Average temperatures - Tenerife Sur
Tenerife SurJanFebMarAprMayJunJulAugSepOctNovDec
Tenerife SurJFMAMJJASOND
Min (°C)151516161719202121201816
Max (°C)222223232425282828272523
Min (°F)595961616366687070686461
Max (°F)727273737577828282817773

The rains are very scarce, amounting to less than 150 mm (6 in) per year, therefore at desert levels, and are concentrated, as usual, in the cold half of the year. Here is the average precipitation.
Average precipitation - Tenerife Sur
Tenerife SurJanFebMarAprMayJunJulAugSepOctNovDecYear
Tenerife SurJFMAMJJASONDY
Prec. (mm)152015710014122530133
Prec. (in)0.60.80.60.300000.20.511.25.2
Days22210000122415

Tenerife south from above

The sea in Tenerife is quite cold in winter, even though not prohibitively, and remains cool in summer, as the island is located at a low latitude, almost to the Tropic, but in an area of the Atlantic Ocean where a cold current flows. Those who live in the island are accustomed to swimming in winter as well, which also Nordic tourists do, or those who do not suffer from the cold. However, the sea temperature ranges from 19 °C (66 °F) in the period from January to April, to 23 °C (73 °F) in August, September and October.
Here are the average sea temperatures.
Sea temperature - Tenerife
TenerifeJanFebMarAprMayJunJulAugSepOctNovDec
TenerifeJFMAMJJASOND
Sea (°C)191919192021222323232220
Sea (°F)666666666870727373737268

When to go

For swimming and sun bathing, the best time is from May to mid-October. In the first period (May-June), the air temperature is a bit lower and the sea is a bit colder, but the days are slightly longer. In the Canaries, in summer the days are shorter than in Europe (while in winter they are longer), but due to the time zone adopted, the sun sets late enough anyway.
To visit the cities and for excursions to the interior, you can choose April, May and the first half of October.
In winter you will appreciate the mild temperatures and you can often sunbathe, although we can not exclude a few days of bad weather. If you want you can combine a morning excursion on Mount Teide, in search of snow, with an afternoon on the beach.

What to pack

In winter: light clothes for the day, sweater and jacket for the evening, possibly a raincoat or umbrella. To climb Mount Teide, down jacket, hat, gloves, hiking boots.
In summer: summer clothes, but also a scarf for the wind, a light sweatshirt, a light jacket for the evening and windy afternoons; comfortable shoes for hiking. To climb Mount Teide, hiking shoes, sweatshirt and jacket for the highest altitudes.