El Hierro, where it lies

In El Hierro, the smallest and south-westernmost of the Canary Islands, the climate is subtropical oceanic on the coast, that is, very mild and sunny most of the year, with rainfall concentrated from October to March. In contrast, the climate in the interior varies according to altitude and slope exposure: the slopes exposed to the northeast trade winds (namely, the northern and eastern ones) receive a moderate amount of rainfall and are green, while in the rest of the island (that is, along the coasts and in the south), rainfall is scarce and the landscape is arid.
In the capital, Valverde, which is located in a slope at about 600 meters (2,000 feet) above sea level, temperatures are spring-like most of the year: cool (or even a little cold) in winter and pleasantly warm in summer, as you can see in the following table.
Valverde - Average temperatures
Min (°C)121112121315171817161413
Max (°C)171718181921232424222018
Min (°F)545254545559636463615755
Max (°F)636364646670737575726864

Along the coasts, temperatures are higher, and are very mild in winter and warm in summer, when, however, highs remain below 30 °C (86 °F), as you can see from the temperature of the airport, which is located on the coast, east of Valverde.
El Hierro - Average temperatures
Min (°C)171617171819212122212018
Max (°C)212121222324252627262422
Min (°F)636163636466707072706864
Max (°F)707070727375777981797572

On the coast, the rains are definitely scarce: at the airport, they amount to just 200 mm (8 in) per year. Here is the average precipitation.
El Hierro - Average precipitation

The prevailing winds, the north-east trade winds, cause mists and clouds on the slopes exposed, while they temper the heat along the coast during the summer. The trade winds blow with greater intensity in the afternoon, when they are often moderate or even quite strong.
The days of calima, characterized by abnormal heat and light wind, and dominated by warm air masses from Africa, are relatively rare, and are rarer than on the eastern islands of the Canaries because El Hierro is located at a greater distance from the African continent. However, they are more frequent in spring and summer. In practice, these are the only days when the temperature exceeds 30 °C (86 °F) and the heat is unpleasant. The calima is also felt in the highlands, and it's often felt even more than in the plains because warm air is lighter and tends to rise, while the coast can be mitigated by the cool sea breeze.
The amount of sunshine is very good throughout the year, however, in addition to the low pressure systems that can pass over the island from October to March, cloud banks can form in summer on the Atlantic Ocean and then affect the island. On the northern coasts, and especially on the mountain slopes exposed to the north, local clouds and fogs can form. The southern coast is therefore the sunniest one.
Here are the average daily sunshine hours at the airport, which, being on the northeast coast, is a little less sunny than the southern coast.
El Hierro - Sunshine

El Hierro is a volcanic island, in fact, there are several volcanic craters and areas with lava soil. The highest peak is Pico Malpaso, 1,501 meters (4,925 feet) high.

El Hierro, beach

The sea in El Hierro is quite cold in winter, though not prohibitively, while it is warm enough for swimming from August to October. The island is located at a low latitude, a bit north of the Tropic of Cancer, but as mentioned, a cold current flows in this area of the Atlantic Ocean, though the bulk of it flows along the coast of Morocco, so in this western part of the Canaries, the sea is slightly warmer. The sea temperature ranges from 19 °C (66 °F) between February and April to 24 °C (75 °F) in September. Here are the average sea temperatures.
El Hierro - Sea temperature
Temp (°C)201919192021222324232221
Temp (°F)686666666870727375737270

Best Time

The best time for a beach holiday in El Hierro 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, the days in summer are shorter than in Europe (while they are longer in winter), but owing to the time zone adopted, the sun sets late enough anyway.
If you want to visit cities or to go on excursions to the interior, you can choose April, May, and the first half of October.
In winter, you can appreciate the mild temperatures and can often sunbathe, although we can not exclude some days of bad weather.

What to pack in the suitcase

In winter: bring light clothes for the day, a sweater and a jacket for the evening, and possibly a raincoat or umbrella. When going mountain climbing, bring a down jacket and hiking shoes.
In summer: bring summer clothes, but also a scarf for the wind, a light sweatshirt, and a light jacket for the evening and windy afternoons. When going mountain climbing, bring hiking shoes, a sweater and a light jacket.