Climatic conditions can vary greatly across this relatively small piece of land. They can also change very quickly. While one part of the island might be dull and overcast, less than a mile away, the sun could be splitting the stones and vice versa.
In high altitude areas, it can get very hot when the sun shines, but when it goes behind the clouds, the temperature can drop significantly; important advise for walkers and trekkers! Likewise, during the day it remains hot but in the evenings it really does get a lot colder.
On coastal areas, you can always sit out in shorts and t-shirts in the evenings. As you head inland the temperature drops and you'd probably need a jumper and long trousers.
In the mountains the weather can vary from day to day, or even, from one minuete to the next. Take Los Muchachos (the highest point) for example. Somedays you can see all the way across to Tenerife, La Gomera and El Hierro from the summit. On other days you'll see nothing more than an endless blanket of mist a fog. The next day, the journey up to the summit might be crystal clear with brilliant sunshine, whereas the actual summit itself will be shrouded in mist and fog.
If you are going walking, it's a good idea to bring lightweight rainproof clothing with you. It might start off hot and sunny but can change rapidly. Mist can fall very quickly and it can become very cold and sometimes hard to navigate. It's very important to be well prepared.
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