In reply to yorkshire_lad2:
We were there a week ago and just used the bbc weather app.
Even though the island is small it does seem to have its own micro climates.
Depending on where you are staying you just need to stick your head out of the window really. The summit can either be in cloud or bathing in sunshine above any cloud.
If you don't have a permit for the summit (free but register online before) you can still go to the summit but have to be up and down before the security arrives at 9am.
Take a warm layer as it quite chilly on top. 28c in los Cristianos but only 6c up the mountain when we were there.
Super place enjoy your trip.
Post edited at 11:02