The sky is so gray because of the smoke coming from the chimneys.
The last stop of the overview excursion around the island was viewing point to Garachico. It was the principal port of Tenerife till 1706 when a long eruption poured lava into the bay and effectively destroyed it.
I already mentioned it was always raining since we started to descend from volcano Teide. And so all we could see was:
We were supposed to see a more beautiful view: