During the fourth night, all was going smoothly. The air was dry, the skies were clear, and I was running as quickly through my observing programme as I could. Suddenly, after a couple of hours, the telescope control system started beeping - the humidity was higher than the telescope could take and I had to close everything down quickly. A few minutes later it had dropped right back down. I opened up again and carried on.
It did this a couple more times during the night. At 5.45am I closed up and I couldn't open again. By dawn, the mountain was in thick cloud. The visibility was about ten metres, and the NOT quickly disappeared from sight as I drove back to the Residencia.