This is a rare sight, thankfully. I turned on the irrigation pump at 8 pm last night as it was already close to freezing. The ice protects the tree from the extreme cold of the air. As the water freezes and changes state from liquid to solid it gives up its latent heat, (remember those physics lessons way back when).
The water continues to be applied as I write this at 9.30 am. We must apply more water until all the ice melts.
It is forecast to be this cold again tonight and then warm up to freezing for Wednesday night. That is good news as the more days of freeze we have has a cumulative effect on the tree and fruit. Think warm thoughts.