No idea, but I tend not to fly fish when it is below 30. I can deal with a little bit of ice on the guides, but prefer not to. That said, I have done other forms of fishing in cold weather, and I used to do a lot of ice fishing, years ago.
One year I was trying to set up a homemade ice shanty in heavy wind. Wind caught it and started blowing it across the lake. Fortunately I hadn't gotten it completely setup so it collapsed. Unfortunately win also caught a bucket full of "gear." I was able to gather some of the stuff off the ice since there was no lid on the bucket, but the bucket along with few items that did not fall out of it made it all the way across the lake to where a 20 foot stretch of ice was all broken up from the wind.
Temperature was maybe in the teens with sub-zero wind chill. Did I mention it was windy that day?
I still fished, but instead sat on my collapsed shanty.;-)
Worst fishing experience was when it was upper 30s and raining and i didn't have an ice shanty. I stayed warm enough, or so I thought. Sat there on a bucked in the middle of a lake for a couple hours, all by myself. I felt fine until I went to stand up. My legs had gotten wet, and my knees stiffened up. I couldn't straighten my legs for a little bit. I'm sure I looked funny at first walking back with bent knees, but they eventually started working again. ;-)
I had good boots, so my feet never got cold. That is the key.
The cold never used to bother me. It still doesn't bother me all that much, but as I get older, it is starting to.
This morning I had single digit temperatures when I went out to stoke the outside boiler. The air was so crisp and fresh, I actually enjoyed it.
It's been a record warm December. I actually found broccoli growing in the garden on New Years day and still had buds on the rose bushes. A coupe weeks ago, I had dandelions in the side yard, and the bees were our flying around. We need some cold weather, but back into the mid 40s later in the week.
Ponds are finally froze over.
Gulf and Pete, shut your pie holes.