Don't take games at grounds where there's no cover and it's likely to rain is the only solution. As far as protecting yourself against the cold, make sure you wear a coat with deep pockets and put some pocket warmers in your pockets. Alternatively you could use Tiger Balm on the inside of your wrist but it does make you smell a bit lol.
I'm about to start experimenting using Dragon software which is a voice to text system that works on most operating systems. I'm going to trial it on a game I just go to watch to see if it can produce notes like I have when I am assessing. If it works, I'll just pop in the headphones and dictate my way through a game, then at half time and full time I'll draft the notes into a feedback/de-brief document.
That's the technique used by the FL observers for FL referees except they use iPads.
Edited to say how impressed I am that you are out assessing at age 21. I found my whole game improved massively once I began assessing as I began recognising my own areas of development much more quickly.