30's is cold for you, for me that would be cooler temps. Today it was about 7 degrees and -7 with the wind chill and while I find it cold, ie my face gets frost on it, it is not as cold as it can get. Below 0 is when you really struggle breathing wise. The temps you like to run in are indeed optimal in terms of air as it is so much better but below a certain temp, really cold ie below zero, it can burn and be a real struggle to get your air. Any way not a bad day for me, 13.8 miles and a lot of ice to scrape off my beard,lol.