Shouldn't matter..... Temperatures at 30,000+ feet are pretty darn cold to start with.... Even August in Texas if you are at 30,000+ feet it's going to be pretty darn cold, the type of aircraft that travel to those kind of altitudes have a ton of ice warning systems and heating elements to melt said ice.... and freight aircraft have far fewer restrictions on flight as passenger aircraft do... They can usually do things passenger aircraft can't. The Polar routes were mainly used because it was quicker and shorter to go over the caps (Depending on the starting and ending points) Hong Kong to Chicago is probably shorter going over the cap vs Memphis..... They usually take the quickest route.... Might be shorter to go over the caps but quicker to use the jet stream and push you across the ocean????