m m a, Bart and Andrew - Cool trials and experiments : ) I was recently staying in a dorm room with the last shared bath on the building's plumbing circuit. To be the first one up in the morning guaranteed a cold shower. I was really glad I had some awareness of the goodness of this. Also, a few of us on the retreat did walking meditation on the snow, snow melt and cold walk ways barefooted. It was a really refreshing way to keep the mind in meditation (in the world, in the senses, in the sensations, watching arising and passing thoughts...).
Bart:
I had a chronic pain in lower left leg/left knee. It still pops up when I sit crosslegged, but other than that, it's rarely ever present. I'm not sure whether this pain is gone because of my change of jobs (I now sit much more often), the intu-flow practice I'm doing, the cold showering, or perhaps because of resolved psychological issues, (Wim Hof mentioned cold habituation would help with knee problems)
Is the pain in your lower left knee sharp/zingy (like nerve pain) or achey like muscular pain?
Regardless, there is no good reason to keep such a pain.
You can add some pillowing under that knee and see if that reduces the pain (it may translate to an ache somewhere else (like hips or lumbar spine, but the pillowing should not create nerve pain somewhere else).
Nerve pain (sharp and/or zingy) should especially be acknowledged and relieved...one can do permanent damage by keeping a nerve pinched for too long. Those nerves are our bodies' allies: they are there and 'speak loudly' for
good reason!
Sitting in a chair is perfect, too. The chair will allow a person to do breathing meditation straight away (and it still offers back and bum aches). Sitting cross-legged is not required. It's way more important to start the practice and do it regularly than to develop resistance to practice because of sustaining unnecessary pains/damage.
A lot of achy pains do resolve as the mind settles, but that can take hours and even days even if a person is very willing and into meditation. Again, having a gentle consistent practice supports and reinforces the practice, versus developing aversion to it.