In high-altitude trekking and mountaineering difficulty in adjusting clothing to weather conditions, levels of exertion and respiratory fluid loss from hyperventilation in dry cold air commonly causes dehydration.
Dehydration at high altitude increases the likelihood of altitude sickness, hypothermia and frostbite. So you should drink at every opportunity and learn to gauge hydration by volume and colour of urine (it should be a light yellow straw colour).
Only clean water should be used for fluid replacement, so the use of water purification drops or a water filter should also be considered and keep a mental note of how much water you drink during the day.