Once you feel thirsty, you are already low on fluids. The best indicator of proper fluid levels is urine output and color. Ample urine that is light colored to clear shows that the body has plenty of fluid. This is what an average person will need on a daily basis in general temperate conditions. In the winter time you are losing moisture through evaporation to the dry air and especially through respiration.
Dry air entering the lungs heats up and is exhaled saturated with moisture.
1quart 1.8 liters At high altitude the body looses more fluid. Another factor in overall fluid balance is the replacement of salts lost to sweat. In most cases the salts found in normal food consumption is adequate for salt replacement. Use lukewarm fluids. Discontinue the fluids if the person becomes nauseated or vomits.