Hygrometer
The forecast says 70% humidity. That sounds damp. Your lips, hands and throat have all independently concluded otherwise by the middle of the first afternoon.
The forecast is not wrong. It is just answering a question about a ratio when what you need is a quantity.
70% of very little is very little
Relative humidity is a percentage of what the air could hold at its temperature, and that capacity collapses as the air gets colder.
| Temperature | At 70% RH, the air holds |
|---|---|
| 20°C / 68°F | 12.1 g/m³ |
| 10°C / 50°F | 6.6 g/m³ |
| 0°C / 32°F | 3.4 g/m³ |
| −5°C / 23°F | 2.4 g/m³ |
| −15°C / 5°F | 1.1 g/m³ |
At −15°C, "70% humidity" means the air is carrying about one eleventh the water of a mild day at the same reading. It is among the driest air you will ever stand in, and the number describing it sounds humid.
That gap between the reading and the reality is precisely what grams per cubic metre exists to fix.
Altitude makes it worse twice
It is colder. The dominant effect — the capacity curve above does most of the work.
The pressure is lower. At 2,500 metres the air is roughly three quarters of sea-level density, so there are fewer molecules of everything per cubic metre, water included. Even at an identical relative humidity there is less water present.
Together, mountain air in winter is about as dry as terrestrial air gets outside a desert.
Then you go indoors
The lodge is where it becomes genuinely severe, and it catches people who thought going inside was relief.
Heating adds no water. It raises the amount the air could hold, so the percentage collapses:
| Outside | Water carried | Indoors reads |
|---|---|---|
| 0°C at 70% | 3.4 g/m³ | 19% |
| −5°C at 70% | 2.4 g/m³ | 13% |
| −15°C at 70% | 1.1 g/m³ | 6% |
Six percent is drier than the Sahara. You are sleeping in it.
What it does
- Skin and lips. Continuous water loss to the air, made worse by wind and reflected sun.
- Airways. A dry, sore throat and a cough that arrives on day two.
- Fluid balance. You lose more water than usual through breathing alone, on top of the exertion and the altitude response.
- Static. The same threshold as any winter, crossed emphatically.
The full set is the usual dry air pattern, just more extreme.
What helps
- Drink more than thirst suggests. Respiratory water loss is invisible and substantial.
- Barrier products on lips and exposed skin, applied before going out rather than after cracking.
- A humidifier in the room if one is available. It addresses the cause rather than the symptom.
- Do not over-ventilate the room in a hard freeze — you are exchanging your air for something drier still.
- Read the g/m³ figure, not the percentage, so the air stops looking deceptively damp.