Why is cold air heavier

Why are heated rooms so dry in winter?

Winter is not only cold, but also dry - at least in a warm room. The reason: The cold air that comes in from outside hardly contains any water vapor.

A desert climate often prevails in heated rooms in winter. Mouth and nose dry out - and it may seem obvious to let in fresh air. The frosty outside air cools the room down and provides a short-term remedy. But as soon as you close the window and the temperatures in the room rise again, the desert climate returns. Even if there is fog outside at 1 degree Celsius, i.e. the outside air is very humid, it appears extremely dry when warmed up to room temperature. The reason: cold air that comes in from outside hardly contains any water vapor.

If the radiator in the room warms the fresh air, the amount of water vapor it contains does not change. But the warm air can in principle absorb more water. This is because as the temperature rises, the average speed of molecules in water and air increases. This means that more H.2Dissolve O-molecules from the liquid molecular structure and pass into the air.

Desert climate

This not only applies to the hot water in a saucepan, but also to the surface of tiny water droplets in warm air. On the other hand, water molecules that are already buzzing around do not combine into droplets so easily when they are moving quickly. Warm air can therefore contain more free water molecules - or water vapor.

The maximum number of water molecules that can be absorbed by the air before they condense into droplets therefore depends on the temperature. The so-called relative humidity describes the ratio of the water vapor actually contained in the air to the maximum possible amount. In fog, for example, the humidity is one hundred percent: In this case, the air contains more water vapor than it can absorb - and water droplets form. Absolute dryness, on the other hand, corresponds to a relative humidity of zero percent. The air in the desert has a relative humidity of ten to thirty percent. In midwinter you can sometimes even measure values ​​below ten percent indoors.

From all this it is clear: Frequent ventilation does not help against dry room air. On the contrary! Because that would keep adding water-poor air. The only solution is to humidify the air in the heated rooms - be it with the help of indoor plants or technical devices.

An extreme form of the winter effect can be observed on long-haul flights. The air that is directed into the cabin of the aircraft is almost anhydrous. Because outside the temperatures are below minus 50 degrees Celsius. That is why the motto on such flights is always: drink, drink, drink!