Why does the air in a heated building in the winter have such a low humidity?

Question

These are chem questions dealing with vapor pressure:

Also, why does rain often form when humid air is forced over a mountain range?

Why does moisture condense on the outside of a cool glass in the summertime?

Answer 1

The air at 15 degrees Celsius can't contain more than 14 grams of water per cubic meter. When it does, the relative humidity is said to be 100%

When it is colder, the air must contain less water. In the winter, there might be only 2 or 3 grams of water per cubic meter to have it saturated. But when it enters your building, being warmed to say, 20 degrees Celsius, its relative humidity then becomes very low.

If rains falls when humid air rises over a mountains (a.k.a. the orographic effect) it is because as pressure diminishes, the so-called adiabatic effect cools the air and humidity condenses as rain.

The adiabatic effect of compression (heat) and release of a gas (cold) is what you have in your refrigerator.

Answer 2

When a body of air cools water vapour condenses as the specific heat is released. The result is the water pressure in the air drops and the air becomes feels drier.

Answer 3

When temperature is low the number of air molecules in 1 cubic square feet would be more due to low energy levels which causes the molecules to come close, thus the same air space holds water vapors, thus colder the air more its compressed, when air is heated the molecules move apart, but numbers remain same causing high energy vapor molecules to move away from each other thus resulting in less water molecules per cubic square feet and this giving you feeling of low humidity.

2. There are many reasons for rain clouds to form over mountain range.

a. Pressure that forces all the water vapors to compress due to limited space when its trapped between air flow and mountain range.

b. Cooler temperatures when vapors are forced upward due to air flow that pushes against mountainside.

3. The answer has to do with thermodynamics.

When we heat things they expand (i.e. the molecules go away from each other) when its cooled they come back. But there are 2 factors the fogging of the glass in summer.

1. When water vapor in air touches the glass it vapors cool down.

2. These vapors at higher energy level were keeping away from each other (expansion), now stick together at lower energy level, thus they form condensation.

Answer 4

Because the air is being dried out by the heat. This is very common. This is why I have a humidifier in my home. It makes the air much more comfortable in the winter and summer, it's healthier too