[kapital]

At some temperature water has certain vapour pressure. Why is this not always achived?(real water parcial pressure is less that wapour pressure and so we have to define relative humidity)

[Stepan]

This is incorrect. Vapour above the liquid water can equal to saturated vapour pressure for given temperature within the accuracy of measurements.    Matter of fact it can be less or can be more - depends how you measure. When it is more, people usually blame sub-micron droplets. When it is equal, the relative humidity is 100% - this is what you get in head space of closed  bottle with water. It takes  20-30 minutes to achieve equilibrium. 

[kapital]

This is incorrect.

What exactly?

[Stepan]

Why is this not always achieved?

As any dynamic process it is achievable as long as you give it sufficient time. Depends on the size of vessel, it can take as little as few seconds to achieve 100% Relative humidity. 

[kapital]

Ok, thank you for your answer.