It was only the psychrometry came; I read about vapour pressure is equal to the saturation pressure at 100% relative humidity. While before even in the textbooks both terms are used frequently as same. I fully understood what saturation pressure is; learned during phase change phenomenon of water. Again I learned about vapour pressure defined in closed container and through physics forums I understood about non equilibrium phenomenon in open container. Somehow I am not able to relate humidity, vapour pressure and saturation pressure but I think I know them independently well. Please help me.