The gradient between isobars determines the wind speed. The amount of water in the air is called humidity . The actual amount of water vapor in the air is the absolute humidity . It is saturated . The ability of air to hold water changes depending on the temperature.
Water gets into the air by evaporation or sublimation.
Evaporation is changing from a liquid to a gas. To get the water out of the air it either condenses or sublimes. Water needs a surface to condense on. Particles that allow condensation to occur are called condensation nuclei . If condensation occurs at ground level, you'll get fog .