Amount of Water Vapor in the Air
There is a limit to how much water vapor can enter the atmosphere. As more water vapor enters the atmosphere, the amount of pressure exerted by that water vapor increases. We call this pressure the vapor pressure. The higher the temperature of the atmosphere, the more vapor pressure it can withstand. When the vapor pressure maximum is reached, no more water can enter the atmosphere. At this point, we say that the atmosphere is completely saturated.
Because the maximum vapor pressure increases with temperature, warmer air can hold more water vapor before becoming saturated.