We usually express mixing ratio as the number of grams of water vapor in each kilogram of air. When that limit is reached, we say that the air is saturated with water vapor. The maximum capacity of air to hold water vapor depends on the temperature of the air.
Warmer air is capable of holding more water vapor than cooler air is .
Since the maximum capacity of air to hold water vapor changes when the temperature changes, it follows that relative humidity depends both on the amount of water vapor in the air and on the temperature of the air. The temperature at which this would happen is called the dew-point temperature . However, water vapor requires a solid surface onto which to start condensing. In the atmosphere, microscopic, solid particles of dust, salt, ash, pollen, etc. suspended in the air provide solid surfaces onto which water vapor can start condensing. We call these tiny, solid particles condensation nuclei .