The term humidity describes the amount of water vapour in a given parcel of air.
How much water vapour that parcel of air can hold depends on its temperature. As the temperature of air drops, the maximum amount of water it can hold reduces. So warm air can hold more water vapour than cold air.
For simplicity, the amount of water vapour in a given parcel of air is described as a percentage of the total water vapour that air could hold. This is known as Relative Humidity.
For example, a parcel of air with 50% Relative Humidity would be holding half of the maximum amount of water vapour it could possibly hold.
Since cold air cannot hold as much water as warm air, the relative humidity of a given parcel of air will increase as temperature decreases. If that parcel of air with 50% relative humidity were to cool down, the relative humidity would rise to (for example) 75%.
When a parcel of air contains the maximum amount of water it can hold at its current temperature, the relative humidity is 100% and the air is said to be saturated.
The temperature at which saturation occurs is called the Dew Point.
The Dew Point depends on the amount of water present in the air in the first place. The less water present, the cooler the Dew Point.
As you will learn in future lessons, humidity is arguably the most important single factor influencing the weather.
For example, imagine a parcel of air being heated during the day through contact with surface. The air warms up and begins to rise. As it rises, it expands and cools and eventually dew point temperature is reached. Any further cooling will cause the water vapour to condense and form visible water droplets, creating clouds.
We will learn more about this process in a later lesson.