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In the previous video you have already seen that water can occur in 3 phases; as water vapour, liquid water or as ice. This video explains the relative humidity.

Suppose there is a package of air. There is a certain amount of evaporated water in the air: water vapour. The exact amount of water vapor in the air is the absolute humidity. However, this type of humidity is not the most important type of humidity.
That is the relative humidity. This is the ratio between the amount of water vapor currently present in the air and the amount of water vapor present in the air before the water vapor condenses. This is expressed as a percentage. If we add extra water vapor to the air at a relative humidity of 100%, then water vapor will also condense and form clouds.

The relative humidity could also be 50%. At that moment there is only half the amount of water vapor in the air, compared to the amount of water vapor that can be present in the air.

The relative humidity can change in 2 ways:
The first way is by adjusting the amount of water vapor in the air. When water evaporates, the amount of water vapor in the air increases and thus the relative humidity rises. For example, by taking a long and hot shower, water evaporates, so that the relative humidity can rise sharply in the shower and some of the water vapor condenses.

The second way is by changing the temperature of the air. Remember the following rule: Warm air can contain more water vapor than cold air. Sometimes this is explained as if warm air can hold more water vapor. While this is true in itself, it's also a bit misleading, because it's not the warm air that holds the water vapor, but the water vapor itself moves much faster when it's hot, making it less likely to condense.

This also means that if the air gets warmer, but the same amount of water vapor remains in the air, the relative humidity decreases. The maximum number of possible particles of water vapor that could be present in the air increases, but the number of particles present remains the same. This lowers the relative humidity.

When the air cools, but the same amount of water vapor remains in the air, the relative humidity rises. The maximum number of possible particles of water vapor that could be present in the air decreases, but the number of particles present remains the same. This increases the relative humidity.

We see this in practice, for example: When we exhale on a cold winter day, we also exhale water vapor. Because the air in a warm environment suddenly cools down enormously, the relative humidity rises and a cloud forms. Because this cloud mixes quickly, the water quickly evaporates again.

For the same reason, fog often forms early in the morning in the fall and winter. The ground can then cool down very much at night and so does the air layer above it. As a result, the relative humidity rises to almost 100% and fog is created. As it gets warmer during the day, the temperature of the air rises and the relative humidity decreases, causing the fog to disappear.

Now you know that high relative humidity leads to condensation of water vapor, which can create clouds and eventually precipitation. The following video explains in which 3 ways air cools and how clouds are formed.