Humidity is the actual amount of moisture in a certain volume of air.

Relative humidity is the percentage of the maximum amount of moisture that's possible in certain volume of air for a particular temperature.

Example of Relative Humidity: Suppose a 5 ft. cube of air could could hold a maximum 10 oz. of moisture at 80 degrees. Now suppose that the cube of air actually contains 5 oz. of moisture. The Relative Humidity would be 50%.

The idea of humidity refers to how much water is mixed in with the air around us. Everywhere on Earth there is always at least some water mixed in with the air; in some places, like the desert, it's just a little; in other place, like by the ocean or in a rainforest, it's a lot.

The air can only carry around a limited amount of water vapor (evaporated water, or water in its gas form). At some point, there can be more water vapor than the air can hold, and the water turns into little droplets, which in nature is clouds, fog, or rain.

Just "humidity," also known as "absolute humidity," means "what is the concentration of water vapor (evaporated water) in the air right now?" In other words, it would give you a measurement of how much water you would get if you could take a box of air and squeeze all the water out.

This is expressed in units of mass/volume. So, for example, you could express the absolute humidity as 5 grams per liter (5 g/L) if there were 5 grams of water vapor floating around in every liter of air.

Again, the air can only carry around a limited amount of water vapor. How much it can carry is determined by the atmospheric pressure and the temperature. There is a mathematical equation that lets you figure out the maximum amount of water that the air can carry based on the pressure and temperature. Relative humidity is how much water the air is actually carrying divided by how much water the air could possibly carry. People, especially weather forecasters, usually express this as a percentage. So, if the air in a particular place is carrying exactly half the amount of water that it could theoretically carry, the relative humidity would be 50%. If it were carrying all the water it could theoretically carry, the relative humidity would be 100%. The relative humidity can never be higher than 100% or lower than 0%.

Relative humidity has no units, because it is just a fraction.

So, ways they are alike:

1. They both talk about how much water vapor is mixed with air in a particular place.

Ways they are different:

1. Humidity has units (mass/volume) but relative humidity is just a fraction or percentage.

2. Humidity doesn't care about how much water vapor it's possible for the air to have, but relative humidity does.

3. Humidity doesn't tell you anything about how close the air is to having rain or fog, but relative humidity does.

Specific humidty is the amount of water in unit amount of air by weight

and

Relative humidity is the amount of water in unit amount of air by pressure(partial pressure)
Relative humidity is one means of describing the amount of water in the air. Relative humidity describes the amount of water in the air as that precentage of the total amount air at that temperature should be able to hold.

Absolute humidity describes the amount of water in the air by saying how many grams of water per milliliter of air are present.

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