Answer:

Absolute Humidity:
The actual amount of water vapour present in the atmosphere is known as absolute humidity. It is measured in grams per cubic meter. It is the mass of water vapour in a given volume of air, regardless of temperature.

Specific Humidity:
Specific humidity is also a measure of the amount of water vapour in the air. It is defined as the mass of water vapour in a given mass of air. It is measured in grams per kilogram.

Relative Humidity:
Relative humidity is the ratio of the amount of water vapour present in the air to the maximum amount of water vapour that can be present in the air at a given temperature. It is expressed as a percentage. Relative humidity depends on temperature, so it varies with the time of day and the season.

Differences between Absolute Humidity, Specific Humidity and Relative Humidity:
- Absolute humidity is the actual amount of water vapour present in the air, whereas specific humidity is the amount of water vapour present in a given mass of air.
- Relative humidity is the ratio of the amount of water vapour present in the air to the maximum amount of water vapour that can be present in the air at a given temperature.
- Absolute humidity is measured in grams per cubic meter, specific humidity is measured in grams per kilogram, and relative humidity is expressed as a percentage.
- Absolute humidity and specific humidity are independent of temperature, but relative humidity depends on temperature.

In conclusion, the actual amount of the water vapour present in the atmosphere is known as absolute humidity, which is measured in grams per cubic meter. It is different from specific humidity and relative humidity as they are measured in grams per kilogram and expressed as a percentage respectively.