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Unit of Humidity

What is humidity?

Humidity is the amount of water vapour present in the air or any other gases, and generally, water vapours are invisible to the human eye.

What are the factors on which humidity depends?

Humidity is dependent on the temperature and pressure of the system, which is in consideration. For example, if there's the same amount of water vapour in cool and warm air, then humidity is higher in the cool air as compared to warm air.

What is the dew point?

When air is cooled to that temperature, it becomes saturated with water vapour, which is known as the dew point. When the airborne water vapour is cooled further, it condenses to produce liquid water (dew). When air cools to its dew point due to contact with a colder-than-air surface, water condenses on the surface.

If the temperature increases, the amount of water vapour that is needed to bring saturation also increases.

In total, there are three main ways in which we can express humidity.

a) Relative humidity –

The ratio of the actual water vapour pressure to the saturation vapour pressure is relative humidity, which is the quantity of water vapour in the air represented as a percentage of the highest amount that the air could retain at a given temperature. So, in most cases, relative humidity is measured in terms of percentage.

A greater percentage indicates a more humid air-water combination. The air is saturated and reaches its dew point when it has a relative humidity of 100 per cent. Only the invisible water vapour is taken into account when calculating relative humidity. The relative humidity of the air is not affected by mists, clouds, fogs, or water aerosols.

b) Absolute humidity –

The whole mass of water vapour present in a particular volume or mass of air is known as absolute humidity. It doesn't take the temperature into account. When the air is saturated at 30 °C, absolute humidity in the atmosphere ranges from near zero to about 30 g per cubic meter. The unit of measurement of absolute humidity is gram per cubic meter.

If the volume is not constant, the absolute humidity fluctuates when the air temperature or pressure changes. As a result, it is inappropriate for chemical engineering calculations, such as drying, where temperature varies greatly.

c) Specific humidity

Specific humidity is defined as the ratio of the mass of water vapour to the total mass of the water vapour in the air (or moisture content). The mixing ratio, which is defined as the ratio of the mass of water vapour in an air parcel to the mass of dry air in the same parcel, is roughly equivalent to specific humidity.

As the temperature drops, so does the amount of water vapour required to attain saturation drops. Generally, it is stated in terms of grammes of vapour per kilogram of air.

Effects of humidity

In order to provide human comfort, and ensure health and safety, while meeting the environmental requirements for the machinery, sensitive materials (historical documents) and technical processes, some methods are employed. These methods also known as climate control refer to the control of temperature and relative humidity in buildings, vehicles and in other closed environments.

Animal and plant life –

Humidity is an important abiotic element in defining any ecosystem, including tundra, marshes and the desert, and it is a determining factor that allows animals and plants to survive in a particular area.

Through perspiration and evaporation, the human body disperses heat. The main routes of heat transfer from the body are heat convection to the surrounding air and thermal radiation.

The rate of perspiration evaporation from the skin reduces under high humidity. In periods of excessive moisture, the atmosphere cannot disperse heat from the body's surface by conducting it to the air if it is thus warm or warmer than the skin.

When so much blood reaches the outside surface and the working muscles, the brain and other inside organs get lesser. Physical strength diminishes, and tiredness happens earlier than others. Heatstroke or Hyperthermia can also impact alertness and mental function.

Global climate–

Humidity affects the temperature in two prominent ways.

First, water vapour includes "latent" energy in the environment. This amount of heat is removed from the surface fluid through transpiration or evaporation, which cools the surface of the earth. It is the most important non-radiative surface cooling effect. The average net radiative warming on the surface is around 70 percent.

Second, the most high percentage of all greenhouse gases found in the atmosphere is water vapour. Water vapour allows entry to the most amount of solar energy like other greenhouse gases. But, it absorbs infrared radiation released (radiated) upwards by the surface of the earth, which is why wet zones are not chilly at night at all, whereas dry desert zones are quite cool at night. This is the selective absorption that causes the greenhouse effect.

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