# Absolute Humidity

We have studied in detail the atmosphere and layers of the atmosphere in our previous sessions. The atmosphere is the main component of the Earth which makes life possible on it. It contains various gasses like nitrogen, oxygen, and argon. It also consists of minor traces of other gases along with water vapour. Water vapour is measured using a parameter known as humidity. Do you know what is exactly meant by humidity, absolute, and relative humidity? In this session, let us learn about these parameters.

## Water Vapour and Humidity

The amount of water vapour in the air varies from one place to another. We know that water can exist in three different states as ice (solid), water (liquid), gas (water vapour). Water vapour is also known as moisture. Through environmental processes like evapouration, the amount of water vapour is increased in the atmosphere. Major natural sources of water like the sea, rivers, lakes, and ponds aid in the evaporation process. The process of boiling water also contributes to the formation of water vapour. Water vapour is transparent in appearance and is one of the main constituents of the atmosphere.

We experience changes in the atmosphere in the form of weather. Some days are sunny, some days are cloudy and rainy. We have heard people saying coastal regions are too humid. Why is this so!. It is because of the concept of humidity.

The Earth’s atmosphere is divided into four layers, and each layer differs in density, constituents, mass, pressure, and thickness. Changes in any of the factors affect the atmospheric condition of the Earth. Humidity is one of the factors that can affect these changes in the weather and climate. Humidity is defined as the concentration of water vapour existing in the air.

High levels of humidity are seen in warm air. This is because warm air can hold more water at higher temperatures. Higher the water vapour content of the atmosphere means more latent energy is accessible for the generation of storms.

Humidity is defined as the ratio between the vapour pressure (air) to the vapour pressure at saturation. There is no official SI unit to measure the humidity. However, the amount of water vapour that exists in the air is measured using a specific device known as hygrometers. It is a device that is specially designed to calculate the relative humidity in an open or enclosed area.

After understanding water vapour, humidity, and how humidity is measured, let us have a look at how humidity is classified.

Humidity is classified into three categories, they are:

• Absolute humidity
• Relative humidity
• Specific humidity

Let us know about each of these along with their units.

## What is Absolute Humidity

It is estimated as the exact quantity of water vapour or moisture available in the air, irrespective of the air’s temperature. When the content of water vapour is high in the air, the value of absolute humidity is also high. This is used in measuring the weight of water vapour per unit volume of air. The unit of absolute humidity is g.m^-3 expressed as grams of moisture per cubic meter of air.

Consider the example: a maximum of about 30 grams of water vapour can be present in a cubic meter volume of air with a temperature in the middle 80s.

In the atmosphere, the range of absolute humidity starts from near 0 to roughly 30 grams per cubic metre. Absolute humidity is mathematically represented as:

$$\begin{array}{l}\Delta H = \frac{mH_{2}O}{V_{net}}\end{array}$$

The value of absolute humidity differs with air temperature and pressure changes if the volume is not fixed. It is also affected by the water distribution on the Earth and seasonal changes. As it is the measure of absolute moisture in the air, it relatively remains constant.

### What is Relative Humidity?

Relative humidity is the ratio of the absolute humidity at a given time to the highest possible humidity, which depends on current air temperature. It measures water vapour relative to the temperature of the air. It is represented in percentage. Higher the percentage of relative humidity means that the air-water mixture is highly humid. The relative humidity is affected by the temperature and geographical location of the particular region. When the quantity of water vapour changes, relative humidity also changes. In rainy seasons, the relative humidity is 100%. It is given by the formula:

$$\begin{array}{l}RelativeHumidity = \frac{actual vapour density}{saturation vapour density}\times 100\end{array}$$

### What is Specific Humidity?

The ratio of the mass of water vapour to the total mass of the air parcel gives specific humidity. Specific humidity measures the weight of water vapour per unit weight of air. The unit of specific humidity is g.kg-1, which is showcased as grams of water vapour per kilogram of the air.

You might be interested in reading: Effects of Humidity

See the video below to learn the composition of the air.

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## Frequently Asked Questions – FAQs

### What is humidity?

Humidity is defined as the concentration of water vapour present in the air.

### Through which process is the amount of water vapour increased in the atmosphere?

Through the evaporation process the amount of water vapour is majorly increased in the atmosphere.

### What are the types of humidity?

The types of humidity are:
• Absolute humidity
• Relative humidity
• Specific humidity

### What is the unit of absolute humidity?

The unit of absolute humidity is g.m^-3.

### What are the layers of atmosphere?

The layers of atmosphere are:

• Exosphere
• Thermosphere
• Mesosphere
• Stratosphere
• Troposphere