What Are the Differences Between Absolute, Relative, and Specific Humidity?
You may have often heard people remark that the humidity is unbearable. Have you ever wondered what humidity means? Humidity is nothing but the measure of water vapor in the air. Humidity, combined with high temperatures, can increase the distress caused due to heat. Since humidity is a measurable entity, there is a SI unit of humidity.
Nevertheless, before learning the humidity unit, you must learn about the various ways in which humans measure humidity.
Absolute Humidity
In this form of measurement, you can consider the mass of water vapor present in a particular volume of air. Due to such a direct and specific approach, this humidity measurement unit is the most scientifically accurate method.
Absolute humidity unit is expressed in grams per cubic meter of air.
Example 1
Your measurements reveal that 7 grams of water vapor exist in 2 cubic meters of air. In such a case, you can say that the absolute humidity is equal to 3.5 grams/m3.
While scientifically accurate, this unit of measurement is difficult to understand for the general population. Therefore, other forms of units for humidity are more popular.
Relative Humidity
Relative humidity refers to the amount of water vapor present in the air relative to the maximum water vapor that it can hold. In most cases, the relative humidity unit is expressed in percentage.
Example 2
Suppose the air is capable of holding 10g/m3 of water vapor. Now consider, weather analysts determine that at present the air holds 8g/m3. From this, they can easily derive the relative humidity (RH), through the following formula –
\[RH = \frac{8}{10}\times{100}\]
Therefore, \[RH = 80\]%
This is the most common method used as a measure of humidity. Weather forecasts in the news also tend to report humidity predictions in the relative humidity format. Doing so eases comprehension significantly.
Quick Exercise
Q. 22 grams of water vapor exist in 6 m3 of air. This volume of air can hold a maximum water vapor content of 25 grams. Calculate relative humidity and absolute humidity.
Solution
Specific Humidity
Specific humidity is a measurement method, which is similar to the absolute method. The only difference is that instead of considering air volume, this calculation considers the total air mass. Thus, the specific humidity unit is g/kg. This means that such humidity is expressed as the amount of water vapor (in grams) present in each kilogram of air.
Specific humidity remains constants as long as moisture in the air is not removed or added. Therefore, the figure remains constant, even with changes in pressure and temperature.
True or False
Relative Humidity can never be 100%.
Ans. False
Relative humidity can be 100% in many cases, especially during the monsoons when air is saturated with moisture.
Dew Point
Apart from understanding the various units for humidity, you should also learn about dew points. Dew point is the temperature at which water vapor content in air reaches the maximum point. At a dew point, the rate of water condensation is equal to the rate of evaporation.
You can also define it as the temperature when air reaches its saturation point when it comes to holding water. Temperature below dew point will result in condensation, in the form of clouds, fog or dew (you can see it on leaves and blades of grass).
Humidity is a complex topic, which determines weather, temperature and even precipitation. Vedantu’s unique online classes and material can help students gauge the concept better. Download our app on your phones and begin learning in a more interactive and convenient way!
FAQs on Units of Humidity in Physics: Absolute, Relative & Specific
1. What is absolute humidity and what is its unit?
Absolute humidity is the measure of the actual amount of water vapour present in a given volume of air, regardless of the temperature. It is a direct measurement of the mass of water vapour per unit volume of air. The standard unit for absolute humidity is grams per cubic metre (g/m³). For example, an absolute humidity of 10 g/m³ means there are 10 grams of water vapour in every cubic metre of air.
2. How is relative humidity defined and what is its importance?
Relative humidity (RH) is a ratio, expressed as a percentage, that compares the amount of water vapour currently in the air to the maximum amount of water vapour the air could hold at that specific temperature. Its importance lies in how it relates to our perception of comfort and weather phenomena like dew or fog. Because it's a ratio, it is calculated as:
RH = (Actual water vapour density / Saturation water vapour density) × 100%.
Since it is a ratio of similar quantities, it is a dimensionless quantity.
3. What does specific humidity measure and how is it different from absolute humidity?
Specific humidity measures the mass of water vapour in relation to the total mass of the moist air parcel (which includes both dry air and water vapour). It is typically expressed in units of grams of water vapour per kilogram of air (g/kg). The key difference is:
- Absolute Humidity relates water vapour mass to the volume of air (g/m³).
- Specific Humidity relates water vapour mass to the mass of the air (g/kg).
4. What is the key difference between relative and absolute humidity?
The key difference lies in what they measure and their dependence on temperature. Absolute humidity is a direct measure of the mass of water in a volume of air and is an absolute value (e.g., 12 g/m³). In contrast, relative humidity is a percentage that shows how 'full' the air is with water vapour relative to its maximum capacity at that temperature. Therefore, even if the absolute humidity remains constant, the relative humidity will increase if the temperature drops, as colder air can hold less water.
5. What does it physically mean when the relative humidity is 100%?
When the relative humidity reaches 100%, it means the air is completely saturated with water vapour and cannot hold any more at its current temperature and pressure. This is the dew point. If the air cools down any further, or if more water vapour is added, the excess moisture will be forced to condense into liquid water, forming dew, fog, or clouds.
6. Why do weather reports use relative humidity instead of absolute humidity?
Weather reports use relative humidity because it better indicates the likelihood of precipitation, dew, or fog. More importantly, it directly correlates with human comfort. The rate of evaporation from our skin, which cools our bodies, depends on the relative humidity. High relative humidity slows down sweat evaporation, making us feel hotter and 'stickier' than the actual air temperature might suggest. Absolute humidity does not provide this context as it doesn't account for temperature's effect on the air's moisture-holding capacity.
7. What are the three main types of humidity units used in physics?
The three primary types of humidity and their corresponding units are:
- Absolute Humidity: Measured as the mass of water vapour per unit volume of air, with units of grams per cubic metre (g/m³).
- Relative Humidity: A dimensionless ratio expressed as a percentage (%), indicating how saturated the air is with water vapour.
- Specific Humidity: Measured as the mass of water vapour per unit mass of moist air, with units of grams per kilogram (g/kg).
