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Relative Humidity - Absolute Humidity, Relationship Between Relative and Absolute Humidity, Dew Point, Practice Problems and FAQs

You must have heard or seen weather reports stating that a given area has a relative humidity of this percentage, that there may be a chance of cloud formation, a possibility of rainfall, etc. These are the fundamental components of a daily weather prediction, and relative humidity is an important factor.

Moisture is a source of evaporation, therefore humidity simply helps in estimating the amount of water vapour in the air. After condensation, humidity causes the creation of clouds, which causes precipitation. This precipitation can come in the form of rain, snow, hail, sleet etc.

So, let’s explore more about the concepts of relative and absolute humidity.

• Relative Humidity
• Absolute Humidity
• Difference between Relative and Absolute Humidity
• Relationship between Relative Humidity and Temperature
• Dew Point
• Importance of Measuring and Monitoring the Relative Humidity
• Practice Problems
• Frequently Asked Questions-FAQs

Relative Humidity

The relative humidity of an air-water mixture is defined as the ratio of the partial pressure of water vapour in the mixture to the equilibrium vapour pressure of water on a flat surface of pure water at a particular temperature. Usually expressed as a percentage. The higher the percentage, the moist the mixture of air and water.

At this point, you may be wondering how relative humidity is calculated.

The equation for calculating relative humidity is as follows:

Relative humidity

The ideal relative humidity for health and comfort is between 30-50 % humidity. This means that the air carries between 30-50 % moisture.

Relative humidity is an important indicator used in weather forecasts because it is an indicator of the likelihood of precipitation, dew, or fog. In hot summer weather, elevated relative humidity prevents sweat from evaporating from the skin and raises human body temperature.

Humans are very sensitive to moisture because the skin relies on air to dissipate moisture. The sweating process is an attempt to cool the body and maintain its current temperature. If the relative humidity of the air is 100 %, the sweat will not evaporate into the air. As a result, high relative humidity makes it much more hotter than it really is. Low relative humidity allows sweat to evaporate and ease your discomfort. For example, if the temperature is 24oC and the relative humidity is zero percent, the temperature feels like a little less than 24oC to our body. When the temperature is 24oC and the relative humidity is 100 %, it feels like the temperature is more than 24oC.

Absolute Humidity

Absolute humidity represents the moisture content in the air and is expressed in grams per cubic metre or grams per kilogram. Absolute humidity in the atmosphere ranges from almost 0 to about 30 g cm-3.

Mathematically, absolute humidity is defined as the mass of water vapours divided by the volume of a mixture of air and water and it is expressed as:

Absolute Humidity $\left(AH\right)=\frac{{m}_{{H}_{2}O}}{{V}_{mixture}}$

Absolute humidity changes with temperature, and pressure changes if the volume is not fixed. The mass of water vapour per unit volume in the equation is also defined as volumetric humidity.

Difference between Relative and Absolute Humidity

The water vapour that exists in the air is called moisture. The actual amount of water vapour present in the atmosphere is called absolute humidity. The percentage of moisture present in the atmosphere compared to the total volume at a particular temperature is called relative humidity.

Relationship between Relative Humidity and Temperature

The ability of air to retain water vapours depends entirely on temperature. Change in temperature increases or decreases the ability to retain moisture which affects the relative humidity. The relationship between humidity and temperature is inversely proportional. As the temperature rises, the relative humidity decreases. This will dry the air. As the temperature drops, the air becomes more humid. Therefore, the relative humidity rises. The relative humidity is generally high on the ocean and less on the land.

Dew Point

People feel most comfortable at a relative humidity of 30 to 50 percent. Humidifiers and dehumidifiers help maintain the humidity at a comfortable level. They also, importantly, help dry internal structures such as drywall and wood to prevent deterioration due to moisture.

If you are an outdoor person, or if you are very sensitive to dampness, it is very important to understand the difference between relative humidity (RH) and the dew point. A dew point is a temperature at which the relative humidity reaches 100%. At this temperature, air cannot hold water in the vapour form.

The greater the dew point, the more uncomfortable it will be. For example, a temperature of 27oC and a dew point of 15oC mean almost around 50% of relative humidity. This is not a reading that most people will enjoy unless they have easy access to a comfortable and cool air conditioner. When you see the weather forecast, "It's 29.4o C outside, but with the increased humidity it feels like 33.3o C." 33.3o C is a combination of temperature and dew point, also known as the heat index.

As a thumb rule, the closer the dew point is to the actual temperature, the more likely it is that the air will feel too humid, but keep in mind that temperature affects the perception of humidity. For example, on a winter day at 1.6oC with 94% relative humidity, the dew point is 0.7o C. However, because the air is pretty cool, you are much less likely to complain about the hot humid conditions.

Importance of Measuring and Monitoring the Relative Humidity

For example, consider a chocolate bar. When the relative humidity in the warehouse rises above a certain level, moisture is generated on the surface of the chocolate and the sugar melts. As the water evaporates, the sugar forms larger crystals and causes discolouration. While chocolate still remains fit for consumption after this, the same does not apply to all the food items.

Humidity can also damage highly moisture sensitive products such as medicines. Humidity can change the characteristics of the product and make it ineffective. Therefore, products such as medicated tablets and dry powders are stored under controlled conditions at accurate humidity and temperature levels.

The relative humidity is paramount in building an automated system that focuses on human comfort such as air conditioners. The ability to measure and control relative humidity helps maintain a comfortable environment within the building. This helps optimise the efficiency of the HVAC system by indicating the amount of outside air intake that needs to be adjusted according to the outside air temperature.

Practice Problems

Q.1. If the actual vapour pressure is 10 Pa at 20oC compared to the saturation vapor pressure 17.3 Pa at the same temperature. What is relative humidity?

Solution: Relative humidity

Relative humidity

Relative humidity = 57.8 %

Q.2. A closed space contains 47.7 g of water vapour. If 12.5 g of water vapour is added to this closed space, the system becomes saturated. What is the amount of water vapours (in grams) required to saturate the air in the closed space?

Solution:

Mass of Water vapour when the system is saturated

= Mass of Water Vapour + Mass of Water Vapour Added

= (47.7+12.5) g

=60.2 g

Q.3. A room contains 72 g of water vapour. Volume of water vapour is 10 m3 and the temperature is 298 K. Calculate the relative humidity of the room. The room is sealed, and no mass transfer is allowed. Given that saturated vapour pressure at 298 K=3.17 kPa.

Solution:

We know that PV=nRT for an ideal gas and we are considering water vapour as an ideal gas.

$PV=\frac{w}{M}RT$

$P=\frac{wRT}{MV}$

Here P is actual vapor pressure, w is the mass of water vapours, R is the universal gas constant, T is the temperature in Kelvin scale, M is the molar mass of water vapours, V is the volume of the water vapours.

P=991.03 Pa

P=0.99103 K Pa

Relative humidity

= 31.26 %

Q.4. Given that water vapour mass in the air is 47.7 g and water vapour mass for saturation is 60.2 g. Find the relative humidity.

Solution: Relative humidity

=79.24 %

Q. How to maintain relative humidity with an air conditioner?
If the amount of water vapour is higher (high percentage of relative humidity) than expected, the air conditioner removes the water vapours and maintains a comfortable environment.

Q. What is blooming?
Blooming is a phenomenon which occurs due to prolonged periods of high relative humidity. It involves the germination and growth of moulds. It can often be confused with dirt or dust. This can be overcome by controlling humidity and the dry mould can then be dusted off.

Q. What is saturated air?