Relative Humidity

This article explores what relative humidity is and why it matters. We describe how much water vapour is in the air using relative humidity. It tells us how much water vapour is in the air compared to how much it could hold at that temperature. Relative Humidity is represented as a per cent.

What Is Relative Humidity?

Air contains water vapour. It plays a significant role in the weather phenomena. The moisture present in the atmosphere is derived from water bodies through evaporation and plants through transpiration.

Therefore, there is a constant exchange of water between the atmosphere, the continents and the oceans through transpiration, evaporation, precipitation and condensation.

Absolute Humidity vs Relative Humidity

Water vapour present in the air is called humidity. The actual amount of water vapour present in the atmosphere is known as absolute humidity. It is described in terms of grams per cubic metre. The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature is known as relative humidity.

Relative Humidity and Temperature Relationship

Air’s ability to preserve water vapour depends entirely on its temperature. With the change of air’s temperature, the ability to retain moisture either increases or decreases, affecting relative humidity. The relation between humidity and temperature is inversely proportional. If temperature increases, it will reduce relative humidity; thus, the air will become drier. When the temperature decreases, the air will become wetter; therefore, the relative humidity will increase. The relative humidity is greater over the oceans and least over the continents.

What Is Dew Point?

The air retaining moisture to its full capacity at a given temperature is saturated. The temperature at which saturation occurs in a given air sample is the dew point. It suggests that the air at a dew-point temperature cannot hold any additional water.

When the air reaches the dew-point temperature at a particular pressure, the water vapour is in equilibrium with liquid water, meaning water vapour is condensing at the same rate at which liquid water is evaporating.

The hygrometer is used to calculate the Dew point temperature manually. It operates by cooling a shining, smooth surface, such as a mirror, until air’s water vapour condenses on it. You will obtain the dew point temperature when this happens.

Relative Density Calculation

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

The equation for calculating relative humidity is as follows:

Actual Vapour Density/Saturation Vapour Density x 100

The actual vapour density calculates the current water vapour (g/m3) in a space. The saturation vapour density measures the maximum amount of moisture that the air can hold at that temperature (g/m3).

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

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Importance of Measuring and Monitoring of Relative Humidity

By far, the most prominent reason to monitor relative humidity is to control moisture around a final product. This means ensuring that the relative humidity never rises too high in most cases. If the relative humidity in a storage facility increases above a particular level and stays above that level for an adequately long period, a phenomenon called blooming can occur. For example, letâ€™s consider a bar of chocolate. If the RH in a warehouse facility increases above a certain level, moisture forms on the chocolate surface, dissolving the sugar. When the moisture evaporates, the sugar forms larger crystals, leading to discolouration.

Humidity can have a severe impact on building materials. Let’s say that you are lengthening your property, and you lay a concrete subfloor before setting down the hardwood flooring. Assume that the concrete isn’t dry before you lay the floor. In that case, it can create massive problems because the moisture from the concrete will inherently migrate to a drier area â€“ the flooring material. This moisture migration can cause the floor to blister, swell or crack.

Humidity can affect also products with extreme sensitivity to moisture, such as pharmaceuticals. Humidity can modify the product’s characteristics until it becomes ineffective, which is why products such as medical tablets and dry powders are stored in controlled conditions at precise humidity and temperature levels.

Finally, relative humidity is paramount in building automation systems focused on human comforts, such as air conditioning. The ability to measure and control RH helps maintain a comfortable environment inside a building. It helps to optimise the efficiency of HVAC systems by indicating how much outside air being taken in needs to be conditioned, depending on the temperature outside.

Q1

What is Relative Humidity?

The percentage of moisture present in the atmosphere compared to its full capacity at a given temperature is known as relative humidity.

Q2

What affects Relative Humidity?

Airâ€™s ability to preserve water vapour depends entirely on its temperature. With the change of airâ€™s temperature, the ability to retain moisture either increases or decreases, affecting relative humidity.

Q3

Is relative humidity higher in summer or winter?

Since cold air carries lesser moisture, it is easier to saturate a parcel of cold air. Therefore, relative humidity is higher in winter than in summer.

Q4

What Relative Density is comfortable?

The ideal Relative Humidity for health and comfort is between 30-50% humidity.

Q5

Why is humidity important in the pharmaceutical industry?

Humidity can modify the productâ€™s characteristics until it becomes ineffective, which is why products such as medical tablets and dry powders are stored in controlled conditions at precise humidity and temperature levels.

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