Understanding Relative Humidity: Real-Life Applications in Physics

You must have heard or checked the weather forecasters saying that there is a relative humidity of this much percentage at a certain region, or there can be chances of a cloud formation or even the chances of rainfall, etc. These are the basic points of daily weather forecasting and in this relative humidity plays a vital role. In this article, we will be talking about this concept of Geography or Geophysics, i.e., Relative Humidity. In this article, we will learn about what humidity is, the difference between the two types of humidity, relative humidity meaning, its formula or percentage and examples, various uses or effects etc. You will also learn the relationship of relative humidity with pressure or temperature as well.

Humidity basically helps in determining the water vapour in the air, i.e., Moisture is a source of evaporation. Humidity after condensation leads to the formation of clouds and these clouds lead to precipitation and this precipitation can occur in any form such as rainfall, snowfall, hailstone, sleet, etc. The difference between the humidity of two regions is generally known as humidity gradient. It also helps in understanding the relation between temperature and humidity on the basis of its particular type. There are two types of humidity i.e. Absolute and Relative. In this article, we will be talking about Relative Humidity and its related aspects.

Relative Humidity Meaning

It is basically an amount of water vapour present in the air. It is that percentage that specifies how much moisture can be held by air and this moisture can be increased with an increase in the temperature but on the other hand temperature and relative humidity are also inversely proportional. The higher percentage of relative humidity in the air says that it is more humid. It is not dependent on the height but on the capacity of the air to hold moisture. When the humidity percentage reaches 100%, it is called saturation which helps in the formation of the clouds. It is always expressed in the form of percentage or can be written as φ or RH. It is majorly high at midnight and in the early morning and it reduces rapidly after the sun rises and then it is lowest and then again it starts increasing up to midnight. The effect on relative humidity with respect to change in temperature or pressure is mentioned below which shows that there is an inverse relationship between temperature and relative humidity whereas there is a direct relationship between pressure and relative humidity.

Relationship Between Temperature and Relative Humidity

Humidity Percentage

We can calculate the relative humidity with the help of the following formula. It is basically a ratio of the amount of water vapour present in a particular parcel of the air and the total capacity of the air to hold the water vapour and it is expressed in percentage form with symbol φ or RH.

Relative Humidity = Amount of water Vapor present (V)/Total Capacity of Air to Hold (V) x 100

Example: Suppose, if there is 100% water vapour present in a piece of air at a certain level, and its capacity of holding water vapour is 200, then according to the above formula, the relative humidity will be 50%.

When the relative humidity is 100%, it means that it is saturated air where saturation generally means a point where water vapour present is equal to the capacity and this saturation helps in the formation of clouds. On the other hand the temperature at which this air is saturated is known as dew point. Suppose, in a particular region where there is a temperature of 30° with a relative humidity of 50% and on another hand, at a different location there is a temperature of 40° with a relative humidity of 33%. These two regions show the relation between the relative humidity and temperature where an increase in temperature leads to a decrease in relative humidity and these both are inversely proportional to each other.

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Effects and Uses

It can lead to various effects on various things whereas it can be useful in multiple things which are mentioned below:

• It helps in weather forecasting,  as this concept is useful in understanding the rainfall patterns or dew points.

• The temperature feels hotter than it actually is with the presence of more humidity in the air.

• It helps in animal husbandry and keeping certain foods at a certain point.

• It is very important to monitor the moisture around any product or even while construction of the buildings.

• It helps in forming human comfort products such as air conditioners. It helps in maintaining and controlling the moisture.

• Humidity can alter the characteristics of some pharmaceutical products that are sensitive to humidity and thus it becomes very important to take care and maintain a certain level.

• Besides all these mentioned uses or effects, any person can also check the relative humidity today of their specific region.

Difference Between Absolute Humidity and Relative Humidity

Both determine the water content i.e moisture present in the air but the absolute does not determine this with respect to temperature whereas the relative one does this with respect to the temperature. The absolute is determining moisture in grams per cubic meter of air whereas the relative is determined in the form of percentage which shows the moisture present in the air against its total capacity. The former does not give an exact idea of humidity present in the air or the point of saturation but the relative humidity helps in determining all this and thus it can be called saturation humidity as well.

Additional Information

Difference Between Temperature and Humidity

Both these concepts are different but interrelated to each other. The former is the measure of the heat or coldness whereas the latter one is a measure of the water present in the air. The former i.e. temperature is governed by the solar radiation coming from the Sun and an increase in the solar radiation will lead to an increase in the temperature. The high temperature along with the high humidity will make us feel sweaty and also we will feel the temperature hotter than it actually is. Different geographical locations and their features play a vital role in their relationship such as in a tropical climate region, if the temperature is high the humidity will also be high whereas if we talk about desert regions, their temperature is high but humidity usually remains low.

Relative Humidity and Dew point

Relative humidity might often represent itself as the moisture-holding capacity of air. Such an understanding of the phenomenon might sometimes prevent us from completely deciphering what happens physically in terms of air humidity. In the scientific sense, the air does not "hold" water vapour as it does not exert any attractive force or capture influence on water molecules. They are actually lighter and have a greater speed value than the nitrogen and oxygen molecules that make up the bulk of the air. The thermal energy of water molecules in the air at a room temperature of 20°C when compared is over 600 m/s or over 1400 miles an hour! Therefore when one talks of air holding water in it, it only refers to the moment for which these molecules act as an ideal gas and will remain as a constituent of air.

At such high speeds, water molecules act as molecules of an ideal gas. At an atmospheric pressure of 760 mm Hg, this represents the vapour pressure contributed by the water molecules.

But water vapour is not similar in its properties in comparison to other air constituents like oxygen and nitrogen. Oxygen and nitrogen are always gases at Earth temperatures, and always act as ideal gases. However, water has a boiling point of 100°C= 373.15K and can exist in solid, liquid and gaseous phases on the Earth. This means that water molecules will always remain in a process of dynamic exchange between their various phases.

At 20°C, as the vapour pressure in the air reaches 17.54 mm Hg, water molecules start to enter the liquid phase thereby escaping from its gaseous form, leading to the conclusion that the vapour is "saturated". As the air approaches saturation, it is scientifically known to come closer to its "dew point". Since water molecules are polar they demonstrate a net attractive force on each other and therefore slowly begin to depart from ideal gas behaviour. As they aggregate and enter the liquid phase, they form water droplets in the atmosphere to make clouds, or near the surface to form fog, or on surfaces to form dew (as seen in leaves early in a foggy morning).

In order to understand this better, we can use a closed flask at 20°C filled with liquid water in it but no air at all, and then saturate it to vapour pressure 17.54 mm Hg. At that point, it would have a vapour density of 17.3 gm/m3 of pure water vapour in the gas phase above the water surface. And if the air is removed and the container is locked with liquid water in it, one might incur a case where there is only 8.65 gm/m3 resident in the gas phase at that given moment. This is referred to in scientific language as a flask with 50% relative humidity because the resident water vapour density is half of its saturation density. In the same way, if the air is present at a vapour pressure of 8.65 gm/m3 at 20°C, it is said to represent 50% relative humidity.

Conclusion

Thus, we can conclude that relative humidity is one of the important concepts that need to be studied for a good understanding of the weather, temperature or moisture present and also for understanding saturation, dew point, clouds formation or precipitation etc. It also plays an important role in maintaining and keeping products at a certain temperature or moisture levels. This article will help in understanding this concept which you usually study in Geography or Geophysics. 

With this article, you will not only be able to understand the Relative Humidity but also the Introduction of humidity, the difference between temperature and humidity and also a difference between relative humidity and absolute humidity, etc that will help you better understand this concept.