Temperature vs 'Feels-like' Temperature: Why Is Relative Humidity Crucial?

If you have ever paid close attention to the humidity levels in the weather forecast, it might have left you feeling confused.

For instance, on a hot summer day, the forecast might say that the humidity level is 75 percent – and you will be sticky and sweaty. But during winters, the same 75 percent might mean that your skin is very dry. You might be wondering how the same reading can mean two different things in different seasons, right?

This topic gains pertinence amid a brutal heatwave currently gripping Delhi, with some parts of the city experiencing temperatures as high as 46.2 degree Celsius on Monday, 22 May.

Not only the national capital, several states in North India have been undergoing heatwave-like conditions of late.

Chandigarh has been experiencing rapidly-rising temperatures, with the Celsius going beyond 40 degrees. Punjab and Haryana exceeded Chandigarh – with temperatures soaring as high as 44 degrees Celsius.

It all comes down to the fact that most weather channels report relative humidity (RH). But what is relative humidity? And why is it monitored? But first, what is a heatwave?

What is a Heatwave?

A region is said to be undergoing a heatwave if it experiences abnormally high temperatures, which are uncommon in the region during the summer. Heatwaves in India are most prevalent between March and June, and may even extend to July.

According to the IMD, a heatwave is declared when at least two of the below-mentioned criteria are met in a sub-division for at least two consecutive days:

• The maximum temperature reaches at least 40 degrees Celsius in the plains and at least 30 degrees Celsius in hilly areas.

• When the departure from the normal temperature in a region is 4.5 to 6.4 degrees Celsius.

• When actual maximum temperature is equal to or more than 45 degrees Celsius.

What is Relative Humidity?

In simple terms, relative humidity is the amount of water vapour actually in the air, expressed as a percentage of the maximum amount of water vapor the air can hold (saturation) at the same temperature.

To understand this, think of a sponge. What happens to the sponge when you squeeze it, or put it out in the sun? It becomes totally dry right?

You could then say that the relative humidity of the sponge is 0 percent. Now, when you start using it to absorb water for instance from a surface, you will reach a point when the sponge will no longer be able to absorb any more water.  

At this juncture, the RH of the sponge is 100 percent. In other words, we say it is saturated. The air around us behaves much like that sponge as when the air which already contains a lot of moisture, will not easily accept more moisture.

Now, you might automatically assume that once RH reaches 100, it might rain (think of the water being squeezed out of that sponge). But that necessarily is not the case.

As Dr Rathin Datta, former scientist at the Meteorological Department (MeT) told The Quint, “When RH reaches 100 percent, it simply means the air is saturated with water vapour at the prevailing temperature.”

And does the capacity to hold water vapour depend on temperature?

“Yes, the capacity of air to hold water vapour varies with temperature,” added Dr Dutta.

Why is Relative Humidity Monitored?

The reason the RH is monitored is that it has a bearing on how we feel.

You might have observed that on some particularly hot summer days, you can cool off easily by say just lying under the fan.

But then, there are also days, when we feel sticky and hot, and nothing seems to have a cooling effect. So, what explains this difference in experiences?

Well, it can be attributed to the difference in RH. Explained Dr Dutta:

"In the first case, the RH is relatively low and there is not a lot of moisture in the air. We sweat and the sweat is released via our skin to the surface. There the liquid evaporates. What happens is when water changes phase from liquid to vapour, it absorbs heat from the surface on which it lies. As a result, when sweat evaporates, it absorbs heat from the skin, thereby cooling it."

In the second instance, the RH is high, which means it is more humid.

So what happens when humidity is high is that the air is so laden with water vapor that there is simply no room for much else. When you sweat when it is humid, it is hard to cool off as your sweat cannot evaporate into the air like it needs to and you therefore continue to feel hot.

“The  combination of high temperature and high relative humidity is therefore deadly, as it can cause heatstrokes,“ he added.

Here are some ways to prevent heatstrokes:

• Wearing comfortable and loose clothing

• Applying sunscreen with an SPF (Sun Protection Factor) of at least 15 before stepping out

• Covering your head with a scarf, hat or any other piece of clothing

• Keeping yourself hydrated through a regular intake of water and fruit juices

• Not staying in a locked car for too long.

• Avoiding taxing activity in the heat as much as possible

• Avoiding going out when the heat level is at its highest (for example, around noon)