Explained | What is relative humidity and why does it matter on a hot day?

Relative humidity is a simple concept as weather phenomena go – but it has significant, far-reaching consequences for how we must take care of ourselves on a hot or wet day.

Letting the wind flow freely is the secret to cooling our cities

Humidity is the amount of moisture in the air around us, and there are three ways to track it. The most common of them is absolute humidity: the mass of water vapour in a given volume of the air and water vapour mixture, expressed as kg/m3. The second is specific humidity, equal to the mass of the moisture divided by the mass of air. It is expressed as a dimensionless number (but sometimes also as grams per kilogram, among other similar units.)

The third way is relative humidity: it is important because it factors in the amount of vapour that air can hold at different temperatures. Determining its value is a bit more complicated: it is the vapour density of the air divided by the saturation vapour density at the dry-bulb temperature.

Why does relative humidity matter?

On a hot day, our body uses sweat to cool itself. Sweat is released via our skin to the surface. There, the liquid evaporates. When water changes phase from liquid to vapour, it absorbs heat from the surface on which it lies. So when sweat evaporates, it absorbs heat from the skin, cooling it.

The higher the relative humidity of air, the more it is filled with moisture. When air already contains a lot of moisture, it won’t easily accept more. This means that the sweat on your skin can’t evaporate. At the same time, the body keeps sweating because it is still expecting to cool itself.

As a result, if the relative humidity is high, you can sweat on a hot day even when you are sitting still – while your body also keeps accumulating heat. This can quickly become dangerous.

A relative humidity of 30-60% is generally considered to be comfortable. Environments that have lower levels than this typically use humidifiers to increase the humidity. When the level is higher, a fan will help move the air around you and help sweat evaporate better. 

In both cases, drinking water is important.

What does relative humidity imply physically?

Warmer air can hold more moisture than cooler air. So at the same absolute humidity, the relative humidity of warmer air will be lower than that of cooler air. It is like saying two vessels can hold the same amount of water – just that the bigger vessel will be less full than the smaller vessel.

This is why, for example, while Chennai had an absolute humidity of 70% at 2.30 pm on April 23, its relative humidity was lower, around 60%, because it had an ambient temperature of 32 C.