Relative humidity is a simple concept as far 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.

What is humidity and how can it be measured?

• 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 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 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 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 as 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 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.

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.
• 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.
• The change in the capacity for moisture as the air warms is also non-linear, meaning it doesn’t increase by the same amount as the air warms. For example, the difference as it warms over 20º C from -20º C to 0º C is small, but there is an enormous improvement in its vapour-carrying capacity as it warms over 20º C from 30º C to 50º C.

Why does the wet-bulb temperature matter?

• A more direct way to understand the implications of relative humidity for your wellbeing is in the form of the wet-bulb temperature (also known as the adiabatic saturation temperature). It is the lowest temperature a surface — like your skin — can reach when water evaporates from it. The wet-bulb temperature is equal to the dry-bulb temperature when the relative humidity is 100%.
• A wet-bulb temperature in an environment of 32-35ºC or higher can be quickly lethal, even if you are not doing any physical activity or are in the shade. (At least one study has shown that even a wet-bulb temperature of more than 29º C can be dangerous.)