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. It is expressed as a dimensionless number The third way is relative humidity: it is important because it factors in the amount of vapour that air can hold at different temperatures. 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 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, if the difference was 6 C and the dry-bulb temperature was 28 C, the relative humidity would have been 59%.