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.
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.
