A better measure of how atmospheric moisture will affect people and the environment is the dew point.
The dew point is, in short, the point at which dew droplets form on objects like grass – in other words, when a relative humidity of 100 percent is achieved.
If you want a real judge of just how "dry" or "humid" it will feel outside, look at the dew point instead of the RH. The higher the dew point, the muggier it will feel.
Taken from Oppressive, humid conditions for southern Australia as heatwave combines with tropical airmass
Dew point is the indicator of the amount of moisture in the air ... it's the temperature that the air needs to cool to in order to form cloud.
When temperatures are very high, relative humidity can under represent how much moisture there is in the air, making dew point the better measure.