How do you define moisture levels in air?
Moisture in air is water vapour – and is present in air in almost all circumsances. The amount of moisture in air is very small in comparison with materials or in liguid. On a normal spring day in NZ, there is about 1 gram of water per cubic metre of air. Water vapour is typically defined in a nuber of ways:
- As a vapour pressure (kPa) – this gives the amount of partial pressure the moisture is exerting in the air. The more moisture, the higher the vapour pressure.
- As a weight by dry air weight (g/kg) – this gives the absolute mass of vapour inside a kg of dry air (approximately 1 cubic metre)
- As relative humidity (%RH) – this is the percentage of water that is in the air divided by the amount of water that the air could possibly hold in its current state.
How ‘wet’ is ‘wet’?
As human beings, we feel the differences of moisture in the air. We tend to give ‘wet’ days words like ‘humid’ or ‘muggy’, and ‘dry’ days words like ‘crisp’ and ‘fresh’. What we actually are feeling is the Relative Humidity (%RH) of the moisture in the air. The reason we feel this is because at high humidity days (close to 100%RH – ie ‘saturation’) when we perspire, our sweat does not evapourate very quickly or easily, because the air around us is already at ‘saturation’ and can not absorb any more water. Conversely, on a ‘dry’ day, the humidity is low (ie 50%RH) and our sweat evapourates easily as the air around us has ‘spare capacity’ to absorb more water vapour.