As I indicated at the end of the recent post about surface tension, I've started a new thread about the amazing properties of water. This time I'll write about saturation, what it is and what it isn't. The reason I included the bit about "what it isn't" is that a close friend once asked what saturation actually was - they thought that if the air were "saturated" it was like walking through a swimming pool. Not an unreasonable deduction based on our everyday meaning of "saturation". But in meteorology the word has a precise meaning and, to understand it, we first need to look at the three forms that water takes.

States of water

1. Liquid: like the dew and rain drops mentioned in the surface tension post, this fluid form is probably our most familiar. It's what we drink, clean with and swim in. Our lives depend on it, and we suffer whenever there's a shortage.

2. Ice: as mentioned in the post on late frosts, occurs at temperatures below 0°C. It is solid and, in meteorology, most often occurs as hail or snow (see types of weather).

3. Water vapour: perhaps the form least familiar to us,  is a gas that is:

  • invisible

  • odourless

  • abundant in many parts of the world, but also

  • very unevenly distributed.

All three forms of water can be found in your kitchen: either turn on your tap, look in your freezer, or feel the air around your face.

Characteristics of water vapour

Examples of water vapour are all around us. If you breathe out on a cold morning you can see the tiny droplets in the air - the droplets aren't vapour, they're actually tiny bits of liquid but, once they've disappeared into the air around you, they've evaporated (become vapour). Or perhaps you've had a hot shower and bathroom surfaces are damp - if you leave the door or window open the room will ventilate and the water will evaporate.

Sometimes, after a clear sunny morning, you may notice bubbly cumulus clouds forming towards lunchtime, seemingly from nothing. These clouds are made of many, many tiny droplets of liquid water that have condensed from water vapour in the air (perhaps originating from your shower!).

Cumulus cloud forming after a clear morningCumulus cloud forming after a clear morning


If you put a dish of (liquid) water on a sill, then the water will eventually disappear by changing to vapour. A real-life disappearing act! Provided the air isn't saturated that is, and here lies the answer to my friend's question. When liquid water is in contact with air there is an exchange of the H2O water molecules between the two forms. Eventually an equilibrium is reached where the number of molecules going from liquid to vapour matches those going from vapour to liquid. Once equilibrium is reached the air is saturated. If you've heard of the term relative humidity (RH), it is at this point that RH is 100%. Nothing like a swimming pool although the air is moist, holding a lot of water in vapour form.

I hope I've explained saturation well for you :-)