Dehumidification - A condensed explanation

When you leave your car outside overnight, have you noticed that in the morning it’s wet? – Or if you walk in long grass early in the morning or late in the evening you end up with damp feet – no matter what season of the year or where you are in this lovely country, this happens almost everyday and we call it ‘dew’. Thank goodness it occurs because without this source of water many plants and trees would not survive, including the vines that we rely on for Spain’s abundant supply of wine.
All air contains water and the amount of water in the air varies from season to season and during the course of each day. The higher the temperature of the air the more water it can contain and vice versa. The amount of water in the air at any particular time can be measured with an hygrometer and the result is expressed as a ‘% relative humidity’ or ‘RH’. This percentage is the ratio of the actual amount of water in the air compared to the maximum it could hold at that temperature.
So, let’s imagine we have the start of a typical Spanish day with not a cloud in the sky, the temperature is not very high and up comes the sun. If we measured the RH first thing in the morning we would probably find it would be moderately high – say 60 – 80%, then as the sun starts warming everything up nicely, even on a warm winter’s day in December/January (and we feel sorry for all those folks back in the UK shivering and getting colds!), the RH probably drops to 30 – 40%. If it is cloudy then the RH may stay around 60 – 80% all day.
So far so good.
Come late afternoon, the sun goes down, the air temperature drops and as it drops the relative humidity rises as you would expect. (Here comes another ‘weather’ word) – As the temperature drops a point is reached where the RH reaches 100% and the air gives up its water as condensation and dew forms on any cool surface, (hence this is called the ‘dew’ point).
So, back to the beginning of this story, this dew will form on the top of your cool car, the cool vines, olive and almond leaves BUT ALSO on any cool surface it can find – INCLUDING those of your villa or apartment – walls, windows, furniture and fabrics. Most of us wish it didn’t happen and start opening windows and doors to ‘air’ the place – by now you will know that what you are actually doing is to bring in air from outside that is warmer, therefore with a lower relative humidity, so that this condensation gets reabsorbed into the air. Problem solved?
This is fine if the outside weather is warm and dry AND if you are IN your villa or apartment to open it up, but on cooler and damp days or if you have gone away and the place is shut, the condensation will largely stay in the walls, furniture and fabrics and will create perfect conditions for moulds, mildew and even fungi. (Incidentally water condensed from the air by definition is distilled water – i.e. pure and, as such, is the ideal uninhibited breeding ground for any organism).
This has been a problem for a long time as evidenced by the origin of the word mildew – it comes from the Old English word ‘mild-aw’ and that takes us back before 1150 AD.
But you can do something about it. In fact there are two solutions:

1) Raise the air temperature sufficiently by heating the inside of your property allowing the water to stay in the air, or,

2) Remove the moisture from the air mechanically thus removing the source of condensation.
For centuries the only solution has been 1 above – light a log or coal fire, turn on the gas fire, switch on the heating or electric fire – all costly and not very easy to do if you aren’t in the property! (Quite often we might even heat our villas in winter with portable ‘Super Ser’ type heaters – these actually ADD water to the air and can make the situation worse!)
The better and more efficient solution is 2. By using a machine to take water out of the air – a dehumidifier to give it its proper name.
These machines have inside