This page looks at the different layers for rough weather outings. If your off to hotter climbs then there is probably little of great benefit. Just remember keep it lose and baggy and don't offend the locals by wearing shorts in towns or off the shoulder numbers. Its amazing the number of women who complain about being touched in markets when they are wearing shorts and cropped tops. Remember this is the equivalent in some countries to walking around in your underwear...
Breathability is the ability for moisture to pass out through a fabric. If you have ever found moisture on the inside of you water proofs due to perspiration then this is because the material is not breathable. If you have a breathable jacket then you should also wear breathable layers underneath. Cotton is not good as it absorbs water and stays damp. If you have a breathable jacket then it is important that you keep it clean otherwise the microscopic holes can become blocked stopping moister from passing through it.
Non Breathable: Usually PVC or PU good for light weight or leisure use.
Waterproof / Breathable Shells These prevent water from penetrating but allow moisture to escape. They are usually made by laminating or coating a nylon or polyester cloth.
Fabric Coated Shells: These work by water being drawn through the fabric by polymer chains, or the have microscopic pores that allow water vapour through but not water liquid. (Triplepoint, Cyclone, Cascade)
Laminated Membranes 2/3 Layer: These are cloths that have a performance membrane bonded to the outer shell. Then a liner fabric (pertex), or drop mesh liner, removes the moisture from the body. 2 layers have a lose lined inner and are more suitable for light use. 3 layer has the liner fabric laminated to the membrane which is laminated to the shell. These are more heavy weight and have a stiffer feel.
To get the maximum benefit from a jacket it is important to clean it regularly. Read the manufactures instructions. In most cases they can be hand washed in soap flakes.Breathable garments usually have a water proof coating that wears of with time although you will not get wet on the inside when the water no longer runs off its worth reproofing it.
When not in use hang them up in a dry airy place.
Probably one of the most popular mid layers is the fleece jacket. These range from the cheap types that are warm but heavy to the lightweight fabrics such as the Polartec range of fabrics. Each range of fabrics come in different ranges for example there is Polartec 100, 200 and 300. Depending on how warm you want the jacket to be. In windy weather you will need a outer wind proof shell to benefit properly from a fleece jacket.
Base layers should be able to wick away moisture from the skin to keep it dry. The most important factor is that it can also trap a layer of warm air next to the body to preserve heat. Synthetic fibres are best such as polyester, polyamide or polypropylene.
Cotton T-shirts aren't really any good as a base layer, the cotton absorbs moisture leaving you feeling damp and cold. This can make it seem that your breathable mid and top layers do not work properly. It is worth taking a change of shirt or any other item of cotton you choose to wear so you can change into warm dry cloths at the end of the day.
Base layers do not have to be skin tight unless your doing really strenuous activities, were they will be more effective tighter.