Whenever you use the term weather proof for any object as a means of description, you are basically saying that the product has got some form of insulation against the elements of the environment. Their surface is resistant to rain, hail and the like and this can be because it is naturally that way or because the material has been treated to become so.
In the case of most weather proof clothing, it is often seen that the material has been treated. What the material is treated for is resistance to water because if it passes through the outfit it can end up damaging the clothes that you wear making being on the outdoors rather difficult. It is one of the main reasons why outdoor gear such as that of camping, skiing, trekking etc are made to be water proof.
To prevent the water from getting into your clothes, the material will be natural or will be synthetically sealed. Many times, this cover is added at the time of making the garment. However, it can also be added at a later point. Once you have a ready-made garment with you, you can use a uniquely formulated spray that will help make it a water proof one. What you have to remember in such cases is that the effect is not permanent and will have to be repeated multiple times. Instead investing in a water proof outfit would be a good idea. In order to make clothes permanently water proof, the surface of the material is merged with a laminate of sorts. Some examples will be polyurethane or polyvinyl chloride.
Weather proof outfits are not all the same and there are varying degrees to which they can be water proof. There are some garments that may be water proof but are also labeled as breathable as far as the structure of the fabric goes. This technically means that the material will be able to hold its own in around 9.8 kilopascals (kPa) and will not let without letting water in. While doing this it also gives way for the material to breathe meaning that any vapor within the jacket will be released.
There are a few weather proof clothing items that are made of directional textile. This means that when moisture falls on these, they are directed away from the body and this is done without any lamination to the material. These materials are uniquely created and help spread water well preventing it from pooling around the body. This has more advantages since in extreme cold, weatherproof materials may not be able to hold onto the abilities.