The water column is one of the most important data that is usually given for the outer layers of outdoor clothing, mainly jackets, but also trousers. In simple terms, the value of the water column indicates how much it can rain so that a given piece of clothing (let's call it a jacket, for example) can still protect you from the rain.
This parameter determines the limit to which the jacket will be effective against water. It is given in millimeters and is measured by placing a cylinder filled with water on the fabric of the jacket. And that is until the first three drops of water run through the fabric.
The higher the better?
The higher the water column, the better, but only up to a point. Some jackets (but also pants or perhaps tents) are already excessively effective for our conditions, and if you are deciding between two jackets of a higher category based only on the level of the water column (while the other parameters are more or less the same), feel free to choose the one with indicated water column lower (and which is perhaps cheaper or is better in the opposite parameters).
However, the above only applies to water columns of around 20,000 mm and higher. Until then, the water column is still an important parameter that you should take into account when choosing a jacket. For tents, a water column of around 3000 mm is completely sufficient, unless you are going on a demanding expedition to tropical regions or, conversely, to snowy or "rainy" mountains.
Important details
However, the water column is not the only data you should look at when it comes to the waterproofness of a jacket. In this regard, the weakest links of jackets, i.e. the seams, are also important. It is ideal if they are welded or covered with a strip of fabric, the so-called flap. The fabric's high water column will do you no good if it leaks into your jacket through the seams.
Waterproofness or reduced wetness of the jacket must then go hand in hand with breathability. The jacket should be membranous so that it can breathe during your physical activity. Otherwise, you could easily wrap yourself in stretch film instead of a jacket - it is also waterproof (and probably has a fairly high water column), but for obvious reasons we do not wear it on the hike. The breathability ensures efficient drainage of moisture away from the body and ensures that we don't get steamy during a hike or sport activity.
When choosing a jacket according to the value of the water column, we should also take into account the constant pressure on the jacket when walking - for example, with a backpack that we carry on our back. So you should give yourself a certain reserve. In places where the jacket is compressed, it is simply not as effective.