Now that we are used to the hot days of summer it's time to understand how to keep our hard-won catch cool until we get home. The modern way is to use an insulated bag with a cool pack from the freezer but many of us still use a bass bag. This, when thoroughly wetted, will keep its contents at a lower temperature than the water it has been dipped in.
To use it properly the fish need to be placed in a plastic liner (any polythene bag will do) which is then placed inside the bass bag. The bass bag is then thoroughly wetted and then placed in a shady spot on the bank. As the water on the bag slowly evaporates the contents are cooled.
The very small particles of all matter are called molecules by scientists. These molecules (especially in liquids and gasses) move around at speeds ranging from slow to very fast. This normally need not concern us but the large scale effect of the average speed of the molecules in a sample of liquid or gas is its temperature. So in hot water, the average speed of the molecules is higher than in cold water.
At the surface of the water, the very highest speed molecules are fast enough to leave the surface where they may be blown away by the movement of the surrounding air. If you remove the highest speed molecules from the water then the average speed of those remaining must be less i.e. it has a lower temperature.
This is why wet skin feels colder than dry even though the air temperature is the same. It also explains how the bass bag works. However, you must remember to keep the bass bag wet and out in the air. Simply dangling it in the water will not have the same effect.