Humidity is often a concern with keeping reptiles, but often, the problem is getting and keeping humidity high enough. At times, though, there can be difficulty in keeping the humidity low enough. For snakes in particular, humidity that is too high can lead to scale rot (a bacterial infection which can lead to death if left untreated); therefore, it's important to make sure the humidity is in an optical range for your specific species.
To begin with, it's always important to do your research prior to owning any animal; different snakes will have different requirements. It's best to mimic your snake's natural habitat as closely as possible, and they all live in varying microclimates.
Humidity is defined as the amount of water vapor in the air. The amount of moisture in your snake's enclosure is determined by a combination of temperature, ventilation, and the amount of water vapor in the air.
Snakes usually live in a more humid, warmer environment. The natural moisture in the air helps their skin and respiratory tract to stay moist. It is best to mimic their natural habitat as closely as possible.
The easiest way to measure humidity is by using either an analog or digital hygrometer. If you are using a hygrometer with a probe, a good way to measure is to place the probe where your snake spends most of its time (on the substrate, tree branches, ets.)
Luckily, if you're finding that your humidity needs to be lowered, there are a few ways in which you can achieve this:
As stated previously, the best thing to do is plenty of research and knowing a snake's individual needs before purchasing, which allows you to set up an enclosure in advance and make any necessary adjustments before possibly subjecting your pet to an incorrect temperature or humidity level.