Ball pythons need some humidity to maintain proper respiratory health and to shed their skin correctly. According to TimeandDate.com climate records from known ball python habitat in Africa, humidity levels are generally between 60-80% during the day, rising to 80-100% at night. It’s best to replicate these conditions in your pet’s enclosure, although occasional short dips and spikes outside of the given ranges are not likely to be harmful. 

Keep tabs on both temperature and humidity with a digital thermometer/hygrometer — the humidity probe should be placed in the middle of the enclosure to monitor average humidity, although it is helpful to have another humidity probe in the humid hide to make sure it stays moist.

Of course, even when you use the right substrate, maintaining high levels of humidity can be tricky, especially if you live in a dry climate or if the enclosure is well ventilated. Here are some tips:

• Use a thick layer of moisture-retentive substrate — at least 4″. The more substrate you have, the more moisture it can hold. 
• Daily misting (every evening and then morning if needed) with a traditional spray bottle can give you hand cramps, so use a pressure sprayer like the Exo Terra Pressure Sprayer to make life a lot easier. Alternatively, consider installing an automatic misting system!
• Install a humid hideout somewhere in the middle to cool end of the enclosure. This makes sure your snake always has somewhere humid to go when needed. Simply line a reptile hide or cave with moistened sphagnum moss to encourage high humidity, and replace the moss frequently to prevent mold from developing.
• Convert your ball python’s enclosure into a bioactive vivarium. Bioactive setups offer a powerful combination of thick substrate, leaf litter, and live plants, so in a tropical context, this approach can go a long way toward stabilizing your humidity levels while also eliminating substrate waste. Bioactive kits like this one by the Bio Dude make the conversion process a lot easier (and increase your likelihood of success!).