From UroWiki
Exo-Terra analogue hygrometer
A hygrometer is a digital or analogue sensor for measuring relative
humidity in the air. A digital hygrometer is recommended as they are proven to be more accurate than analogue models, which can be off by as much as 20%. Many models of digital
thermometer also come as dual-function units with built-in hygrometers, although you may have to look around a little to find one. Normally, they are readily available on the likes of
eBay.
