Here's what Taylor say about their Hygrometer:

When it comes to helping players take good care of their acoustic guitars, few things are more impactful than reinforcing the importance of relative humidity and temperature to a guitar’s condition. The new digital hygrometer from TaylorWare provides a clear readout of climate information for virtually any space, from the inside of a guitar case to a studio or just a player’s home. Designed with large readouts for both relative humidity and temperature for easy reading at a glance, the hygrometer also features a kickstand for flat surfaces as well as screw holes for easy wall-mounting. Accurate to within 5 percent, the digital hygrometer comes with the required 2 AAA batteries and can switch between Fahrenheit and Celsius.