The hygrometer operates with a single battery (included) and reads humidity and temperature in Fahrenheit or Celsius; it also records the most recent highest and lowest settings of both readings. It can attach with included Velcro strips to the interior of case, or it can simply be stored in accessory pocket inside case. The Stretto effectively humidifies the entire case, so if the hygrometer is placed inside a pocket, its readings will still be accurate for entire case.