Relative humidity is the amount of moisture in the air displayed as a percentage of how much moisture the air could hold at a given temperature. Storing firearms, ammo and reloading components within a certain humidity range will help prevent formation of rust, mold and mildew, and will also increase shelf life. The large LCD on this handy unit makes reading current temperatures and humidity easy. The unit's magnetic back makes it easy to attach to steel surfaces such as gun safes. It also comes with a stand for use on a shelf or table. Temperature can be displayed in Fahrenheit or Celsius.