There is a replaceable CR2450 battery in the WEATHERmeter. It is designed to provide approximately 500 hours of use on low power, and about 300 hours on high power, which should be many years of normal sampling. If your battery dies, however, you can easily replace it. Just peel off the silicone "skin" to reveal a battery door. Slide the battery door open, replace the battery, and put the skin back on.
* Note. If the Battery is removed the compass will need to be re-calibrated