Use a volt meter to see if the battery is depleted or not. If you replace the battery and the alarm stops then it was a weak battery, If it keeps on then there something setting it off. You can use the information in the owners manual Oregon Scientific BAR629HGU Manuals to troubleshoot it.
Heavens Kaitlan you don't ask much do you? I think you need to spend a happy hour or two online with each question, unless there's a Professor of Climatology reading this.