Final answer:
An unstable atmosphere is characterized by a warm, humid air mass that has low surface atmospheric pressure. Moist air rises and condenses, causing clouds and potentially leading to weather events like storms.
Step-by-step explanation:
An unstable atmosphere is characterized by a warm, humid air mass. In this type of atmosphere, the surface atmospheric pressure is low and there is a tendency for moist air to rise and condense, producing clouds and potentially causing weather events such as storms or heavy rainfall. The movement of moisture and air within various weather fronts plays a role in instigating these events.