What defines atmospheric instability?

Atmospheric instability is primarily characterized by the lapse rate, which is the rate at which temperature decreases with an increase in altitude. When the lapse rate exceeds a certain threshold—specifically, generally around 5.4 degrees Fahrenheit per thousand feet—this indicates that the temperature of the rising air parcel is significantly warmer than the surrounding air. This temperature difference causes the warmer air to continue rising, leading to instability in the atmosphere.

The other options do not correctly define atmospheric instability. For example, an increase in air density with altitude (the second option) is typical of stable conditions, not unstable ones. A constant temperature with altitude would imply a neutral stability, while cooler surrounding air than the rising air (the last option) does signify instability but doesn't capture the essence of what defines it as definitively as the lapse rate does. A solid understanding of lapse rates and their implications is crucial for recognizing when atmospheric conditions will lead to instability and, consequently, potential weather phenomena like convection and turbulence.