When a parcel of air is warmer (less dense) than its surroundings, it rises and cools. Eventually, the parcel of air will cool to the same temperature as the surrounding atmosphere and it will stop rising. The rate at which temperature decreases with altitude determines the stability of the atmosphere.
To understand stability, let’s imagine we’re ascending up through the atmosphere in a hot air balloon and we take a temperature reading every thousand feet. This will result in one of a few situations:
In a stable atmosphere, we will record very little decrease in temperature as we ascend in our hot air balloon.
In these conditions, a rising parcel of air will decrease in temperature faster than the surrounding air, so it does not have to rise particularly high before reaching an altitude where the atmosphere is the same temperature as the parcel itself.
Stable conditions are typically associated with high pressure systems and any cloud will have little vertical extent (flat, widespread cloud).
In an unstable atmosphere, we will record rapidly decreasing temperatures as we ascend in our hot air balloon.
In these conditions, a rising parcel of air will remain warmer than the surrounding air as it ascends, so the parcel will continue to rise.
Unstable conditions are typically associated with low pressure systems and troughs. Clouds will have extensive vertical development in an unstable atmosphere (tall, towering clouds).
The rate at which the temperature of the atmosphere decreases as altitude increases is known as the Environmental Lapse Rate (ELR). For example, if we record a temperature decrease of 2°C every 1000ft as we ascend, the ELR would be 2°C/1000ft.
This ELR of 2°C/1000ft is in line with the International Standard Atmosphere (ISA) – but in the real world, the ELR will constantly be changing as temperature changes throughout the day.
A third case can exist, known as conditional stability, in which the stability depends on humidity. If humidity is low, cloud is less likely to form. Rising air will reach a height where the surrounding air is the same temperature and stop rising.
When humidity is high, clouds are likely to form. This formation of cloud is the result of condensation, which releases latent heat into the atmosphere (from the earlier lesson on The States of Water). This extra heat keeps the rising parcel of air warmer than the surrounding air, meaning it continues to rise.
So when the atmosphere is conditionally stable: