What is Instability?

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When discussing severe weather setups in the Spring we often take it for granted when we mention the words instability, lift, and wind shear. Its as if we expect people hearing those terms to know exactly what we mean.

So today, we’re going to take a look at one part of that combo and answer the question, what exactly is instability?

The answer to that question could actually get quite complex, but we are going to stick to a narrower definition that’ll help explain why instability is important for thunderstorms to form.

But First…Some Basics

To answer this question, I need you to think of the atmosphere as something that’s dynamic, fluid, and ever-changing. Can we agree to do that? Ok then!

The atmosphere tends to naturally cool slowly with height. So the further up and away from the surface you are, the cooler the air is.

Using a hugely scientific phrase, ‘warm air rises’…you’d think that because the atmosphere naturally cools with height, air from the surface would always be rising, but that’s actually not the case.

As air rises from the surface, it too cools as it goes upwards. As the air rises and cools, if it encounters air as warm or warmer above it, the air will quit rising.

If this happens, and air is failing to rise, the atmosphere is considered stable.

So How Does it Become Unstable?

What has to happen for the atmosphere to become unstable is the air at the surface has to become BOTH warm and moist while the air above the surface also has to cool.

Usually, air heating up simply happens thanks to a warm, sunny day occurring.

And why does air need to be moist you ask?

A piece of air with more moisture within it cools less as it rises vs. a drier piece of air.

So, the atmosphere is potentially more unstable in a warm and moist atmosphere than a hot and dry one because the air cools slower and can rise further up when its more moist.

As this warm and moist air rises from the surface, it if finds the atmosphere above the surface is cooling as well, then the atmosphere has become unstable.


Unstable air is when the atmosphere is cooler above than it is below. By its very definition, that means the air is unstable and prone to BIG changes.

When this happens, the warm and moist air at the surface lifts into air which is cooler than it is. Because of this, our air rising is buoyant, or it can continue to move upwards. Buoyant air will rise until it reaches a level of equilibrium, or stability, with the air around it.

As that warm and moist air rises, it eventually will form into clouds and continue to rise. Its at that point we see the atmosphere give berth…to a thunderstorm.