Instability.

We talk about it a lot. We talk about as if you know what it is. But you may not know, we may be assuming, and you may be lost.

So let’s explain. First…a definition from the officially official experts:

The tendency for air parcels to accelerate when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted. Instability is a prerequisite for severe weather – the greater the instability, the greater the potential for severe thunderstorms.

NWS Glossary

The basics of instability is that warm air in the atmosphere tends to rise, so when especially cold air is above warm and humid air — expect instability.

Here’s a video to explain it deeper:

Why is Instability Important?

You may have heard the term in science class, warm air rises.

This may or may not have been because of a crude joke or seven in middle school due to certain…ahem…nevermind.

With weather, it’s the same concept. Instability is when warmer air exists underneath cooler air. When this is the case, the air below will have a tendency to rise — thus the air is unstable.

Storm towers need instability to grow, so instability is one of the essential ingredients to thunderstorm formation.

How to Measure Instability

Instability is commonly measured by an index called convective available potential energy, or CAPE. 

CAPE has several different types, from surface-based CAPE, to mixed-layer CAPE, to most-unstable CAPE.

But what do the numbers measuring CAPE mean exactly?

CAPE values of 0-1000 are considered positive CAPE, but pretty weak. You can get storms with this CAPE — but they’re not going to be extremely robust. 

1000-2000 CAPE values are considered weakly unstable. 

2000-3000 CAPE values are considered moderately unstable.

3000-4000 CAPE values are considered strongly unstable. 

4000+ CAPE values are considered extremely unstable.

For our considerations, anything over 2000 is considered pretty robust and should be monitored especially closely.