Severe 101(Part 1): What is CAPE?

During severe weather season, we’re looking for two main things. How much potential is there for an updraft to spin (shear) and how much buoyancy is available (instability) to have the air rise. Synoptic scale forcing is another piece of the puzzle but typically if you have a severe weather outbreak…you’ve got the forcing.

CAPE or Convective Available Potential Energy is how meteorologists quantify instability. The larger the number, the more acceleration upward overall.

The idea behind instability is a simple one. Less dense air will rise through denser air, much like the oil settling on the top of a vinaigrette that hasn’t been shaken. Colder air is more dense than warm air, and water vapor is less dense than air. Going back to high school chemistry, you know this. Water is one oxygen and two hydrogens, yielding a molecular weight of 18.02 g/mol and dry air has a molar mass of 28.97 g/mol.

So the more water vapor in your air, the more buoyant that air is.

To stay with the HS chem theme, we can use the ideal gas law to show how temperature effects the density of air.

PV = nRT

Re-arranging and making D = n/V (mass over volume…density) we get:

(P/RT) = D

If we hold Pressure constant and since R is constant, you can see that if you raise the temperature the density will drop.

This is why warm air rises.

Next, I think it’s helpful to see graphically what CAPE is before looking at the math. I’ve pulled the Birmingham upper air sounding from 4/28/11 at 00z to demonstrate and to show you what scary looks like.

The subscript on the temperatures can be ignored, for now. It stands for virtual temperature and considers water vapor concentration as well. This is a concept I’ll dive into in a later post.

What this formula is telling you is the difference between the parcel temp and environmental temp is the driving force of convection, and is considered for every point in the vertical direction from the layer of free convection (LFC, or Zf in the formula) and the equilibrium level (EL, or Zn in the formula).

g is the acceleration due to gravity. dz just tells you to do the math from the bottom point to the top point. Sum it together (or, in other words, to find the area under the curve) and you have the potential energy of convection lifting the parcel over the entire area in question.

Finally, when I think about what is “high CAPE” or “low CAPE” I have ranges in my head:

Low: 0-1000 J/kg

Medium: 1000 – 2500 J/kg

High: 2500-4000 J/kg

Extreme: > 4000 J/kg

That’s all for this edition of Severe Weather 101. Next time, I’ll discuss the virtual temperature and how it is used and the differences between “fat” and “skinny” CAPE.

Jacob Caddell