Explainer: Atmospheric Instability Measured through CAPE and Caps

There's a lot of talk about a potentially major tornado outbreak across parts of the Plains states this weekend, with tornadoes possible from Texas to Nebraska on both Saturday and Sunday. There's a good chance that the hype is just that — hype. But there is also a chance that the hype is warranted.

To understand why there's uncertainty in the forecast, it requires a little beefing-up of your weather knowledge. (Also, please see the update at the very bottom of this post.)

The Cap

Warm air is less dense than cool air, so when air warms up at the surface, it begins to rise. This process, called convection, is the spark that triggers all thunderstorm activity on earth. The rising air must remain warmer than the air around it to continue rising, or else it will sink and a thunderstorm won't form.

A cap — short for capping inversion — occurs when cool air lies below warm air in the lower part of the atmosphere near the surface. As cool air is denser than warm air, the cool air cannot rise — in other words, it's capped.

Since this cooler air cannot rise on its own, it requires help from either a forcing mechanism (like a cold front) or the surface temperature warming up enough that the air is able to rise on its own.

We can identify caps using products called Skew-T charts.

Skew-T Charts

Explainer: Atmospheric Instability Measured through CAPE and Caps

Skew-T charts allow meteorologists to analyze the vertical makeup of the atmosphere. Skew-Ts are created using data taken from the hundreds of weather balloons released around the world twice a day. A Skew-T chart shows a vertical slice of the atmosphere, telling us temperature, dew point (moisture), pressure, and wind data from the surface until the balloon pops well above 50,000 feet.

The Skew-T chart above is a forecast that was generated by a weather model (the NAM) for eastern Nebraska for Sunday evening. Don't worry if you don't understand how to read it — they can be confusing even for meteorology students.

For the purpose of this post, here's what you need to know:

  • The thin red line that squiggles from bottom to top is the temperature of the atmosphere (also called the environmental temperature).
  • The thin green line that squiggles from bottom to top is the dew point, or the temperature to which the air temperature would need to drop in order to reach 100% humidity.

Explainer: Atmospheric Instability Measured through CAPE and Caps

The thin lines that move across the chart horizontally are called isobars, or lines of constant air pressure. Air pressure, measured in millibars on the scale to the left, drops rapidly with altitude. We use specific pressure levels to analyze different weather features — for instance, the jet stream is typically located between 200-300 millibars.

Explainer: Atmospheric Instability Measured through CAPE and Caps

The lines that move across the chart from bottom-left to top-right are isotherms, or lines of constant temperature. Temperatures are measured in Celsius, and the scale for temperatures is located along the bottom of the chart.

A Bubble of Air

Now, picture a bubble of air the size of a basketball, but instead of calling it a bubble, we'll call it a "parcel" because we're adults. This parcel of air starts near the surface and warms up through daytime heating. We'll assume that the humidity is less than 100%, so the parcel of air is not fully saturated with moisture.

Dry air warms and cools much faster than moist air. A parcel of dry air will cool by about 10°C for every 1 kilometer it rises through the atmosphere. Once this parcel of air cools to its dew point, its humidity reaches 100% and the parcel is now fully saturated with moisture.

Since moist air warms and cools more slowly than dry air, the now-saturated parcel of air will cool more slowly as it rises through the atmosphere. This saturated parcel of air will cool at a rate of around 5°C per kilometer.

We can use Skew-T charts to track the path of a parcel of air as it rises through the atmosphere. I've drawn the path of a rising parcel on the following Skew-T chart showing the predicted atmosphere over eastern Nebraska this Sunday evening. The parcel of air appears as the thick black line.

Explainer: Atmospheric Instability Measured through CAPE and Caps

Remember that the environmental temperature is shown by the thin red line, and the temperature of the parcel of air is shown by the thick black line.

When the parcel of air is to the left of the temperature line, it is cooler than the environment around it.

When the parcel of air is to the right of the temperature line, it is warmer than the environment around it.

CAPE Isn't Just for Superman...

This brings us to two terms that are vital for understanding severe weather: CAPE and CIN.

CAPE stands for Convective Available Potential Energy. It's essentially the fuel that feeds a thunderstorm its energy. The more CAPE available, the stronger the storms can get. CAPE is measured in joules per kilogram (don't worry about it), and values that get above 2,000 are generally ideal for strong thunderstorms.

CIN is the exact opposite of CAPE. It stands for Convective Inhibition, and it measures how much the atmosphere resists rising air. The CIN essentially measures the strength of the cap — the more CIN, the stronger the cap, and the less likely thunderstorms are.

Let's look back to our parcel of air that's rising through the atmosphere.

Explainer: Atmospheric Instability Measured through CAPE and Caps

When the parcel of air is to the left of the temperature line — cooler than the environment — the area between the parcel line (black) and the temperature line (red) measures the amount of CIN in the atmosphere.

When the parcel of air is to the right of the temperature line — warmer than the environment — the area between the parcel line (black) and the temperature line (red) measures the amount of CAPE in the atmosphere.

The above Skew-T shows that, while there is a lot of potential CAPE to fuel thunderstorms, there is also quite a bit of CIN that's suppressing convection. Unless the atmosphere warms up enough to overcome the effects of the cap (CIN), air will not be able to rise and thunderstorms are not likely to form.

Let's look at another example.

Explainer: Atmospheric Instability Measured through CAPE and Caps

This Skew-T is from earlier in the week in Texas. In this map, there's virtually no CIN. There is a very, very small amount near the surface — so small that I had to zoom in to highlight it in blue. The atmosphere is almost all rising air, so thunderstorms had a very easy time forming this day.E

How Does the Cap Erode?

The most common way to break the cap is to warm up the air at the surface. If the surface air warms up enough, the parcels of air will be able to rise and potentially create thunderstorms. When convection breaks the cap, it can be dangerous — this causes very strong updrafts and, consequently, very strong thunderstorms.

Getting to the Point...

So, where am I going with this?

There is the potential for a tornado outbreak on both Saturday and Sunday across areas from Texas to Nebraska. The social media hype machine is in full blast, working people up with hype without telling them the caveats.

There are two huge problems that could render the fretting moot:

  1. Almost all of the convection will have to fight a very strong cap, as demonstrated by the Nebraska Skew-T I used as an example above. If the surface doesn't warm up enough and the cap doesn't erode, thunderstorms will have a very hard time forming.
  2. The thunderstorms are also going to have to fight dry air, and dry air is a death sentence for tornadoes.

If the cap erodes and if enough moisture is present in the area, there will be enough instability and wind shear from Texas to Nebraska to trigger very strong severe thunderstorms, including the potential for tornadoes. That cannot be ruled out. But the forecast is uncertain right now. If and when storms form, the Storm Prediction Center notes that the biggest risk will be hail the size of golf balls or larger.

One of the biggest question marks in the field of weather forecasting is whether or not capping inversions will erode. Meteorologists have good tools available to get a reasonable idea if they will or not, but sometimes it comes down to watching and waiting.

Whenever you see pages sharing images on Facebook and Twitter that make it sound like the sky is falling and doom is here, fact check them. Go look at what Gary England or James Spann or the Storm Prediction Center are saying about the weather. More often than not, those heavily-followed pages on social media are run by high schoolers or conspiracy theorists with a backwards understanding of how the weather works.

This situation comes down to following an old cliché: prepare for the worst, but hope for the best.

Update 7:40 PM CDT, 4-26-2014 | The Storm Prediction Center notes that there's a strong chance for a violent tornado outbreak across the Arklatex region of the southern U.S. on Sunday. The severe weather over Texas and Oklahoma was nicely staved off by the cap on Saturday, but this will not be the case on Sunday across portions of Oklahoma, Arkansas, and Louisiana.

Update May 27, 2014 | I've changed the title of this post to more accurately reflect how it's essentially a primer on CAPE and convective inhibition, using storms on April 26, 2014 as a case study.

[Top image by the author, all other images via TwisterData]