Today’s the Friday before a three-day weekend, and just about everyone is checked-out until the day after Memorial Day. Since it’s just you and me around these parts, how about we get our nerd on while nobody’s looking? By popular request, here’s an explainer on how to analyze instability a SKEW-T chart by hand.

First, to understand what’s going on and that the exercise isn’t an arbitrary doodle, we have to do some basic weather geekery.

SKEW-T Charts

SKEW-T charts are populated with data collected by weather balloons as they ascend to the heavens, looking at temperature, dew point, wind speed and direction, and air pressure to give us an idea of what’s going on through a small slice of the atmosphere. If you do this in enough locations—there are a few hundred weather balloon launch points around the world—it gives us a pretty accurate depiction of what the weather is doing at that moment.

Not only do meteorologists feed this data into weather models to give them a starting point to run their simulations, but we can observe the data itself (both raw and charted) to see what could happen at a specific location. This is incredibly useful in severe weather situations, as these SKEW-T charts give you a great visual indication of wind shear and instability through that layer of the atmosphere, telling you what kind of thunderstorms could develop and how intense they can grow.

Traditional SKEW-T charts show temperature, dew point, and wind speed, but some of them (like the ones available from the SPC and shown above) are packed with hodographs and more metrics than you’ll ever need to evaluate the atmosphere.

Rising Air

‘The whole process involves theoretical parcels of rising air—the example usually given in meteorology courses is imagine a parcel of air the size of a basketball. As these parcels of air rise through the atmosphere, they don’t instantly cool down to the temperature of the environment around them, and as such they’re typically warmer and less dense than the environment, giving them positive buoyancy.

The rate at which these parcels of air cool as they rise into the sky depends on how much water vapor is in the rising air. Warm air expands and cool air contracts, so warmer air can hold more water vapor than cooler air. As the rising air ascends, cools, and contracts, it will be able to hold less and less water vapor, eventually allowing the parcel to reach saturation, or 100% relative humidity.

Why does that matter? It affects how quickly the rising air can cool off as it rockets skyward. Water has a higher heat capacity than air, so it takes longer to absorb and release heat, so saturated air cools more slowly than unsaturated air.

Lapse Rates

The rate at which air cools with height is known as the “lapse rate.” If you read the forecast discussion written by your local NWS office, you’ll often see meteorologists talking about steep lapse rates, which are conducive to thunderstorm development.

Rising air that isn’t saturated cools at what’s called the “dry adiabatic lapse rate,” which is about 9.8°C per kilometer. In other words, if the temperature of a parcel of rising air is 10°C at one kilometer above ground level and it cools at the dry adiabatic lapse rate, it will be about 0.2°C by the time it reaches two kilometers above ground level. Once the rising parcel of air reaches saturation, it begins cooling at the “moist (or saturated) adiabatic lapse rate,” which is somewhere around 5°C per kilometer.

It’s important to know these variables because rising air has to stay warmer than the environment in order to continue rising to the top of the atmosphere—if rising air takes longer to cool off, the better its odds of staying warmer than the environment, and the longer and faster it can rise.

CAPE and CIN

CAPE is an acronym for Convective Available Potential Energy, or the fuel that feeds thunderstorms their energy. CAPE is measured in joules per kilogram (j/kg), and higher levels of CAPE equate to stronger updrafts and more intense thunderstorms.

CIN is short for “Convective Inhibition,” or the opposite of CAPE. CIN is an area of the atmosphere where the environment is warmer than the parcels of rising air, preventing the air from rising beyond the area of CIN. This is usually called a “cap,” because it caps the atmosphere like a ceiling, preventing air from rising beyond the warm layer.

Surface temperatures can rise sufficiently through the day that rising air can surpass this inversion, “breaking the cap.” Thunderstorms can turn severe in a hurry if rising air can break the cap on a high CAPE day.

The Chart Itself

It can be daunting to figure out what’s going on on a SKEW-T chart, especially if you’ve never really looked one over before. For most purposes, the only two lines you really need to know are the environmental temperature and environmental dew point, which are two solid lines that trace from bottom to top. These are the variables directly measured by the weather balloon—temperature is always to the right, and dew point is always to the left (unless the air is saturated, in which case they overlap).

The other lines—the ones I usually say not to worry about—show the dry and moist adiabatic lapse rates, as well as the mixing ratio, which is important in calculating CAPE by hand.

Finding CAPE and CIN

For this example, we’ll use a model-generated SKEW-T chart for the Texas/Mexico border on Tuesday.

We start by finding the temperature and dew point at the surface. (The quality will get a little wonky because, to do this in a photo editing program, you have to zoom all the way in. This is what you would see if you used Paint or Paint.NET.)

The surface temperature is the starting point for the rising parcel of air. We want to lift this parcel through the atmosphere, but it’s not saturated yet, so we trace its path into the atmosphere parallel to the nearest dry adiabat:

Since the rising parcel of air started at the surface dew point, we trace the surface dew point into the atmosphere using the mixing ratio line until it intersects the parcel path. The point where they intersect is known as the Lifted Condensation Level, or LCL. This is the point where rising air reaches its saturation point, and begins cooling more slowly as it rises.

The point where a parcel of rising air becomes warmer than the environment is known as the Level of Free Convection, or LFC, and this it’s at this point that the air can rise freely through the atmosphere until it once again becomes colder than the environment. Sometimes, like we see above, the LCL and LFC basically overlap with one another.

Once you’ve found the LCL, you trace the path of the parcel of air parallel to the nearest moist adiabat until it crosses the environmental temperature. The point where the parcel temperature equals the environmental temperature and stops rising is known as the equilibrium level, or EL.

The entire area where the difference between the environmental temperature and the parcel temperature is positive (that is, the parcel line is to the right of the temperature line) is the area of CAPE. The longer and thicker the CAPE, the greater the instability, and the stronger the thunderstorms you’ll see. In this instance, the CAPE for the Texas/Mexico border on Tuesday was over 4,000, and storms did indeed form.

To sum it up:

  • Trace the surface temperature parallel to the nearest dry adiabat.
  • Trace the surface dew point parallel to the nearest mixing ratio line.
  • The point where the two lines intersect is the LCL, and air rises at the moist adiabatic lapse rate from this point.
  • Trace the path of the parcel parallel to the nearest moist adiabat until it crosses the environmental temperature.
  • CAPE consists of the positive area between the parcel and the environemental temperature from the LFC to the EL, while CIN consists of the negative area between the two (if the parcel is cooler than the environment).

Historical Examples

One of the worst tornadoes in American history occurred on May 27, 1997 in central Texas, killing more than two dozen people as the F5 leveled homes and businesses in the town of Jarrell.

It takes an intense supercell to sustain that kind of a tornado, and the CAPE value at Dallas-Fort Worth that evening? 6,706 j/kg. Usually it only takes 2,500 j/kg to sustain a hefty supercell.

Here’s another example, this time from Springfield, Missouri, on May 22, 2011.

The weather balloon and its instruments had a hard time holding it together through the atmosphere, but it gave us just enough to know that southwestern Missouri was dealing with an extremely unstable atmosphere, as the CAPE over Springfield was 4,471 j/kg that evening.

This sounding was taken two hours after the catastrophic EF-5 tornado leveled much of south Joplin, Missouri, about 70 miles west of Springfield.

[Images: Bentley Smith and Nicholas A. Tonelli via Flickr | SKEW-T charts via SHARPpy, TwisterData, and University of Wyoming, with modifications by the author]

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