CAPE, CIN And The Lifted Index

The previous lesson described stability qualitatively. Forecasters need numbers: how much energy is available to a thunderstorm updraft, and how hard is it to get that updraft started? Those questions are answered by CAPE, CIN, and the Lifted Index, all calculated by following an imaginary air parcel upward on a sounding.

What Are the LCL, LFC and Equilibrium Level?

Picture a parcel of air lifted from near the ground:

1. It rises dry adiabatically, cooling at 9.8 C per kilometer, until it saturates. That height is the **Lifted Condensation Level (LCL)**, which is about where cumulus bases form.
2. Above the LCL it cools at the slower moist adiabatic rate.
3. At first it may still be colder than the environment, so it has to be forced upward by a front, a hill, or an outflow boundary.
4. Eventually it may become warmer than its surroundings. That height is the **Level Of Free Convection (LFC)**. From here on, the parcel rises on its own.
5. It keeps rising until it is again cooler than the environment, usually near the tropopause. That height is the **Equilibrium Level (EL)**, where storm anvils spread out.

A quick rule: the LCL height in feet is roughly 400 times the difference between temperature and dew point in degrees Fahrenheit (or about 125 meters per degree Celsius of dew point depression).

What Is a High CAPE Value?

CAPE, Convective Available Potential Energy, is the total buoyant energy a parcel gains between the LFC and the EL. On a sounding diagram it is the area between the parcel's path and the environmental temperature curve, where the parcel is warmer. It is measured in joules per kilogram (J/kg).

Typical values:

• Below 500 J/kg: weak instability; showers and weak storms.
• 500 to 1,500 J/kg: moderate instability; ordinary thunderstorms.
• 1,500 to 2,500 J/kg: strong instability; severe storms possible with other ingredients.
• Above 2,500 J/kg: very strong; values of 4,000 to 6,000 J/kg are seen on some Great Plains outbreak days.

In theory, the maximum updraft speed is roughly the square root of twice the CAPE. At 2,000 J/kg that gives about 63 meters per second, or 140 mph. Real updrafts are slower because of mixing, water loading, and pressure effects, often roughly half the theoretical value, but still strong enough to suspend large hail.

What Is CIN in Weather?

**CIN*, Convective Inhibition, is the energy that must be *supplied to lift a parcel from the ground to its LFC, through the layers where it is colder than its surroundings. It is also measured in J/kg and is often shown as a negative number.

• 0 to minus 25 J/kg: weak; storms can form easily.
• minus 25 to minus 100 J/kg: moderate; a trigger like a front or outflow is needed.
• More negative than minus 100 J/kg: strong; storms are unlikely unless the lid is removed.

What Is a Capping Inversion?

Large CIN often comes from a Capping Inversion, a layer of warm air aloft, commonly between 850 and 700 hPa. Over the southern Great Plains, this warm layer is often the elevated mixed layer blown northeastward from the high deserts of Mexico and the southwestern United States.

A cap is a double-edged sword. It keeps storms from forming early in the day, letting heat and moisture build beneath it, so CAPE grows. If a front or dryline finally breaks the cap late in the day, a few isolated storms can erupt into that stored energy with nothing to compete with them. Isolated storms often become the strongest supercells. If the cap holds, the day ends with nothing but haze. Forecasters sometimes call these "loaded gun" soundings.

What Does a Negative Lifted Index Mean?

The **Lifted Index (LI)** is an older, simpler measure. Lift a surface parcel to 500 hPa (about 18,000 feet) and subtract its temperature from the environmental temperature there:

LI = environmental temperature at 500 hPa minus parcel temperature at 500 hPa

• Positive: stable.
• 0 to minus 2: marginal instability.
• minus 3 to minus 5: moderately unstable.
• minus 6 to minus 9: very unstable.
• minus 10 or lower: extremely unstable.

The LI looks at only one level, so it misses details that CAPE captures, but it is quick and still printed on many sounding displays.

Instability Is Only One Ingredient

Severe storms need moisture, instability, lift, and, for organized storms, Wind Shear. Huge CAPE with no lift produces no storms. Moderate CAPE with strong shear can produce tornadic supercells. Always read these numbers together.