In short: CAPE is the fuel. It measures how much energy a rising bubble of air gains because it is warmer than the air around it. More CAPE allows a stronger updraft. CIN is the lid that keeps the fuel from lighting. Neither one, on its own, tells you what kind of storm you get: that is the job of wind shear, covered in Wind Shear & Helicity.

A thunderstorm is a column of rising air. Air rises on its own only when it is warmer, and so lighter, than the air beside it. Forecasters picture a small bubble of air, called a parcel, and lift it on paper. As it rises it cools. If it ends up warmer than its surroundings, it keeps going. CAPE adds up how much warmer, over how deep a layer.

CAPE: the energy an updraft can tap

CAPE stands for convective available potential energy. It is measured in joules per kilogram (J/kg). On a Skew-T it is the area between the lifted parcel’s temperature and the real air temperature, from the level where the parcel first turns warmer than its surroundings to the level where it stops being warmer.

1 2 3 4 5 6
  1. 1
    CAPE. The parcel (dashed, amber) is warmer than the air (solid). The bigger this area, the stronger the possible updraft.
  2. 2
    CIN. The parcel is cooler than the air. It has to be pushed through this layer.
  3. 3
    LCL. The parcel saturates. This is cloud base.
  4. 4
    LFC. The level of free convection. Above it, the parcel rises on its own.
  5. 5
    EL. The equilibrium level. The parcel stops being warmer. The anvil spreads out near here.
  6. 6
    The axes. Height runs up the page, temperature from cold (left) to warm (right). Drawn without the skew for clarity.
A parcel's trip up through the atmosphere, simplified. Real soundings are drawn on a skewed chart, but the areas mean the same thing.

How to think about the number:

  • It is a ceiling, not a forecast. In theory, CAPE converts to an updraft speed. In practice, real updrafts reach only part of that, because they mix with drier air around them and carry the weight of rain and hail.
  • Big is common in spring on the Plains. Values of 2,000 to 4,000 J/kg are routine on major severe days there. The Gulf Coast in summer can top that and produce only ordinary afternoon storms, because shear is weak.
  • Small can still be dangerous. In the cool season, the Southeast often sees damaging storms and tornadoes with a few hundred J/kg, because the wind shear is extreme. Forecasters call this “high shear, low CAPE.”
  • Shape matters as well as size. The same total can be spread thin through a tall column or packed into a fat layer near the ground. CAPE packed low helps an updraft accelerate quickly near the surface, which matters for tornadoes. That is what the sounding’s 3CAPE row (CAPE in the lowest 3 km) is for.

In MesoRadar: the National tab, group HRRR Instability, has CAPE (Surface-Based), CAPE (Mixed-Layer) and CAPE (Most-Unstable), one frame per forecast hour (Pro). These are the HRRR's own parcels (its mixed layer is 90 mb, a little shallower than SPC's 100 mb), so values can differ slightly from SPC's mesoanalysis; the For forecasters notes under each map give the exact layers. The Skew-T tool and the HRRR groups.

Surface-based, mixed-layer, most-unstable

The three CAPE values lift three different parcels. The SPC definitions, in plain words:

VersionWhich air is liftedWhen it is the one to read
Surface-based (SBCAPE)The air right at the groundAfternoons, once the surface has warmed. Overstates CAPE when only a thin surface layer is warm and moist
Mixed-layer (MLCAPE)An average of the lowest 100 mb, about the lowest kilometerThe steadiest all-round number. A storm feeds on a deep layer, not a thin skin
Most-unstable (MUCAPE)The most buoyant air in the lowest 300 mb (about the lowest 3 km)Night, or north of a warm front, where storms feed on warm air above a cool surface layer. These elevated storms can drop large hail

When MUCAPE is large but SBCAPE is near zero, the most buoyant air sits above the ground. Storms that form there are called elevated. They can still make large hail and frequent lightning, but they feed on air above the cool surface layer, not on the air at the ground, so tornadoes are much less likely.

CAPE, surface-based (SBCAPE)

Quick read

This map lifts the air right at the ground, the air a storm rooted at the surface would take in. It reads highest on a hot, humid afternoon and falls off after sunset as the ground cools. Lift and wind shear do not show on it.

  1. 100
  2. 500
  3. 1000
  4. 1500
  5. 2000
  6. 2500
  7. 3000
  8. 4000
  9. 5000

J/kg. Clear below 100. Typical values, not severity classes: a few hundred J/kg can feed damaging storms in the cool season when the shear is strong; 2,000 to 4,000 J/kg is common on big spring days on the Plains (see CAPE: the energy an updraft can tap).

For forecasters

Definition
Convective available potential energy: the positive buoyancy of a lifted parcel, integrated from the level of free convection (LFC) to the equilibrium level (EL). CAPE = g ∫ (Tv,parcel − Tv,env) / Tv,env dz over LFC to EL.
Units
J/kg (m²/s²)
Parcel
Surface-based: the parcel starts at the HRRR's lowest level, at the ground.
How it is made
Native HRRR output (NCEP Unified Post Processor). MesoRadar reads the value from the HRRR file and only colors it; nothing is recomputed. GRIB field CAPE, level "surface".
Caveats
Overstates the energy when only a thin skin of air near the ground is warm and moist, which is common on summer afternoons and rare at night. The post-processor's parcel method differs in detail from SPC's, so the same sounding can give a somewhat different number on SPC's mesoanalysis or in the Skew-T tool.
Model
HRRR, about 3 km grid, hourly runs. NOAA GSL HRRR.
Sources
SPC mesoanalysis help, surface-based CAPE/CIN; Thompson, Edwards, Hart, Elmore and Markowski 2003.

CAPE, mixed-layer (MLCAPE)

Quick read

The same energy, but for air averaged through the lowest part of the atmosphere instead of the thin layer at the ground. Because it averages a deeper layer, a thin warm, moist skin at the ground does not inflate it.

  1. 100
  2. 500
  3. 1000
  4. 1500
  5. 2000
  6. 2500
  7. 3000
  8. 4000
  9. 5000

J/kg, on the same colors as the other two CAPE maps. Clear below 100. Usually lower than surface-based CAPE on a warm afternoon.

For forecasters

Definition
CAPE (LFC to EL) of a parcel with the mean temperature and moisture of a layer next to the ground.
Units
J/kg
Parcel
The HRRR's mixed layer is the lowest 90 mb, the mean of its three lowest 30 mb layers (GRIB level "90-0 mb above ground"). SPC's mesoanalysis mixes the lowest 100 mb, so values here can differ slightly from SPC's MLCAPE.
How it is made
Native HRRR output (NCEP Unified Post Processor). MesoRadar reads the value from the HRRR file and only colors it; nothing is recomputed. GRIB field CAPE, level "90-0 mb above ground".
Caveats
A deep mixed layer smooths out a shallow moist layer at the ground; where moisture is shallow, this map runs well below surface-based CAPE.
Sources
SPC mesoanalysis help, 100 mb mixed-layer CAPE/CIN; HRRR GRIB2 inventory (wrfsfc file).

CAPE, most-unstable (MUCAPE)

Quick read

The energy of the most buoyant air near the ground, wherever it sits in the lowest 300 mb, about the lowest 3 km. It catches storms that feed on warm, humid air above a cool surface layer, such as at night or north of a warm front. Those storms can drop large hail even when the surface-based CAPE is near zero.

  1. 100
  2. 500
  3. 1000
  4. 1500
  5. 2000
  6. 2500
  7. 3000
  8. 4000
  9. 5000

J/kg, on the same colors as the other two CAPE maps. Clear below 100. At least as large as the surface-based value almost everywhere, because the air at the ground is one of the parcels it tries.

For forecasters

Definition
CAPE (LFC to EL) of the most unstable parcel near the ground. The map uses the HRRR's most-unstable CAPE: the post-processor searches the model levels in the lowest 300 mb, picks the one with the highest theta-e, and lifts it. This is close to SPC's MUCAPE.
Units
J/kg
Parcel
The highest theta-e level in the lowest 300 mb (GRIB level "255-0 mb above ground"; the file labels it 255 mb, the search depth is 300 mb), the same search SPC uses. Values can still differ a little from SPC's mesoanalysis, which starts from its own analysis of the air rather than the HRRR's.
How it is made
Native HRRR output (NCEP Unified Post Processor). MesoRadar reads the value from the HRRR file and only colors it; nothing is recomputed. GRIB field CAPE, level "255-0 mb above ground". The Supercell Composite map uses this same field. Earlier versions of the map used the HRRR's best CAPE (level "180-0 mb above ground", six 30 mb layer averages in the lowest 180 mb), which averaged each 30 mb layer and so often read lower, sometimes below the surface-based value.
Caveats
A large gap between this map and surface-based CAPE means the best air is above the ground: storms that form there are elevated, and much less likely to produce tornadoes.
Sources
SPC mesoanalysis help, most-unstable CAPE; HRRR GRIB2 inventory (wrfsfc file).

CIN: the cap

CIN, convective inhibition, is the energy needed to shove a parcel up through a layer where it is cooler than its surroundings. The usual cause is a warm layer of air a mile or so up, called a cap. A cap is why a hot, humid day with huge CAPE can stay sunny until evening.

  • A little CIN is a feature. It holds storms back until the strongest lift arrives, so the few storms that do form get the whole warm, moist air mass to themselves. Isolated storms are often the most dangerous kind.
  • A lot of CIN wins. SPC’s Significant Tornado Parameter shows how forecasters weigh it: CIN weaker than about 50 J/kg (surface-based in the fixed-layer version, mixed-layer in the effective-layer one) costs nothing, and the term falls to zero at 200 J/kg. Beyond that, storms rooted at the surface are unlikely.
  • CIN changes by the hour. Afternoon heating, moisture flowing in, and rising air ahead of a front or upper-level wave all erode it. That is why the hourly frames matter.

In MesoRadar: CIN (Surface-Based) and CIN (Mixed-Layer) sit beside the CAPE rows in HRRR Instability (Pro).

CIN, surface-based (SBCIN)

Quick read

The energy needed to push air from the ground up through the stable layer above it, the cap, before it can rise on its own. The map colors the size of that barrier: deeper colors mean a stronger cap and fewer storms rooted at the ground.

  1. 10
  2. 25
  3. 50
  4. 100
  5. 150
  6. 250
  7. 400

J/kg. CIN is a negative number; the map colors its size, so "50" here is CIN of −50 J/kg. Clear below 10. For reference, SPC's tornado parameter starts to penalize CIN beyond about −50 J/kg and drops to zero at −200 J/kg.

For forecasters

Definition
Convective inhibition: the negative buoyancy of a lifted parcel, integrated from its starting level to the level of free convection (LFC).
Units
J/kg (reported ≤ 0)
Parcel
Surface-based: the same parcel as surface-based CAPE.
How it is made
Native HRRR output (NCEP Unified Post Processor). MesoRadar reads the value from the HRRR file and only colors it; nothing is recomputed. GRIB field CIN, level "surface". MesoRadar flips the sign for the colors only, so the scale runs from weak to strong like the other maps; the value you read is the size of CIN.
Caveats
CIN changes fast with afternoon heating, moisture flowing in and lift ahead of fronts; read it hour by hour, not as one frame. Where there is no CAPE at all, CIN is not meaningful.
Sources
SPC mesoanalysis help, surface-based CAPE/CIN; SPC help, fixed-layer STP (the CIN term).

CIN, mixed-layer (MLCIN)

Quick read

The cap again, measured for air averaged through the lowest part of the atmosphere. It often reads stronger than surface-based CIN on a hot afternoon, and it is the version SPC uses in its effective-layer tornado parameter.

  1. 10
  2. 25
  3. 50
  4. 100
  5. 150
  6. 250
  7. 400

J/kg, on the same colors as surface-based CIN; the map colors the size, so "50" means −50 J/kg. Clear below 10.

For forecasters

Definition
CIN of the mixed-layer parcel, from its starting level to the LFC.
Units
J/kg (reported ≤ 0)
Parcel
The HRRR's lowest 90 mb mixed layer (GRIB level "90-0 mb above ground"); SPC mixes 100 mb.
How it is made
Native HRRR output (NCEP Unified Post Processor). MesoRadar reads the value from the HRRR file and only colors it; nothing is recomputed. GRIB field CIN, level "90-0 mb above ground", sign flipped for the colors.
Caveats
As for surface-based CIN: it erodes by the hour, and means little where there is no CAPE.
Sources
SPC mesoanalysis help, 100 mb mixed-layer CAPE/CIN; SPC help, effective-layer STP.

Cloud base: LCL, LFC and EL

Three levels on the parcel’s path have names. The Skew-T tool lists all three for the surface parcel in its LCL / LFC / EL row.

  • LCL, the lifting condensation level. Where the rising parcel cools enough to saturate. It is cloud base. A low LCL means humid air near the ground. Low cloud bases go with tornadoes: SPC’s tornado parameter gives full credit to LCLs below 1,000 m and none above 2,000 m (the surface-based LCL in the fixed-layer version that the map uses, the mixed-layer LCL in the effective-layer version). A high LCL with dry air below it favors strong downdrafts and damaging wind instead. The Skew-T tool’s LCL is the surface parcel’s (tagged SB), which often reads a little lower than the mixed-layer one.
  • LFC, the level of free convection. Where the parcel first becomes warmer than the air around it. Below it, something has to lift the air: a front, a dryline, an outflow boundary, or rising terrain. A low LFC is easy to reach.
  • EL, the equilibrium level. Where the parcel cools back to the temperature of its surroundings. This is roughly where the anvil spreads out. A strong updraft overshoots it, which shows up on satellite as a bubbling dome above the anvil.

Lapse rates: how fast it cools with height

A lapse rate is how quickly temperature drops as you go up, in degrees Celsius per kilometer. Steep lapse rates mean cold air sits over warm air, so a lifted parcel stays warmer than its surroundings for longer.

  • SPC’s reference points: a lapse rate below 5.5 to 6 °C per km is stable, above 9.8 °C per km is absolutely unstable, and anything between is conditionally unstable, which means it is unstable if the air is moist enough.
  • 700 to 500 mb lapse rates (about 3 to 5.5 km up) of 7 °C per km or more usually mean a plume of hot, dry air from the desert Southwest or Mexico has drifted over the Plains. Forecasters call it an elevated mixed layer. It builds CAPE and, underneath it, the cap.
  • 0 to 3 km lapse rates that are steep mean the low levels are well mixed. That weakens the cap and can favor strong gusts.

In MesoRadar: the Skew-T tool's More fold lists Lapse rate 0-3 km AGL, Lapse rate 700-500 mb and Lapse rate 3-6 km AGL (Pro).

DCAPE: fuel for the downdraft

Downdraft CAPE runs the idea in reverse. It asks how much energy a parcel of air gains as it sinks, cooled by rain evaporating into it. Dry air in the middle levels over a deep, dry layer near the ground makes for large DCAPE. SPC’s summary is simple: larger DCAPE is associated with stronger downdrafts. It is one of the numbers behind the damaging-wind forecast in Squall Lines, Bow Echoes & Derechos.

Theta-E: heat and moisture in one number

Equivalent potential temperature, theta-e, folds temperature and humidity into one value. Warm, humid air has high theta-e. On a map, a tongue of high theta-e pointing north from the Gulf traces where the most unstable air is flowing, and storms often fire along its edges.

In MesoRadar: Theta-E (2m) is in HRRR Instability (Pro).

Theta-E, 2 m (equivalent potential temperature)

Quick read

Heat and humidity folded into one number, mapped at 2 m. Read it for tight changes over short distances, such as fronts and drylines, where storms often form.

  1. 250
  2. 270
  3. 285
  4. 300
  5. 315
  6. 330
  7. 340
  8. 350
  9. 360
  10. 375

Kelvin. The map paints everywhere. Summer moist axes often reach 345 K and higher; dry or cold air is below 300 K.

For forecasters

Definition
The temperature a parcel would have if all its water vapor condensed, releasing its latent heat, and it were then brought dry-adiabatically to 1000 mb.
Units
K
Layer
2 m above ground.
How it is made
Computed by MesoRadar from three HRRR fields: 2 m temperature, 2 m dewpoint and surface pressure, with Bolton (1980) equation 43, using his equation 15 for the temperature at the lifting condensation level. The dewpoint is capped at the temperature.
Caveats
A surface value only; it does not see the cap or the air aloft. Read it for gradients and axes rather than for a single number.
Sources
Bolton 1980, the computation of equivalent potential temperature (Monthly Weather Review 108); NWS glossary, equivalent potential temperature.

Putting it together

A quick read of the fuel, in order:

  1. Is there CAPE where the lift will be? Read MLCAPE along the front or dryline, not the biggest number on the map.
  2. Is it rooted at the surface? Compare SBCAPE and MUCAPE. A large gap means elevated storms.
  3. Is the cap going to break? Watch CIN shrink hour by hour where the lift arrives.
  4. Where is cloud base? Low LCLs on a day with strong low-level wind shear deserve attention. High LCLs over dry air point to wind.
  5. Now go read the shear. Fuel without organization makes short-lived storms.

Key points

  • CAPE is the energy a rising parcel gains by being warmer than its surroundings, in J/kg
  • Surface-based, mixed-layer and most-unstable CAPE lift different air; mixed-layer is the steadiest
  • CIN is the lid; a little focuses storms, a lot prevents them
  • Low cloud bases (LCL) favor tornadoes; high bases over dry air favor downdraft wind
  • Steep mid-level lapse rates build CAPE; DCAPE sizes up downdrafts
  • CAPE says how strong; shear says what kind

Common questions

What is a good CAPE value for severe storms?

There is no single cutoff. A few hundred J/kg can support severe storms when the wind shear is strong, which is common in the cool season in the Southeast. On a big spring day on the Plains, values of 2,000 to 4,000 J/kg are common. CAPE says how strong an updraft could get. Shear decides whether the storm organizes.

What is the difference between surface-based, mixed-layer and most-unstable CAPE?

They lift different air. Surface-based CAPE lifts the air right at the ground. Mixed-layer CAPE lifts an average of the lowest 100 mb (about the lowest kilometer), which is closer to what a storm really ingests. Most-unstable CAPE lifts the most buoyant air in the lowest 300 mb (about the lowest 3 km), which catches storms feeding on warm, moist air above a cool surface layer.

What does CIN mean?

Convective inhibition. It is the energy a parcel of air must be given to push it up through a warm, stable layer (the cap) before it can rise on its own. Some CIN is common on big days because it stops storms from going up everywhere at once.

Can there be storms with zero CAPE?

Lightning-producing storms need some buoyancy, but it can be small and shallow. In strongly forced cool-season events, lines of storms can produce damaging wind with very little CAPE, which is why forecasters never read CAPE alone.

Sources