CAPE
Also called: Convective Available Potential Energy
CAPE (Convective Available Potential Energy) is the amount of energy a rising parcel of air can draw from the atmosphere, measured in joules per kilogram (J/kg). The higher the CAPE, the faster a storm's updraft can rise, and the stronger thunderstorms can become once something sets them off.
Why It Matters
CAPE is the instability ingredient for thunderstorms. Storms also need moisture and a source of lift, and organized severe storms need wind shear. Forecasters look at CAPE to judge how strong updrafts could become if storms form, and strong updrafts are what hold hailstones aloft long enough to grow.
In theory, the peak updraft speed is the square root of twice the CAPE, so 2,500 J/kg works out to about 70 meters per second. Real updrafts reach roughly half the theoretical value, held back by the weight of rain and hail, mixing with the surrounding air, and evaporative cooling.
Typical CAPE Values
| CAPE (J/kg) | Instability |
|---|---|
| Below 1,000 | Marginally unstable (weak) |
| 1,000 to 2,500 | Moderately unstable |
| 2,500 to 3,500 | Very unstable |
| Above about 3,500 | Extremely unstable |
Bands are for a parcel lifted from the boundary layer. No CAPE value marks the point at which severe weather becomes certain.
How to Read It
- On a Skew-T sounding, CAPE is the area between the temperature of the rising parcel and the temperature of the environment, from the level of free convection (LFC) up to the equilibrium level (EL).
- The number depends on which parcel is lifted. Surface-based CAPE (SBCAPE) lifts air from the surface, mixed-layer CAPE (MLCAPE) uses the average of the lowest 100 mb, and most-unstable CAPE (MUCAPE) lifts the most unstable parcel in the lowest 300 mb.
- CAPE usually peaks in the mid to late afternoon, after the sun has heated the ground.
- Shape matters as well as size. The same CAPE in a tall, thin profile gives slower updraft acceleration than in a short, fat one.
What It Does Not Tell You
- High CAPE does not mean storms will form. The air still needs a trigger such as a front, a dry line, or an outflow boundary.
- A cap can hold everything down. Convective inhibition (CIN) is the energy a parcel must overcome to reach its level of free convection, and with high CIN and little lift, storms may never start even when CAPE is high.
- CAPE says nothing about storm type. Wind shear decides whether storms stay short-lived or organize into supercells and squall lines.
- Low CAPE is not an all-clear. Modest CAPE with strong wind shear can still produce tornadoes.
Frequently Asked Questions
What is a high CAPE value?
Values of 2,500 to 3,500 J/kg are considered very unstable, and values above about 3,500 J/kg extremely unstable. Thunderstorm environments often exceed 1,000 J/kg, and extreme cases exceed 5,000 J/kg.
Does high CAPE mean tornadoes?
No. CAPE measures how strong an updraft could be, not whether a storm will rotate. Tornadoes depend heavily on wind shear and on a storm forming in the first place. Some tornado days have modest CAPE and strong shear, and some days with very high CAPE produce no storms at all.
What is the difference between CAPE and CIN?
CAPE is the energy available to a parcel of air once it is rising freely. CIN, convective inhibition, is the energy the parcel has to overcome first to reach its level of free convection. CAPE tells you how strong storms could be, and CIN tells you how hard it is to get them started.
What time of day is CAPE highest?
Usually mid to late afternoon, when the ground is at its warmest. CAPE falls in the evening as the surface cools.
Related Terms
Lifted Index
A stability index comparing the temperature of a rising air parcel to the environment at 500mb (about 18,000 feet). Negative values indicate instability. Values below -6 suggest strong instability and severe thunderstorm potential.
Wind Shear
A change in wind speed or direction over a short distance, either horizontally or vertically. Strong wind shear is essential for supercell and tornado development but is also a significant hazard to aircraft during takeoff and landing.
Supercell
A supercell is a thunderstorm with a deep, persistently rotating updraft called a mesocyclone. Supercells are the most dangerous type of thunderstorm: they produce nearly all significant tornadoes in the United States and most hail larger than golf balls, along with damaging winds and flash flooding.
SPC Outlook
Severe weather forecasts issued by the Storm Prediction Center categorizing risk levels from Marginal (1) to High (5). Outlooks cover tornadoes, damaging winds, and large hail for 1-3 days ahead and highlight where organized severe weather is most likely.
Temperature Inversion
A layer of the atmosphere where temperature increases with altitude instead of decreasing. Inversions trap pollutants and moisture near the surface, causing fog, smog, and haze. They also cap thunderstorm development.
Skew-T
A Skew-T Log-P diagram is the chart meteorologists use to plot a weather balloon sounding: temperature, dew point, and wind from the ground to the upper atmosphere. Its temperature lines are tilted 45 degrees, which gives the chart its name. A hodograph, often shown beside it, plots how wind speed and direction change with height.
CAPE: Guides and Explainers
These explainers connect this definition to real-world forecasting and decision-making.
In-depth guide
Moisture, Instability, Lift, and Wind Shear: The 4 Storm Ingredients
Learn the four thunderstorm ingredients meteorologists look for: moisture, instability, lift, and wind shear, plus how they combine to create severe storms.
Weather Education
What Is Storm Surge? Surge vs Storm Tide, and Why Category Doesn't Predict It
Storm surge explained: how surge, storm tide, and inundation differ, what decides how high the water gets, why a Category 2 can flood worse than a Category 4, and which NHC products to act on.
Weather Education
What Is a Heat Dome? How Stalled High Pressure Bakes a Region
Learn what a heat dome is, how a stalled upper-level high-pressure ridge traps sinking, warming air, why heat domes last for days, and how to stay safe in extreme heat.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Guide
Lightning Safety Guide
Know when lightning becomes dangerous, how far away it can strike, and when to move indoors.
Guide
What Causes Thunder and Lightning?
How lightning forms through charge separation, what causes thunder, and how to stay safe.
Tool
Pressure Tendency Calculator
Calculate 3-hour barometric pressure tendency from rising or falling readings.
Feature
Conversational AI
Ask about rain timing, alerts, and what the forecast means in plain language.
Sources
Explore Other Terms
Tornado
A tornado is a violently rotating column of air in contact with the ground, extending from a thunderstorm. Tornadoes are rated from EF0 to EF5 on the Enhanced Fujita scale by the damage they cause. Most last less than 10 minutes, but the strongest can stay on the ground for more than an hour.
Sleet
Frozen raindrops that bounce when hitting the ground. Sleet forms when snowflakes partially melt falling through a warm layer, then refreeze in a cold layer near the surface. Unlike freezing rain, sleet is frozen before it hits the ground.
Wet-Bulb Temperature
Wet-bulb temperature is the lowest temperature air can reach by evaporating water into it. It is always at or below the air temperature, and the drier the air, the larger the gap between the two. Forecasters use it to judge heat stress and how much falling rain or snow can cool the air.
Velocity
Doppler radar measurement of how fast precipitation is moving toward or away from the radar. Velocity data reveals wind patterns, rotation in thunderstorms, and potential tornado signatures that reflectivity alone cannot show.
IFR
Instrument Flight Rules govern flights conducted under instrument procedures and can be used in clear weather too. Separately, the IFR weather category means a ceiling from 500 to less than 1,000 feet and/or visibility from 1 to less than 3 statute miles, with lower values classified as LIFR. Legal operating requirements depend on airspace, regulations, equipment, and pilot qualifications.
Tropical Depression
An organized system of clouds and thunderstorms with defined circulation and maximum sustained winds of 38 mph or less. Tropical depressions can strengthen into tropical storms and eventually hurricanes.
Browse by Category
In the WeatherAI App
WeatherAI Pro includes NWS radiosonde soundings for supported U.S. stations, with Skew-T and hodograph analysis and supported stability and wind-profile values.
Availability, coverage, update frequency, and alert delivery vary by data source, region, subscription plan, device permissions, and service conditions.