CAPE

Unit: Joules per kilogram (J/kg)

CAPE (Convective Available Potential Energy) measures the amount of energy available in the atmosphere for vertical air movement (convection). Higher CAPE means the atmosphere has more potential for vigorous updrafts — the kind that build thunderstorms, and in fire contexts, pyroconvective plumes.

Why it matters for fire weather

CAPE is relevant to fire weather in two key ways:

Dry thunderstorms. High CAPE combined with a dry lower atmosphere can produce lightning without rain reaching the ground. The rain evaporates in the dry air below the cloud base (a process called virga), but the lightning still reaches the surface. These dry thunderstorms are a leading natural cause of wildfire ignition across southern Europe and western North America.

Pyroconvection. When a large fire generates enough heat, it can tap into atmospheric instability (high CAPE) to create its own convective column. This can lead to fire-generated thunderstorms (pyrocumulonimbus), extreme spotting from embers lofted kilometres into the atmosphere, erratic wind patterns at ground level, and in some cases fire whirls. The GRAF fire analysis methodology classifies these as convection-driven fires — the most unpredictable and extreme fire behaviour category (Castellnou et al., 2009).

How it works

CAPE is calculated from atmospheric soundings (vertical profiles of temperature and humidity). It represents the buoyant energy available to an air parcel rising from the surface through the atmosphere. The higher the CAPE, the more vigorously air can rise — and the more intense the resulting convection.

CAPE typically peaks in the late afternoon when surface heating is strongest, and is near zero at night when the lower atmosphere stabilises.

The value depends on the full atmospheric profile, not just surface conditions. A hot surface with a warm layer aloft (temperature inversion) may have low CAPE because the warm layer suppresses vertical motion. Conversely, a moderately warm surface beneath a cold upper atmosphere can produce very high CAPE.

Key thresholds

CAPE thresholds describe the atmosphere’s convective potential. These values are standard meteorological classifications.

CAPE (J/kg)	Atmospheric significance
0–300	Stable atmosphere. Minimal convective potential. Vertical air motion is suppressed.
300–1000	Weak instability. Isolated convection possible if a trigger is present (frontal passage, terrain forcing, or an active fire).
1000–2500	Moderate instability. Thunderstorms likely if triggered. With a dry lower atmosphere, dry thunderstorm risk increases.
2500–4000	Strong instability. Severe convective development possible. During large fires, this level of CAPE significantly increases the potential for pyroconvection.
> 4000	Extreme instability. Explosive convective development. Rare in European climates but can occur during major heat events.

Note

High CAPE alone does not guarantee storms or pyroconvection — a trigger mechanism is needed. But when high CAPE coincides with an active fire or a dry frontal passage, the atmospheric response can be rapid and intense.

How to read it in Wildflyer

CAPE appears in the expert view as part of the atmospheric profile data. Use it alongside boundary layer height and the Haines Index to assess whether the atmosphere could amplify fire behaviour.

Key patterns to watch:

• High CAPE + dry lower atmosphere — dry thunderstorm risk (lightning ignitions without rain)
• High CAPE + active fire — pyroconvection potential
• Rising CAPE through the morning — the atmosphere is destabilising, which may produce afternoon convective activity

Sources

• Castellnou, M., Miralles, M., & Molina, D. (2009). Wildfire management in Mediterranean-type regions: paradigm change in Southern Europe. Wildfire, 18(3): 18–23.
• Doswell, C.A. & Rasmussen, E.N. (1994). The effect of neglecting the virtual temperature correction on CAPE calculations. Weather and Forecasting, 9(4): 625–629.