Convective Variables

Convective variables describe the vertical movement of air in the atmosphere and the conditions that govern it. In wildfire applications, these variables are used to assess smoke dispersion, identify when atmospheric conditions may contribute to erratic or extreme fire behavior, and understand how a fire's own heat output can interact with the surrounding atmosphere. Convective variables are derived from numerical weather prediction models.

Convective Available Potential Energy (CAPE)

Convective Available Potential Energy (CAPE) is a standard atmospheric stability metric used to characterize the energy available for convection. Higher CAPE values indicate greater potential for deep vertical mixing. In wildfire contexts, CAPE is most relevant as an indicator of pyroconvective potential, particularly in environments where fire intensity is sufficient to interact with an unstable atmosphere.

Most Unstable CAPE (MUCAPE)

MUCAPE is a variant of Convective Available Potential Energy that identifies the single most unstable air parcel within the lowest 300 hPa of the atmosphere (roughly the lowest 10,000 feet) and calculates the total buoyant energy available to that parcel if lifted to its level of free convection. Rather than assuming convection originates at the surface, MUCAPE searches the lower troposphere for whichever parcel has the greatest potential for upward acceleration.

Surface-based CAPE can underestimate instability when a strong low-level inversion caps the boundary layer, suppressing surface parcels while leaving elevated, more unstable air above the cap uncounted. MUCAPE captures that elevated instability and is therefore a more reliable indicator of convective potential across a wider range of atmospheric profiles, including conditions common in the western United States where afternoon inversions and complex terrain are frequent.

Convective Flag

A Convective Flag is a weather-derived indicator such as Pyrocumulonimbus Firepower Threshold (PFT) used to identify atmospheric conditions that could support deep, moist plume growth from a wildfire: conditions where a fire’s smoke plume may become strongly convective and potentially develop into pyrocumulus/pyrocumulonimbus-type behavior. Lower convective flag values indicate more unstable conditions and greater potential for convective fire behavior. In it’s most basic form, the Convective Flag is the ratio of the convective fire potential to surface fire potential. It recognizes that a convective fire needs good burning conditions at the surface in addition to instability.

Convective Inhibition (CIN)

Convective Inhibition (CIN) describes the energy that must be overcome before convection can initiate. When CIN is low and Convective Available Potential Energy (CAPE) is elevated, convective development is more likely.

Lifted Index (LI)

Lifted Index (LI) is a measure of atmospheric instability which compares the temperature of an air parcel lifted upward from near the surface, versus the temperature of the surrounding air at a higher level, such as 500 mb, 650 mb, or 800 mb.

If the lifted air parcel is warmer than the surrounding air, it keeps rising, which means the atmosphere is unstable and more supportive of convection.

Typical interpretation:

Mixing Height

Mixing height is the altitude above the surface to which air is actively mixed by thermal or mechanical turbulence. It defines the depth of the layer within which heat, moisture, smoke, and other pollutants are exchanged vertically.

During daylight hours, solar heating drives surface air upward, deepening the mixing layer. At night, surface cooling suppresses vertical mixing and the mixing height collapses. On days with high mixing heights, smoke from a wildfire can be lofted to significant altitudes and dispersed broadly. On days with low mixing heights, smoke remains concentrated near the surface and fire behavior tends to be suppressed, though conditions can shift rapidly when the mixing layer deepens in the morning.

Mixing height is typically reported in feet above ground level (AGL).

Pyrocumulonimbus (pyroCb) Cloud

A Pyrocumulonimbus (pyroCb) cloud forms when intense heat from a fire creates a powerful convective column that reaches the upper troposphere or lower stratosphere. Once a fire generates a pyroCb, it begins to drive its own weather rather than simply responding to ambient conditions. This transition marks a fundamental shift in fire behavior and significantly reduces the effectiveness of suppression efforts.

Pyrocumulonimbus Firepower Threshold (PFT)

Pyrocumulonimbus Firepower Threshold (PFT) is a meteorological benchmark and convective flag that indicates when a wildfire has reached sufficient intensity to generate its own weather system, specifically a pyrocumulonimbus (pyroCb) cloud. PFT is expressed in gigawatts (GW) of total fire power.

When a fire approaches or exceeds the PFT, the following conditions become possible:

• Downbursts and erratic outflow winds that can rapidly change fire direction without warning.

• Ember cast over long distances, creating spot fires well ahead of the main fire front.

• Collapse of the convective column during a pyroCb can generate powerful downdrafts, producing sudden and extreme surface wind events.

• Conditions that make aerial operations dangerous or impossible.

The PFT is calculated based on atmospheric instability and the moisture and temperature profile of the atmosphere above the fire. A lower PFT indicates an atmosphere that is more susceptible to pyroCb formation; an atmosphere that is stable or dry aloft requires more fire energy to trigger one.

When forecast fire behavior approaches the PFT for the current atmospheric conditions, it signals conditions under which extreme, self-driven fire behavior becomes possible.

Transport Wind

Transport wind is the mean wind speed and direction through the depth of the mixing layer. It describes how smoke, embers, and other fire-related particles are carried horizontally once lofted within the mixing layer.

Transport wind is distinct from the surface wind reported in a standard fire weather forecast. A fire may experience light surface winds while the transport wind at higher altitudes carries smoke or spotting material over a much larger area. High transport wind speeds combined with sufficient mixing height are associated with long-range spotting potential and broad smoke impact.