NASA images capture fleeting pileus cloud above Montana wildfire

NASA has published aircraft images of a fleeting cap-shaped cloud above a smoke-filled cloud rising from Montana’s Sand Creek fire. The August 11 observation gives researchers an unusually detailed view of how a fast-growing wildfire plume can lift moist air into a separate cloud layer, while instruments on the same flight recorded the fire and plume below.

The cloud, called a pileus, appeared over a pyrocumulus, a cloud driven upward by heat from the fire. NASA Earth Observatory published the images on September 29, describing the combination as rarely observed in this level of detail. The publication is new; the aircraft encounter itself took place more than six weeks earlier.

A brief scene above the fire

NASA says the MODIS/ASTER Airborne Simulator, or MASTER, captured the scene from an ER-2 research aircraft at 23:40 UTC, or 5:40 p.m. local time. In the natural-colour view, a thin, circular veil sits over a turret of smoke and water rising from the fire. It was visible in just one of the images collected at roughly 30-minute intervals, NASA says.

The companion view uses shortwave infrared measurements to show features that ordinary colour photography cannot separate as clearly. NASA identifies actively burning spots in pink and yellow, burned ground in brown, clouds in white and smoke in blue-purple. These are displayed colours assigned to different measurements, rather than the colours an observer would see from the aircraft.

How a cloud formed above a cloud

NASA’s explanation begins with a rapidly rising column of air heated by the wildfire. As the pyrocumulus grew, it pushed a more humid horizontal layer above it upward. That displaced air cooled, and some of its water vapour condensed into the smooth pileus. The cap formed above the fire cloud rather than as a separate column rising directly from the flames.

Neil Lareau, INSPYRE’s deputy project investigator, compared that lifting to the way air forced over a mountain can make a cloud. “But in this case, the pyrocumulus itself was the ‘mountain,’” he told NASA. He said the pileus points to what he called “extreme updraft dynamics” within the fire plume, an interpretation of the observed scene rather than a reported measurement of updraft speed.

At the time, Lareau said, the fire was making a rapid run up the western side of Mount Comet as winds aligned with drainage channels leading up the mountain. NASA says pileus clouds commonly last only a few minutes before rising convection overtakes them. That short life helps explain why the cap was clearly present in only one scene from the repeated overpasses.

What the campaign recorded

The flight was part of NASA’s INSPYRE campaign, which studied clouds generated by wildfires and their links to extreme fire behaviour. The Airborne Science Program archive lists the campaign from July 15 to September 8 across the western United States and Canada. Its Sand Creek entries include an August 11 pass from 23:38 to 23:40 UTC, matching the time of the published image.

The archive lists numerous passes over Sand Creek on August 11, as well as flights over other fires. Its stated objective focuses on pyrocumulonimbus clouds, which can carry smoke high into the atmosphere. The newly published image concerns a pileus over a pyrocumulus; NASA has presented it as a detailed observation of cloud formation during that wider campaign, not as a quantified result about smoke transport.

What earlier research can explain

The basic mechanism has precedent beyond wildfires. A 2006 paper in Atmospheric Chemistry and Physics describes pileus forming when humid, layered air is mechanically displaced ahead of rising convection. Its authors examined measurements from a 2002 experiment near Honduras, where deep convection reached the tropical tropopause transition layer. That setting differs from a cloud over a Montana wildfire.

The earlier paper explored a possible route from pileus ice to longer-lasting cirrus near the tropopause. It did not test the Sand Creek cloud or establish any such effect there. NASA’s September account likewise does not give a measured updraft speed, a frequency for pileus over wildfires, or an estimate of wider atmospheric effects from this particular episode.