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Bronco Fire in Arizona produces a pyrocumulonimbus cloud

By Scott Bachmeier •

5-minute GOES-18 Clean Infrared Window (10.3 µm, left) and Red Visible (0.64 µm right) images with overlays of the Fire Mask derived product (semi-transparent red areas), from 2001 UTC on 07 August to 0101 UTC on 08 August [click to play MP4 animation]

5-minute PACUS sector GOES-18 (GOES-West) Clean Infrared Window (10.3 µm) images and Red Visible (0.64 µm) images with an overlay of the FDCA Fire Mask derived product (above) showed that the Bronco Fire in southeast Arizona produced a pyrocumulonimbus (pyroCb) cloud as it was experiencing very active to extreme fire behavior on 07 August 2025. The pyroCb cloud exhibited cloud-top 10.3 µm infrared brightness temperatures (IRBTs) in the -40s C (denoted by shades of blue to cyan) — a necessary condition to be classified as a pyroCb — beginning at 2156 UTC on 07 August.

Note that smoke drifting north-northeastward from the Bronco Fire eventually reduced the surface visibility to 4 miles at Show Low (METAR identifier KSOW) by 0100 UTC on 08 August (below).

Time series of surface observation data at Show Low, Arizona [click to enlarge]

The hazy signature of northward-drifting smoke was apparent in GOES-18 True Color RGB images from the CSPP GeoSphere site (below).

5-minute GOES-18 True Color RGB images, from 2001 UTC on 07 August to 0101 UTC on 08 August [click to play MP4 animation]

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GOES-18 Infrared image at 0016 UTC on 08 August, with a cursor sample of the coldest pyroCb cloud-top infrared brightness temperature [click to enlarge]

The coldest pyroCb cloud-top IRBT was -51.59ºC at 0016 UTC on 08 August (above). On a plot of rawinsonde data from Tucson, Arizona (below) the air temperature of -51.59ºC occurred at an altitude around 11.3 km — not far below the Most Unstable (MU) air parcel’s Maximum Parcel Level (MPL) of 11.6 km.

Plot of rawinsonde data from Tucson, Arizona at 0000 UTC on 08 August [click to enlarge]

The Bronco Fire burned very hot, intermittently exhibiting 3.9 µm infrared brightness temperatures as high as 137.88ºC — which is the saturation temperature of the GOES-18 ABI Band 7 detectors — from 2111 UTC on 07 August to 0016 UTC on 08 August (below).

GOES-18 Shortwave Infrared (3.9 µm) image at 2111 UTC on 07 August, with a cursor sample of the hottest brightness temperature over the Bronco Fire [click to enlarge]

GOES-18 Shortwave Infrared (3.9 µm) image at 0016 UTC on 08 August, with a cursor sample of the hottest brightness temperature over the Bronco Fire [click to enlarge]

Categories: Air quality, Fire detection, GOES-18, Red-Green-Blue (RGB) images