I was driving north from the Navajo Nation toward Durango with the intent of camping on Cumbres Pass in the South San Juan Mountain when I saw something that changed my plans.
Four days previously my route to the Navajo Nation had brought me near Durango, so I expected to see a plume of smoke, but the 416 Fire had flared, so smoke and a roiling cloud were rising to nearly 25,000 feet. I immediately resolved that I was not going to camp downwind of that.
My revised route took me west and north to Cortez, where I had a good view of the smoke and rising cloud. From my vantage 45 miles to the west, some portions of the San Juan Range were visible, but others were obscured by thick, dark smoke extending northward for many miles. Above the mountains, updrafts and downdrafts churned light and dark smoke in a slow motion maelstrom. But the most striking aspect of this view was the cloud, white in some places but pink with smoke in others, exploding upward so quickly I could watch it climb into the sky.
A flammagenitus or a pyrocumulus cloud is a fire cloud created by intense heat from volcanoes or wildfires. Convection winds rising or falling within the cloud generate extreme turbulence at the surface. A flammagenitus generating lightning may have an anvil-shape and these are called pyrocumulonimbus. Most of the time, a pyrocumulonimbus has the combination of conditions that firefighters dread — strong winds, no rain and lightening. The fire causes the cloud, which then fans the fire and might ignite subsidiary fires with lightning.
Pyrocumulonimbus clouds can amplify the impact of wildfires by injecting aerosols, such as smoke, ash and water vapor, into the stratosphere. Most of our weather is in the troposphere, the lower level of the atmosphere, reaching from sea level to 33,000 feet. The stratosphere begins at 33,000 feet and extends to 160,000 feet. Materials that get into the stratosphere are quickly distributed by several jet streams, some of which have wind speeds greater than 100 mph.
Large cumulonimbus clouds can rise into the stratosphere; for example, the thunderstorm that caused the Big Thompson flood in 1988 reached 62,000 feet.
Atmospheric scientists have long appreciated the ability of pyrocumulonimbus clouds to carry plumes of steam, smoke and ash from volcanoes into the stratosphere, where the gases and particulates can be spread around the world. But it was not until 2000 that scientists first watched pyrocumulonimbus clouds pump wildfire smoke into the stratosphere.
Plumes of aerosols in the stratosphere can be detected by weather satellites and also by some types of ground-based radar. In the summer of 2002, 17 pyrocumulonimbus clouds were detected in the West and six of them were in Colorado. Now, looking back at meteorological data, it is clear that the Hayman Fire and the earlier Yellowstone fire both injected large amounts of aerosols into the stratosphere, and that strong winds carried the aerosols around the world. In fact, the Hayman Fire produced two pyrocumulonimbus clouds, a week apart. I am curious to learn whether gases, smoke and ash from the 416 Fire near Durango were injected into the stratosphere.
Volcanic eruptions release ash and sulphur dioxide into both the troposphere and stratosphere and both of these depress surface temperatures by intercepting solar radiation. For example, when Mount Pinatubo erupted in the Philippines in 1991, the ash cloud above the volcano rose to 116,000 feet and was hundreds of miles wide. One of the consequences of the eruption was a dip in earth's average temperature the following year. I wonder if a raging fire season, producing more that a dozen pyrocumulonimbus clouds pumping ash into the sky, could rival the output of a volcano.
A wildfire's heat and rising plumes of air cause it to create its own weather, and now we appreciate that the big wildfires send aerosols — both particulates and gases — all the way around the world.