This is the thunderhead cloud-top of the same fire plume in my [Previous] photo. This is the view from our porch yesterday afternoon. The Zaca wildfire seems to be mostly moving past us through the backcountry to the east. But as I type this, a fire crew is hand cutting a fuel break on the ridge above our house. I guess they are still worried, so I am too. (San Marcos Pass, 4 August 2007)

I call these wildfire clouds "fireheads". I did some research last night and discovered they are properly called Pyrocumulonimbus (PyroCb) clouds. The clouds in our wildfire have been reaching 25,000 - 35,000 feet. They can produce lightning and rain, but the rain probably won't make it to the ground.

Fire and water seem like opposites, but they have interesting connections that produce these clouds:

* It's an old middle school science teacher's trick to hold a dry glass over a candle flame. In a moment, water will condense inside the glass. The chemical "fire equation" is something like this: CnHm + O2 --> CO2 + H20. Fire really does produce water vapor as a product of combustion!

* Fire evaporates more water from the ground and burning plants.

* Hot smoke and water vapor rise by convection into an area of lower atmospheric pressure. In time the smoke cools and gets pushed out of the way by more hot air rising from below. That creates the mushroom cloud shape. (Nuclear bombs work the same way.)

* As water vapor cools in lower pressure, it condenses into droplets, and the smoke provides plenty of particles to act as nucleation sites - like cloud seeding!

The result is that wildfires can make their own pyrocumulonimbus rainclouds! I'm curious why these fireheads only form sometimes. I'm sure they require a very intense firestorm on the ground to generate strong convection. Maybe atmospheric conditions make a difference too?

After I took this photo, I set up my camera on a tripod and started taking a photo every 15 seconds for more than an hour. I ended up taking almost 300 photos until my battery gave out. I've only looked at them as a very fast slideshow, and the motion in the cloud is amazing and beautiful. I'll figure out how to make a proper movie and post it on youtube when I find time.





On August 05 2007 3 Views

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