Ice Capping? Column Collapse?

[abercrombie]

Written in to theysaid:

Hello, I read your site often, can you tell me what the term "ice capping" means?

thanks
Diana

[R5Captain]

Basically, ice capping is just a colorful term firefighters use to describe a fire when the plume from a fire has a distinct, bright white top to it. It takes a certain size fire to generate an ice cap, so it is an indicator from a distance as to exactly how active a fire is and what it is currently doing.

[Bluezebra]

Possibly more important then size of fire is fuel burning. Ice capping typically requires heavy fuels- brush and timber- and making a run, to generate enough heat to push the column high enough to ice cap.

I have never heard of or seen ice capping in grass fires.

[Mellie]

Here's a discussion about pyro-cumulous cloud formation, thanks to Rich Ochoa at NIFC...

http://www.weathernotebook.org/trans...001/07/13.html

"When the smoke column gets very large, the top of the column can be 20, 30 even 50,000 feet. What we see on top there often times we see large cumulo nimbus type clouds. We call these 'pyro -cumulous clouds.' They're actually a cumulous cloud that is fed by the fire."

He says more good stuff about their formation.

But I think this kind of cloud that forms an ice cap on top is technically called a Pileus cloud.

[Mellie]

Just googled Pileus Cloud.

http://www.stormeyes.org/tornado/SkyPix/pileus.htm

Roger Edwards description that goes with his ice capped cloud (Pileus Cloud) picture:

Pileus caps are made of ice crystals high in the troposphere. They form as a slab of air is shoved upward, in the shape of a dome or cap, just above a rapidly rising convective tower. Moisture in the dome condenses directly into an ice fog as the air rises and cools, forming the pileus. Next, the convection shoots right through the pileus layer. The lifted layer above the convective tower can't be too dry (must have high humidity), or the pileus cap won't develop.

More great pileus cloud images and explanations:

http://www.weatherscapes.com/album.p...&subcat=pileus

http://ww2010.atmos.uiuc.edu/(Gh)/ww...dtyp/home.rxml
Pileus (Latin for "skullcap") is a smooth cloud found attached to either a mountain top or growing cumulus tower.

http://www.met.utah.edu/galleries/ho...jpg/photo_view

[Mellie]

Pileus (ice capped) Cloud as differentiated from a Lenticular Cloud:

http://www.hko.gov.hk/aviat/outreach...ion/pileus.htm

Pileus cloud is normally very short-lived. It occurs when a vigorously rising cumuliform cloud encounters a hitherto invisible layer of moisture.

(lenticular cloud Rich took near Tehapiche on the photo page 4: http://www.wildlandfire.com/pics/fire3/lenscloud.jpg )


Here are some stunningly beautiful images of Pileus Clouds:
http://www.cs.cmu.edu/~zhuxj/astro/html/pileus.html

and here: http://www.outdoorphoto.com/photos/s...cat/508/page/1

and from Australia:
http://australiasevereweather.com/ph...hy/pileus.html

[mesocyclone2006]

Intensely-burning fires produce an enormous amount of heat and particulate matter. The local addition of heat at the surface (which rises through the air as long as it is buoyant) steepens atmoshperic "lapse rates"--the rate at which temperature decreases with height. Steeper lapse rates allow for atmoshperic instability--that is, a hot "parcel" of air near the ground generated by a fire is more buoyant than the surrounding air, and it rises and condenses into a cumliform cloud. The numerous smoke particles also aid in the condensation process, allowing for more effective droplet nucleation and in effect "seeding" the surrounding air. Thus...a pyrocumulus cloud is formed by exactly the same process as any other convective (cumliform) cloud, it's just that the heat source at the surface is a wildfire. This is also why pyrocumulus clouds are more common and impressive during periods of instability (when true thunderstorms may be in the vicinity)--the fire is enhancing the dynamics already in place. Pyrocumulus clouds do, on occasion, develop into full-blown thunderstorms, but often result in very little rainfall and cause very erratic and gusty winds when they eventually (and inevitably) collapse. This is where the term "plume-dominated" fire activity comes from--the fire literally drives itself, even in otherwise benign fire weather conditions.

Oh, I almost forgot the question! What is sometimes referred to as an "ice cap" is not quite the same thing as a pileus extension--the "ice cap" is where the pyrocumulus hits the inversion layer (where the parcel is no longer more buoyant than the surrounding air) and flows horizontally outward. It's smooth appearance is due to the temperature at which the cap forms--usually below freezing! This is similar to a glaciated anvil top on a cumulonimbus (thunderstorm) cloud, and once again involves the same principles.

Interesting stuff...

[Mellie]

Thanks Mesocyclone.

Another google search...

Very interesting article by Jim Bishop March 07. A lot of photos and on page 9 a good pic of Anvil top, cumulonimbus, the anvil portion being the ice cap:

http://www.fs.fed.us/psw/cirmount/wk...rms_031607.pdf

Mountain Thunderstorms Their Formation and some Field-Forecasting Guidelines

[AL]

Here's a good page for photos of different kinds:

http://www.cloudman.com/gallery1_3.html

[Portage Pulaski]

Here are some GOES animations of anvil formation on pyrocumulus from space.
Subject is Alaska fires June 2004 at midnight.

The layering of the atmosphere is evident.

http://cimss.ssec.wisc.edu/goes/misc..._g12_anim.html

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Here's a pic of a cumulonimbus from Oklahoma with description:

http://en.wikipedia.org/wiki/Image:Cumulonimbus-tav.jpg

The updraft is the large cloud mass at the center of the photo. The anvil is the flat layer at the top. The downdraft is the rainy area to the right.

Other wikipedia info with photos:
http://en.wikipedia.org/wiki/Cumulonimbus_cloud
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Here's some categorised by height. Scroll 2/3 down for pyrocumulus:

http://www.sky-chaser.com/schcloud.htm