CBs a quick recap

[PompeyPaul]

Cumulusnimbus would suggest that, due to the nimbus part, they have to be raining to be CBs ? On the other hand I thought that a CB was the "cauliflower" cloud:
http://farm3.static.flickr.com/2302/2003030203_7eba2f9df6.jpg

Which does not necessarily mean that it is raining and thus CB ? Just wondering given we're at that time of year that they start appearing...

[goatface]

What you are looking at is "Towering Cu", they usually begin their life from Cu cloud (cauliflower as you put it) then either disapate into nothing or, more often than not, are the precurser for CBs.
Unless you are a very experenced glider pilot and looking for serious thermals, if you see them - avoid them.
They have the same, albeit reduced, qualities of a CB and can make your flying experience an unpleasant one.

There will of course, be the bold, but not old, flyers who know everything and will disagree, but their advice is worth ignoring.

[733driver]

Your picture doesn't show a CB. More lice a towering cumulus (TCu), which may develop further into a CB. A CB will not allways rain, but certainly always has the potential (water content) for qute intense rain, hail etc...

[bookworm]

The cumuliform cloud rises though the freezing level but remains as supercooled water. At some point, depending on the abundance of nucleation sites, the liquid starts to change phase into ice. It is at that point, when the cloud starts to glaciate, that it ceases to be a towering cumulus (as in your photo), and becomes a cumulonimbus. You'll start see some wispiness close to the top: ice crystals.

There always seems to be a perception among pilots that CU is "OK" and CB is "really bad". The working definition of a "CB" for CAT seems to be "something bigger than I'm prepared to fly through". ;) In reality, the convection cells go through phases where the potential for icing and turbulence varies greatly. If the TCU in your photo is penetrated above the freezing level, it could be an extremely bumpy and icy ride. Stay away from building cumulus. A dissipating CB can be quite tame by comparison.

[PompeyPaul]

Yes the picture I posted wasn't a good example, but the first that came up on google. This is the sort of CB I'm talking about:

http://farm2.static.flickr.com/1378/590924938_3adc146d92.jpg?v=0

The main question I had was does a CB HAVE to rain to be a CB (which nimbus suggests). The answer seems to be "NO". See that cauliflower cloud and go running.

[bookworm]

You might find this (http://groups.google.co.uk/group/sci.geo.meteorology/browse_thread/thread/851f97adc5f794cd/613e31f57d2abc85?hl=en&ie=UTF-8&pli=1) of interest. You should believe Grant Petty, not the idiot original poster. ;)

[englishal]

Here's one I took a few weeks ago in the USA...We were at FL350

http://farm4.static.flickr.com/3307/3664362867_ac3a6a11a3.jpg

[flybymike]

All of the above are well defined and clearly visible in clear air surroundings. It is the embedded invisible b*ggers which creep up on you in the haze and murk that you need to "watch out" for.

[SNS3Guppy]

Yes the picture I posted wasn't a good example, but the first that came up on google. This is the sort of CB I'm talking about:



The main question I had was does a CB HAVE to rain to be a CB (which nimbus suggests). The answer seems to be "NO". See that cauliflower cloud and go running.

By technical definition, yes. A Cb cloud produces or is capable of producing rain. While the common definition, often outlined on ATP and commercial tests, says that the mature stage of the Cb is marked by the appearance of rain at the surface, the truth is that the rain may never reach the surface.

From an aviation point of view, whether rain reaches the surface or not, a Cb can still present significant hazards beneath the cloud. Virga may appear and not reach the surface, but the virga also represents not only downdraft, but accelerating downdraft, potential microbursts, etc. Hail is a concern. Lightening, and strong winds, also. Embedded, turbulence, hail, and icing are prime concerns. Hail can be ejected far from the column. All the usual hazards.

Your pictures, however,do not indicate Cb cells. They may be cumulus as in your first shot, and may be towering cumulus. (Cu, and TCu). Cumulus clouds indicate unstable air to some degree, and are marked by vertical development. This means you have some element of convection vertically, which means the cloud has energy. You have vertical shear, you have movement, and some element of turbulence.

Before a thundercell reaches a Cb status, it's a building TCu, and the upstarts on the upwind, or upshear side of a cell, even a building cell, can be damaging, and extremely powerful. While doing reasearch in building cells, I've been rolled over and stalled with enough violence to damage onboard equipment...I entered the top of a building cell which didn't even show up on radar several years ago, at night. It stalled rapidly enough we got the stick shaker and pusher in a Learjet 35, rolled us 90 degrees, stripped the guts out of my personal computer (in a padded bag in back), and broke the headset that one of our operators was wearing in back. That was simply a little cumulus building on the upwind or upshear side of a cell.

When you do see the big cloud forming, it may be hard to visualize, but what you're seeing is a signpost of activity. Air is blowing through the cloud. It's constantly being formed and breaking up. Wind is passing through the cell or the cloud from one side to the other. You can't really see it as you see the cloud, it appears to move very slowly, and it might appear to simply exist at the visible limits of the the cloud vapor...but it's much more, and there's much more going on beyond what you can see.

Where you see the crisp, building top, it's still got energy. When it begins to look fluffy and wispy, loses some of it's definition, it's lost it's vertical energy and has "glaciated," or turned to ice. This doesn't mean the cell has lost it's energy; it's still got more to give, but it typically marks the upper limit of vertical development. This doesn't mean that it can't start building again, but it takes more updrafts and more moisture (which may be supercooled, or may freeze and continue to build with energy from below). Any condensation and fallout as the air rises may release latent energy and cause an upsurge, and the storm can take off vertically again.

Rain at the surface may appear to mark the Cb, but there's far more to the Cb, and you may never see the rain. Rest assured it's there somewhere, even if you don't see it. I've done weather research in places (Saudi Arabia, for example) where one might not expect significant weather, and some of the buildups weren't that high, but still produced significant icing (3" of icing on a learjet) with clouds that didn't extend much above 15,000 to 20,000' and had more energy than one might have expected from a small cloud.

[DX Wombat]

I took this photo some time ago from the caravan site across the road from EGBO.
http://img.photobucket.com/albums/v604/DX_Wombat/P1011447.jpg

[IO540]

Would these

http://i101.photobucket.com/albums/m74/peterh337/clouds1.jpg

qualify as CBs? This was ~ FL140.

[SNS3Guppy]

I would think so. It has the classic anvil structure, has capped out, and has begun building again pushing up through the original column. It still has convective energy, and while it's hard to say what the precip output at the bottom might be, it has the markings of a cumulonimbus cloud. Remember that a Cb doesn't need to be outputting rain to qualify, and the building stages of a Cb are characterized largely by updrafts.

[IO540]

The real good question is the correlation between hazardous weather, and airborne or ground based stormscope imagery.

I think the correlation is very good in one direction i.e. if the stormscope is showing a cell then you definitely do not want to go there.

But can you encounter structurally hazardous weather in the absence of sferics (http://www.blitzortung.de/TOA/Webpages/index.php) activity?

I think you can, and therefore personally I climb above any IMC enroute. With a FL200 ceiling, this has always worked, so far. In Europe, this requires an IR because one is usually in CAS and VFR transits cannot be relied on.

However I have found that any clusters of sferics activity seem to always correlate with a lot of radar return (http://www.meteox.com). OK if you have airborne radar :)

But not the other way round i.e. a strong radar return often does not have any sferics activity. Pilots claim such weather is safe to penetrate since turbulence is what causes the electrical activity. I don't buy that one.

[bookworm]

Would these qualify as CBs? This was ~ FL140.

If you look at the left edge of the cloud on the right, you'll see it's started to get wispy. That's a sign that glaciation has started in a meaningful way.

[Fright Level]

I always understood nimbus to be rain bearing, not rain producing. Although rain may not be falling from the bottom (the upcurrents holding the water in place for the time being), fly through it and the windscreen is covered in rain sized water droplets!

When flying in/around/over TCU, I always look out for the wisps of "cap" cloud over the top of the CU that indicate rapid vertical building.

[SNS3Guppy]

The wisps at the top of the cloud don't indicate that it's building rapidly. A crisp billowing appearance will indicate it's vertical potential and indicate that it hasn't turned to ice yet (glaciated), but you can't tell it's vertical momentum without actually seeing it move.

A rapidly building towering cumulus can quickly overtake you and you can't outclimb it...it can be climbing several thousand feet per minute very easily...what you think you can overfly will easily be thousands, even tens of thousands of feet above you in a few minutes, and can likewise quickly envelop you.

Flying through it to see what's inside is a bad idea.

You may or may not get big raindrops on your windscreen. If droplets are large enough to produce rain, it's already well into the Cb stage. If droplets are inside that are traveling upward vertically with the diameter of large raindrops, then you're in a lot of trouble at that point. Somewhere in that cell you're going to find a column of air, more than likely several, going down. Most likely that water has already made the journey more than once and is still growing, and part of what may be feeding that rising column of air will be latent heat release of the moisture beneath your altitude...which allowed the water to grow to that size of droplet, but also caused an increase in cloud column height and a vertical rise locally in the atmosphere.

When air begins rising, say at 1,200 fpm, it may meet other air which is also rising at say, 1,000 fpm. The column now rises at the rate of 2,200 fpm...the vertical velocity is cumulative. The rate is affected by the surrounding temperature, and will rise more rapidly through colder temperatures, more slowly in warmer surrounding temperatures.

The moisture content in the cloud can vary considerably with temperature and the airmass, but I certainly wouldn't fly through it to find out what's there. I've done that with a lot of scientific equipment hanging off the airplane on hardpoints and probes, measuring droplet size and moisture content and other things, and I've picked up an enormous amount of ice doing it (to say nothing of aircraft damage, hail, severe turbulence, lightening strikes and damage, controllability issues related to the turbulence and icing, and so forth). Best to fly around them...even if it's a towering cumulus. It may hide something inside that you may not like. Remember that every Cb starts life as a TCu, which may be turning nasty at any given time. Even just the rising TCu can be a killer.

All who fly instruments understand that instrument flight can be challenging under the best of circumstances, especially if one isn't flying instruments every day. Instrument flying is a perishable skill, and flying single pilot IFR is one of the highest workloads a pilot can have. Add turbulence to that, even mild turbulence, and the need for a much higher scan rate, and a higher interpretation rate exists. The potential for an upset or an unusual attitude is higher. Certainly punching clouds can be a fun endeavor, no doubt. But in the case of big clouds and tall columns of clouds that are packing energy (think of stored moisture as stored energy)...and clouds made up of energy (rising and falling air currents and columns)...there's more hazard than fun to be had.

I did a lot of column penetrating doing atmospheric research. However, when entering a cloud column or storm, I had the benefit of multiple experienced meteorologists on board providing counsel, other very experienced weather pilots alongside, and a team of meteorologists watching me on specially designed doppler radar with dedicated advanced software enhancements...something none of us get when flying from A to B punching clouds. In my case I also had a lot of excess thrust and performance that none of us have in a light single or twin.

I'd like to touch also on something I think often isn't considered when popping in and out of the clouds, and that's a system failure. This could be a vacum pump failure or an instrument failure. While flying instruments can be a challenge, flying partial panel instruments in turbulence and upset conditions can be much more so...it can also be deadly. I do a lot of night instrument flying lately, including quite a bit in light twins, and despite being current and experienced, I consider a partial panel situation to be a true emergency. Consider that in the context of experiencing an instrument loss inside one of these cells, apart from the inherent danger of the cell itself. Spooky stuff.

[bookworm]

The wisps at the top of the cloud don't indicate that it's building rapidly.

I agree. But they do mean that it is a cumulonimbus from an observer's point of view. The point, which you make very eloquently, is that the observational distinction between CU and CB does not offer a definitive guide to the aviation hazards in the cloud.

I got a sharp reminder of what you write about TCU on the way into Lille a week or two ago. The flight had been almost entirely smooth at FL100 above scattered little CU, but 50 miles out there was a cauliflower CU with its tops at about FL110 or FL120. That's really not very high, is it? I figured I was going down anyway, so hey, what can a little CU like that do? Five minutes later, my wife simply said "don't do that again please". ;) Looking on the radar after, it seems I picked the nastiest little shower over northern France at the time, with turbulence that was on the sporting side of moderate. Welcome to Spring 2009!

[Danscowpie]

Nice photo Wombat - looks like a real caravan cruncher, do you rent it out at reasonable rates? (I have a few CC sites in mind so perhaps you could give me a discount for a 4 week lease). :E:)

[SNS3Guppy]

I agree. But they do mean that it is a cumulonimbus from an observer's point of view. The point, which you make very eloquently, is that the observational distinction between CU and CB does not offer a definitive guide to the aviation hazards in the cloud.


I think I misunderstood what was being indicated by "wispy." If I understand correctly, you're referring to the blow-off, or anvil head on a cell, which often marks a Cb.

The blow-off, pointing to the downshear or downwind side of the cell usually is where vertical development has stopped or stymied, and often glaciated, or turned to ice crystals. it's also where a clear change in the vertical temperature lapse rate has occured, and is typically marked by windshear as well; the movement downwind of the overhang or anvil is evidence of this; it's the ice crystals being blown downwind.

This can happen with TCu, and not just a Cb cloud. While the anvil is the mark of a Cb, it doesn't necessarily mean that the cloud has become a Cb.

Your observations regarding a small cloud are well founded. I thought for a long time that if a cloud was under 15,000' tall, then it simply didn't have the energy or punch to do much damage. Unfortunately, experience has proven me wrong there. I've experienced significant damage from lightening from smaller cells than that, and experienced significant microbursts and other activity out of cells that I really didn't think had it in them. Just as importantly, I've watched cells build which began little puffy fair-weather cumulus clouds and turned into muliple raging Cb's in the time it took me to get on the ground and turn around and look up. (A few years ago I was watching weather develop on my way back from a fire. I estimated two more trips to the fire based on what I was seeing. I landed straight in, back to the weather, and turned around in the pit to start loading. I looked up through my canopy and saw a funnel cloud forming just south of the airport...and quickly tied down the airplane and went to lunch).

There's a common old wisdom with the anvil on a cell formation which says "don't fly under the anvil." I've spent a lot of time penetrating anvil and taking samples beneath and in them...and for the most part have encountered little of significance. However, they can contain ice, and despite being largely ice crystals at higher altitudes, may still contain adequate supercooled water to cause airframe icing. They may also mark the location where the hail coming out the top of a cell is going, and may have hail falling out of the anvil, too. A good policy is to avoid flying under the blowoff, or anvil. A good way to think of the anvil is as a pointer...pointing to the side of the storm where you don't want to be...also the side where turbulence and storm byproducts are most likely to be cast.

A few years ago I sat on a ramp in a small farming community, on Mother's day. I'd flown several passengers into the town in a Piaggio to spend time with their family. Forecasts had significant convective activity, but well to the east and west, with nothing of significance in our area. I noted buildups east and west, but distant, and felt comfortable with the airplane where it was. Shortly thereafter I was contacted by a crop duster on the field (we were using his ramp) to warm me about approaching weather. It was severe. It was producing severe storms, hail, tornadoes, the works.

By the time I rounded up my trusty copilot and pulled the covers off the airplane, the storm was fast approaching, and the cells were everywhere. I turned on the radar at the end of the runway and ahead of us was nothing but magenta, which is very bad...tilting up as far as I could go. I made a downwind departure with an early turnout and began working north. I moved the airplane to another location where we could get it in a hangar. As we were pulling the airplane into the hangar, tornado warnings began going off, and reports began rolling in of multiple tornados.

The storm was the biggest I'd ever seen. It largely missed us; places to the west were hit much worse. I sat at the hotel that night and watched the electrical activity in that mesocomplex, and it was continuous...continuous lightening for hours. Very impressive. What impressed me the most, however, was how rapidly it developed in an area where it wasn't forecast, and what it grew into; it was a monster. I've seen a lot of others that come up quickly and produce 70 mph winds and go away...but this one came up, produced all the damaging and dangerous stuff, and didn't go away...and was much, much bigger.

I like to think of a thunderstorm as being like the finger of God, and us in our little airplanes as being a mosquito. I think it's a good comparison as to just how small and ineffective we are in the face of such things, and it's a constant reminder to me to respect that kind of power and hazard, and to give it as wide a berth as possible.

[bookworm]

I think I misunderstood what was being indicated by "wispy." If I understand correctly, you're referring to the blow-off, or anvil head on a cell, which often marks a Cb.

I guess that my identifying the cloud IO540 showed as a cumulonimbus because it's "wispy" is a bit like identifying this as an elephant because it's "hairy and wrinkly" ;)
http://www.nwf.org/nationalwildlife/images/082005/nwas05_whatisit.jpg

[DX Wombat]

Danscowpie - whilst that CB looks as if it is positioned directly over the caravan site it was most likely somewhere over Shobdon or Leominster a good 28nm away. Being a caravan owner myself, I wouldn't hire it out to you for the purpose of crunching tin tents :* := but if you wished to use it to crunch a few antisocial, anti-aircraft NIMBY neighbours then you would be more than welcome to borrow it. :E :ok: