A wall cloud vs. a shelf cloud

It's Sky Watch Friday post time!! Please visit Tom, Klaus, Sandy and Imac's SKY WATCH BLOG (click here or on the logo) to participate in Sky Watch Fridays. A huge thank you to Dot, the mother of this fabulous blogging event!!! It is so interesting to see skies from all over the world!!! I highly recommend not just checking it out, but considering participation! You are certain to meet an extraordinary group of people!!! Once again, I would like to extend my heart-felt gratitude to the 200+ folks who offered compliments and encouragement regarding the sky watch intro and my regular post last week. It truly is an honor being a part of such an inspirational group of folks! This week, I have chosen a photograph of the birth of a wall cloud, since several people asked about wall clouds vs. shelf clouds after the Sky Watch intro showed my favorite shelf cloud picture last week (click on the link to see a shelf cloud).

A wall cloud, or pedestal cloud, is a cloud formation associated with thunderstorms. It is a marked lowering typically beneath the rain-free base (RFB) portion of a deep cumulus cloud (normally cumulonimbus but on occasion cumulus congestus), and indicates the area of primary and strongest updraft which condenses into cloud at altitudes lower than that of the ambient cloud base. Wall clouds may form as a descending of the cloud base or may form as rising scud consolidates and organizes (such as in this case). Most strong tornadoes form within wall clouds.

Wall clouds can be anywhere from a fraction of a mile wide to over five miles across, and in the Northern Hemisphere typically form at the south or southwest end of a supercell. Wall clouds form in the inflow region of the storm coinciding with the direction of the steering winds. Rotating wall clouds are visual evidence of a mesocyclone.
~Source

Except for the potential for dangerous straight line winds, generally a shelf cloud will not develop tornadoes. A wall cloud, on the other hand, is actually a very dangerous part of the storm. This is a very likely spot for tornadogenesis to occur. This particular cell was a tornadic supercell, and though this wall cloud was just being formed as the accompanying wall cloud dissipated and fed to the inflow of this one (see image below), it actually formed 3 funnel clouds while I was watching it (feel free to click on the image to the right to view it enlarged. You'll see the funnel better that way!) :O)Above, you see the dissipating wall cloud on the right giving way to rain and feeding its remaining scud into the inflow of the newly forming wall cloud on the left. It truly was fascinating and thrilling to watch the birth of this wall cloud and rotation under my own little mesocyclone. On the same day (June 6, 2007), during the same line of storms, not too far from where I was (yet far enough for me to miss it... sigh), a tornado (photograph by Gene Moore) occurred in Kyle, SD.

Wall cloud vs. shelf cloud
Occasionally people see a shelf cloud and think they have seen a wall cloud, which is an easy mistake, since an approaching shelf cloud appears to form a wall made of cloud. Generally, a shelf cloud appears on the leading edge of a storm, and a wall cloud will usually be at the rear of the storm, though small rotating wall clouds associated with mesovortices can occur within the leading edge on rare occasion. Wall clouds will tend to slope in, or toward the precipitation area, whereas shelf clouds as outflow clouds will jut outward from the storm. Wall clouds are inflow features with (often warm) air moving towards them whereas as shelf clouds are an outflow feature with cool air moving away from the storm, often as a gust front.

While I am talking about tornadoes and such, and since the Atlantic basin tropics have settled a bit for the moment, I am going to offer an update on the historic tornadic year we have experienced in the United States so far, which seems appropriately timed as we are facing the secondary tornado season. With a ten year average of less than 1400 tornadoes in ONE YEAR, it is easy to see that 2,091 reports prior to the secondary tornado season (which could offer hundreds more tornadoes to this year's tally) is an outrageous number! Granted, tornado reports overstate the actual confirmed count (they estimate by 30%; however, through June 2008, it looks to be only about 24% overstated). The reason that reports out-number actual tornado counts is because of duplication (several people calling about the same tornado) or false reports (possibly damage resulting from straight line winds reported as a tornado). I have been amazed over the course of the year watching this line rise. We knew right off the bat, in early February, that we were in store for an extraordinarily tornadic year. We'll see what the change in season has in store for us this year.

Have a wonderful day!!!
~Dewdrop