STORM TRACK: November 30, 1984 (Volume 8 Issue 1)

Many of the non-meteorologist readers have asked similar questions over recent months about some of the basic terminology that keeps reappearing in Storm Track. Here is a listing of such terms, which may have given you problems in the past, or which may appear in future references that you read. Some are based on the 'Storm Spotter's Glossary and Supplemental Guide," prepared by Leslie R. Lemon et al for the National Weather Service. In some cases (*), I have added common terms that were not defined here or have substantially modified (#) those that were. Additionally, and in following through on the Storm Research References provided earlier, I have added two schematic sketches on a tornadic storm environment and a cross section of a tornadic thunderstorm. More on these later. Now, in alphabetical order, terms you should become familiar with:

* 1. A lowering - A lowered but smooth bulge beneath an otherwise horizontal cloud base, usually beneath large towering cumulus and within 3 miles of heavy precip.

2. Anvil- The spreading upper portion of a thunderstorm into an anvil shaped plume usually between 20,000 - 50,000 feet high.

* 3. Backshear - That part of the anvil which pushes back against the prevailing upper winds (usually from the west or southwest); indicating a severe storm with updrafts too strong for the prevailing winds to ventilate, or carry, the new condensation downwind in the anvil plume. Thus, the upwind (west or southwest) side of the anvil pushes back against the prevailing wind. The larger the backshear, the more intense the storm. However, in a tropical air-mass situation, upper winds are weak and this rule doesn't apply (the anvil may spread out uniformly in all directions).

* 4. Bounded weak echo region - A radar-weak reflectivity area that indicates a strong updraft on the right flank of an initially non-severe thunderstorm. The "bounded" aspect indicates development of the updraft region into a mesocyclone, with a simultaneous wrapping downdraft, developing on the right rear flank.

# 5. Cb (Cumulonimbus) - Actually a thunderstorm at such a distance that thunder has not yet been heard. Official Government weather reports cannot record a "thunderstorm" until it's audible at the reporting station. Thunder indicates a heightened level of storm intensity; however, some weak Cbs develop with little or no thunder. Nevertheless, the wary pilot will think twice before flying through any reported "Cb."

* 6. Closed low - An upper low that becomes cut-off from the main upper level wind flow and begins to lose strength as it rotates apart from the energy-source winds that originally formed it.

7. Collar cloud - A generally circular cloud ring, sometimes visible, surrounding the upper portion of a wall cloud.

* 8. Diffluence - Normally refers to diffluence in the upper atmosphere (about four miles up),where prevailing winds spread apart across several hundred miles (sometimes with divergent jet streams). Somewhat lower pressure occurs here, between the divergent flows. This encourages the acceleration of any convection which begins within this ares.

* 9. F0, Fl, F2, F3, F4, F5 - A means of measuring tornado strength, developed by Professor T. Theodore Fujita at the University of Chicago, and widely accepted in the meteorological community. F-0 (or F0)= 40-72 mph; light damage to chimneys and sign boards, tree branches broken. F1= 73-112 mph; peels surface off roofs, mobile homes pushed off foundations and moving autos pushed off road. F2 = 113-l57 mph; roofs torn off frame houses, mobile homes demolished, boxcars pushed over and large trees snapped or uprooted. F4 = 158-206 mph; roofs and some walls torn off well-constructed houses, trains overturned, most trees in forest uprooted and heavy cars lifted off ground and thrown. F4 = 207-260 mph; well-constructed houses levelled, cars thrown and large missles generated. F5 = 261-318 mph; strong frame houses lifted from foundation and carried considerable distance to disintegration, auto-sized missiles fly through the air in excess of 300 feet and trees debarked.

10. Flanking line - A line of cumulus towers, merging with the main storm and usually extending to the south or southwest, often associated with a severe storm.

11. Fractus (cumulus fractus or scud) - Low, detached cloud fragments, looking ragged and wind-torn.

12. Gust front - Leading edge of thunderstorm downdraft air, often gusty and cool. Shelf or roll clouds may accompany it and sometimes gustnadoes wrapping up for several seconds at ground level in its turbulence.

* 13. Gustnado - A short lived, small tornado, usually F0 to F1, which sometimes develops along strong gust front boundaries at ground level but rarely extends all the way to cloud base.

14. Hook echo - Radar pattern sometimes seen in the southwest quadrant of a tornadic thunderstorm, looking like the number "6" or like a fish hook. The hook echo is the radar's reflectivity off of the precipitation aloft, wrapping around the periphery of the mesocyclone.

* 15. Inflow bands - Horizontal cloud bands, usually east to south of the main storm base and gradually curving into it, becoming thicker as they approach. Banding indicates increasing organization of the storm, as low level moisture condenses along the increasing inflow winds.

* 16. Inversion (or "cap") - A layer of air between one and two miles above the earth's surface, where temperature rises with height and supresses deep convection. The longer that this persists during a "severe clear" afternoon, the greater the buildup of accumulated moisture and heat beneath it. If the cap isn't broken until mid or late afternoon (by the first strong tower to go up), then the severe potential is much greater.

* 17. Laminar funnel - A smooth sided funnel or tornadic vortex above ground, as opposed to a ragged, turbulent tornado (often characteristic of the strongest).

18. Mammatus (cumulus mammatus or "CM") - Smoothly rounded pouches hanging on the underside of anvils and often occurring with severe weather.

* 19. Meso-cyclone - A radar detectable circulation within the southwest flank of a severe thunderstorm, that incorporates the main updraft just ahead of a wrapping rear-flank downdraft -- producing a rotating core about 2-4 miles across and 6 miles high (can extend almost to anvil top, or 8 miles, for large tornadoes). The wall cloud is generally located near or a little to the southwest of the mesocyclone, near cloud base.

* 20. Meso-low - (This definition is somewhat speculative, since the Editor has seen some professional papers use this term when referring to the mesocyclone. However, he believes that its alternative use, seen in other professional papers, may be more correct). The localized lower pressure falls associated with approaching thunderstorms (as opposed to the normal continental lows and highs seen on your daily media weather map).

* 21. Microburst - An intense, brief and localized downburst of winds from cloud base, often along the southwest flank of a severe thunderstorm, that are sometimes damaging. Debris traces from such events are usually laid out radially, as opposed to concentrically twisting damage tracks from tornadoes.

* 22. Open low - Opposite of the "closed low," mentioned in term #6; or an upper low that is open to and draws its strength from upper level winds that circulate around it.

* 23. Penetrating top (penetrating dome) - That part of the thunderstorm that protrudes above the generally flat anvil deck, and which indicates the strongest updraft. Tornadoes sometimes occur within 20-30 minutes of the collapse of this top (Of course, if you're where you can see this, you're probably 50-100 miles too far away to see the tornado at cloud base).

* 24. Pedestal cloud - (Again, a somewhat speculative definition, since the Editor has not seen this term used much in recent years) A small, inclined cloud edge (like the "cow catcher" in front of an old steam locomotive), which forms at the edge of a "lowering in the cloud base southwest of the main precip core. It is an intermediate stage, before formation of the wall cloud (However, keep in mind that no one stage in storm development automatically leads to the next. At any time, the storm can lose its dynamic support in the atmosphere and simply begin weakening into an ordinary thundershower or hail storm).

* 25. Positive lightning - An occasional and comparatively rare cloud-ground bolt that's positively charged, much stronger than the average, negatively-charged bolt, and which often reaches anvil height, arcing over a long distance.

26. Rainfree base - A horizontal, dark cumulonimbus base with no visible precip or greatly reduced precip) as compared with the main storm's precip core. This marks the updraft region. Wall clouds often form here, within 1-3 miles of the main rain core.

* 27. Rear flank downdraft - Evaporatively cooled and precip driven downdraft that occurs from northwest to southwest to south of the main updraft. "Rear" flank is the side of the storm cell or thunderstorm opposite to the direction in which it is moving.

* 28. Right turner/left turner - When a thunderstorm becomes severe, it tends to veer or turn to the right from its normal path. Facing the direction it is moving and in line with its path, this turn is to your right. When storms split, the left one may conversely become a left turner.

29. Roll cloud - Relativsly rare low-level, horizontal and narrow accesory cloud that is tube shaped, completely detached from the cumulonimbus base. When present, it is located along the gust front and appears to be slowly rolling about its horizontal axis. They do not produce tornadoes.

* 30. Shear- Any boundary between two parallel air streams, that are moving in different directions and/or at different speeds. Shearing tends to produce vorticity between the different air streams, which allows such interfaces to occur. In fact, as Chuck Doswell has pointed out to the Editor, vorticity is the normal way that the atmosphere moves, whether as continent, wide systems or at smaller scales of interaction.

31. Shelf cloud - A low level, horizontal accessory cloud that looks wedge-shaped as it approaches. Its top is usually attached to the thunderstorm base and forms along the gust front.

32. Squall line - Any line or narrow band of active thunderstorms. The term is usually used to describe solid or broken lines of strong or severe thunderstorms.

* 33. Stretching - Refers to stretching of a rotating column of air, as by faster upper level winds, which narrows that column and increases its rotation speed,

* 34. Striations Narrow bands or channels that form just above cloud base, along the low- er, leading side of a cumulus tower, often just above the wall cloud. Their appearance indicates rotation beginning to take place about the updraft.

* 35. Suction vortex - Secondary or sub-vortices that occur within the rotational circulation of the tornado, occasionally visible with larger tornadoes as brief tubes, spinning up from the ground or materializing in the air between the lowering condensation column of the parent tornado and the ground. These produce the maximum damaging winds of 200 - 300 mph.

* 36. Supercell - A thunderstorm that develops in an atmospheric environment that permits it to become and continue being severe for 30 minutes to an hour, or longer, without either (a) the wrapping gust front choking off the moist southerly inflow winds near the surface, feeding the updraft or (b) the overall storm cell moving out of its breeding ground.

37. Tail cloud - A low, tail shaped accessory cloud, extending from the northern quadrant of a wall cloud and often curving back to the east, southeast or south. Motions in the tail cloud are toward the wall cloud, with rapid updraft at the function of the tail and wall cloud.

* 38. Upwind/downwind - Upwind from the thunderstorm is on the side where winds are approaching it. Downwind from the same storm is on the side where winds are moving away from it.

39. Virginate - Refers to an old thunderstorm anvil which has lost its low level convective support and becomes detached from the earlier updraft that fed it. These lower clouds have usually weakened and rained out, leaving an upper level anvil, virginated from its updraft sources, which will drop little further precip and just drift with prevailing winds until it dissipates.

* 40. Vorticity loops - Horizontal vorticity rolls 1-2 thousand feet in diameter) which tend to form at low levels, between different moving layers of air. They are not- continuous loops or tubes but appear and disappear frequently at this level. When strong upirafts develop in building cumulus, these loops, or rather this interface layer is pulled upward, stretching the incipient vorticity and accentuating the rotational component already inherent in the updraft. In this general way, they contribute to the development of tornadic rotation.

* 41. Weak echo regions (WER) - Similar to the bounded weak echo region (term #4) or BWER, except that the rear flank downdraft has not yet developed.

So much for terminology. I had not intended to go so far, but the theme of this issue seemed to require a somewhat fuller treatment than originally planned. My apologies to Steven Leitch in Canada for not including his fine article on "The ROARING Hailshaft" in this issue as I had planned. It will be included in the January issue. Finally, if any of you wish to modify or add to this list of severe storm terminology, please do so.

Two schematic illustrations follow, showing atmospheric dynamics (a) in an overview of a severe storm environment and (b) in a detailed cross-section of an average tornadic thunderstorm. This was an ambitious project, which tried to consolidate and integrate from many sources the major elements from current knowledge on the subject. Each incorporates explanations and illustrations from dosens of authors. It would be difficult to list them all here and would certainly bust this issue's two ounce postage limit. However, some of the principal sources for these illustrations were: (a) For the severe storm environment -- Toby N. Garison, Pennsylvania State University, 1982; Peter A. Browning and Henry E. Fuelberg, Saint Louis University, 1982; and John T. Snow, Purdue University (Scientific American, April, 1984). (b) For the tornadic storm cross-section -- Leslie R. Lemon and Charles A. Doswell III, National Severe Storms Forecast Center, 1978 and 1979; T. Thsodore Fujita, University of Chicago, 1981; and A.B.C. Whipple, Storm (Time-Life Books), 1982.

Prior to this publication, the Editor sent out nine draft copies to storm meteorologists around the country for comment and received one telephoned and seven written replies. My thanks to Don Burgess, Bob Davies-Jones, Chuck Doswell, Tim Marshall, John Weaver, John T. Snow and Howie Bluestein for their thoughtful and detailed (Burgess & Marshall) comments. A common response from several was that my initial efforts were too complicated. In other cases, a few inconsistent comments were received on minor aspects of these sketches. In such instances, a choice was made or else that part of the sketch was dropped. The Editor has tried to clarify each illustration, accordingly, and has simplified some parts. To the best of my knowledge, this is the first time that a single, unified display has been attempted that ties together basic state-of-the-art knowledge on tornadic storms.

Please note that both illustrations portray an ideal setting over the southern plains states. However, there are variables between tornadic storms and between storm environments at the same time of the year, at different times of the year, and/or in different parts of the country. For example, differences can be found between the July-Montana tornado and the April-Alabama twister. In some cases, one or two "textbook" factors will be missing, and still produce a tornado. Later in the spring and early summer months, some storms become tornadic with very weak upper level winds, if low level heating and moisture are abundant. In other cases, the dry front is absent. However, these illustrations do show most of the major ingredients and are, at least, instructive in this respect.