SHEAR SOARING

<body topmargin=0 leftmargin=0 marginheight=0 marginwidth=0> <table cellpadding=0 cellspacing=0 border=0><tr> <td valign="top" colspan=2 height=90 BGCOLOR="#006699" TEXT="#080000" bgproperties="fixed" background="02c285a1erlcgo.png"> <div> <B> |     </B><A HREF="index.htm" TARGET="_top" TITLE=" "><U><B>home</B></U></A></div> <div> <A HREF="id84.htm" TARGET="_top" TITLE="how to make the best of a GREAT situatio"><U>how to make the best of a GREAT situation</U></A>   |   <A HREF="id86.htm" TARGET="_top" TITLE="ALL our local mountains"><U>ALL our local mountains</U></A>   |   <A HREF="id326.htm" TARGET="_top" TITLE="west wind winners"><U>west wind winners</U></A>   |   <A HREF="id87.htm" TARGET="_top" TITLE="SHEARLINES common at Crystal"><U>SHEARLINES  common at Crystal</U></A>   |   <A HREF="id327.htm" TARGET="_top" TITLE="The Big Bow to Big Bear"><U>The Big Bow to Big Bear</U></A>   |   <B>SHEAR SOARING</B></div> <div> </div> </td> </tr><tr> <td valign="top" width=210 BGCOLOR="#FFCC66" TEXT="#080000" background="02ad2c00dkelyh.png"> <div> <B>Most textbooks on soaring deal with elementary training, and cannot go far beyond that.</B></div> <div> <B>The text on the right is a 3 page excerpt from a CD we offer to all our students, covering most aspects of soaring at the intermediate level.</B></div> <div> <B>If you are interested in seeing more of this document, let us know - or better yet, come fly with us ! </B></div> <div> <A HREF="index.htm" TARGET="_top" TITLE=" "><U><B>HOME</B></U></A><B>    </B><A HREF="id327.htm" TARGET="_top" TITLE="The Big Bow to Big Bear"><U><B>BACK</B></U></A><B>    </B><A HREF="id52.htm" TARGET="_top" TITLE=" SAFE MOUNTAIN FLYING"><U><B>NEXT</B></U></A></div> </td> <td valign="top" height=390 BGCOLOR="#FFFFFF" TEXT="#080000"> <div> <B>                                    </B><B>SHEAR SOARING</B></div> <bR> <bR> <div> Shear lift occurs rarely or never in some regions while it is commonplace in others.  Here in southern California shears often present us with an ultimate form of soaring opportunity:  terrific thermals crowded together into convenient, predictable lines.</div> <bR> <div> The word 'shear' is casually used to describe so many different atmospheric concepts that it may be more confusing than meaningful.  Keep an open mind as we discuss phenomena, rather than words, and try to relate everything you already know to the unexpected things you encounter.</div> <bR> <div>      Fundamentally, a shear is some variety of interface between two or more bodies of air moving in different directions, or at unequal speeds.  When multiple air masses move together horizontally, the shear occurs in a more or less vertical plane, often causing at least one of those air masses to rise.       </div> <div>      In terms of soaring potential this upward displacement can display at least as much variety as the other, better known forms of lift.  It may be very rapid and turbulent, but can also be so gentle that the effect is scarcely noticeable.  It may or may not be marked by contrasting degrees of haze and clarity, or by the full range of cumulus type clouds, and also by dust devils or a line of blowing dust.  A shear may occur in only one limited area, or can run continuously over the horizon.  The zone of soarable lift may be no broader than a wingspan, but can also be miles wide!  Whether weak or strong, a shear line may provide solid, smooth lift; it might consist of a series of individual thermals (blue or otherwise); or it could be nothing more organized than intermittent bundles of turbulence rolling upward.  It may extend as a straight line, it may arc off into the distance as a vast, uniform curve, or possibly meander back and forth, in serpentine fashion.  A shear line may be workable only at very low altitude or only far above the ground, it may favor the highest terrain or the lowest, and some shears can remain consistent and strong from near sea level all the way up to very great height.  If wind is stronger on one side of a line than the other, a shear will tend to migrate sideways, but a change in relative wind strengths can reverse this drift.</div> <div>      Two flows can come together at any angle, their interface extending vertically or diagonally.  Shears may even behave as multiple pressure waves, with each line occurring between winds from about the same direction but of unequal speeds.  (Shears of that type are apt to march downwind rather quickly.)  If two air masses collide 'head-on' the lift might be dramatic at even very low altitudes, even if the wind on either side is light.  Sometimes, though, the different directions of flow are so similar that a shear resembles the wake behind a boat, displaying a sort of 'rooster-tail' effect.  </div> <div>      The simplest kinds of shear occur when separate currents within a single air mass move together.  One example is katabatic flow, in which cooling mountain air descends from both sides of a valley and meets in the middle, pushing the valley air upward.  Such lift is commonly a late afternoon phenomenon, and is fairly predictable in some places.  It may not be strong or promise great climbs, yet might offer a smooth, leisurely cruise into the evening with little or no loss of height.</div> <div>      Another kind of shear within a single air mass can occur when wind is divided into separate currents by a hill or other obstruction and then converges again downstream, similar to a confluence of two rivers.  One potential benefit is that any existing thermals from either side of such a junction are pushed together at the interface, perhaps doubling their frequency.  </div> <div>      If merging air masses are more diverse (having different temperatures, densities or humidities), a shear may be more complex, and also probably larger and more powerful.  A sea breeze front is the classic example, in which continental and marine air masses, quite different to start with, heat and cool at different rates throughout the day.  In principle, the same sort of interaction can occur inland where air masses from two distinct geographical regions meet.</div> <div>      In either of these situations, high ridges or broad watersheds can serve to anchor a shear, slowing the advance of either air mass and therefore holding the interface in one predictable line.  It is also likely that at least one side of such a watershed will act as a thermal source, feeding the shear with extra energy. </div> <div>      For our purposes<B> </B>in soaring, the overall intensity of solar heating seems to effect the relative depth and power of a shear.  When thermal conditions are weak or the convective layer does not reach above the highest terrain, mountains may impose outright barriers, forcing interaction between separate air masses to take place at lower levels, perhaps running straight across intervening valleys.  But once convection rises above the highest terrain, mountaintops are apt to be where the most powerful interaction will take place between the stronger winds aloft.  In any case, once a shear exists, increased solar heating will render it more soarable.</div> <div>      Consider the analogy of biscuits baking on a flat pan.  As they expand, their edges push together.  Expansion from either side of the line between any two biscuits is directed toward that line, and since nothing can descend through the underlying surface, anything suspended between the two merging bodies can only go UP!  Still, in even perfect conditions, the line will eventually end where there is no longer a material difference in the movement or density of the air on either side of the line.  </div> <bR> <div>               <IMG SRC="940ab4c0.jpg" border=0 width="427" height="332" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div>      Another variation of shear between dissimilar atmospheres is found downwind of major mountain passes.  The best example may be here in southern California, where high, steep mountains lie parallel to the coast.  Here, marine air is trapped in the coastal plain and detained from encroaching on the desiccated air of the Mojave desert a few miles inland - except where it flows through low areas.  These shears are essentially segments of the sea breeze front; what's noteworthy is their location and behavior.  </div> <div>      Marine air is more dense, so as it flows through a pass it forms a lobe extending like an alluvial fan into the desert.  The lobe's perimeter amounts to a semi-circular line of consistent thermal lift where the warmer, lighter desert air is displaced upward.  As a summer afternoon progresses, the convective lifting of vast quantities of air over the desert interior creates a 'thermal low' near the ground and draws surface air in from all directions.  Meanwhile the marine air, also expanding in the afternoon sun, continues to flow in from the sea.  As a result, the lobe of encroaching marine air expands, pushing the semi-circular shear line further into the desert.  </div> <div>      Such shears are sometimes marked by cumulus, or often by an abrupt changes in coloration of the air.  A cloudless shear may also be marked by haze domes atop the convective layer.  These lumps will appear at the angle above the horizon where the sky turns deep blue and, if they seem to be growing, they probably indicate that the shear is advancing toward your position.       </div> <div>      In a typical scenario, marine air (far more massive in a global sense) finally pushes through late in the afternoon as the desert begins to cool.  The shear then moves more quickly into the desert, massing existing thermals into a line of very predictable lift, and possibly even a wave-like effect.  </div> <div> This `shear passage' usually terminates most thermal activity behind its advance.  Yet on the hottest days the marine air itself is so rapidly heated as it flows into the desert that it, too, becomes unstable and begins to spawn new thermal lift.</div> <bR> <DIV ALIGN="LEFT"></DIV> <div> <B>TECHNIQUES</B></div> <div>      As with other forms of lift, in small or weak shears it is sometimes necessary to remain in one area in order to stay aloft.  Yet the lift can be so sporadic that continuous circling is impractical.  Be ready at any moment to abandon a conventional thermal turn and fly toward the shear's rising side.  If the shear zone is quite narrow, figure eights may prove to be most effective, mixed with occasional full circles or straight runs.  (If using a figure-eight, turn toward the livelier air mass at each end of your eight, just as when turning into the wind each time while ridge soaring.)</div> <div> Shear lift may also bubble up in a somewhat cyclic manner, first in one spot, then in another, and later back where it was before.  A sort of rhythm may exist, allowing you to anticipate and move away from one area when it weakens, arriving elsewhere just as newer lift begins to build.  </div> <div>      Cloud formations caused by such action can be very short-lived, or could last all day.  They usually help in finding lift, but if they're far above you they might be deceptive.  Clouds can appear over an area that is presently inactive at your altitude, while down where you are the lift might be better where there are no clouds.  Here too, though, a useful rhythm may be discernable.</div> <div>      Shear clouds may form at multiple levels, even within one localized area.  You might find yourself soaring thousands of feet above cumulus while more lie overhead.  If clouds are forming at various heights, moving toward their forming side may enable you to soar right on up over their tops.  (This seems to occur more often over high terrain.)  </div> <div>      </div> <div>      When a shear extends in linear fashion, soar it as you would a thermal street.  Unless you intend to climb in an area of concentrated lift, just keep moving.  (If a solid line of cloud marks your shear, search overhead for darker areas on course where the lift is probably strongest.)  As with ordinary thermal soaring, the farther you explore the more lift you're apt to find, and the more you'll <U>learn</U> about this peculiar soaring opportunity - for future as well as current use.  </div> <bR> <div>      Whether you're holding station in an isolated patch of shear or running along a line that extends out of sight, the trick is to remain within that interface where the merging of flows is most dynamic.  A shear's character can vary greatly as you travel along it, or as you (or it) change altitudes.  Therefore, employ multiple models interdependently to visualize what is actually happening, and be ready to alter your assessment and technique at any moment.  Feel and watch constantly for subtle changes in drift vector.  Moving downwind toward the <U>opposing</U> wind can shorten your search for optimum lift.  Be ready to respond immediately to differences in either the feel of the air or your perception of physical conditions.  Porpoise, veer off course as necessary to follow the shear, but stop to climb only when you need altitude or when the lift is much stronger than average.</div> <bR> <div>      Where different winds or dissimilar air masses oppose each other more directly, denser air tends to wedge under lighter air.  The actual interface may then be slanted vertically, with the best lift existing further toward the lighter air mass at lower altitudes and further toward the denser side higher up. While climbing along such a shear, you might begin some distance from a line of clouds, but gradually edge closer to it, eventually arriving directly under it up high.  The same logic works in reverse:  if you fall out of a deeply slanted shear you'll descend into the sinking air of the less active side, necessitating a lengthy downwind run toward the foot of the wedge to relocate lift at some lower altitude.  (If this is bewildering, blame the writer, but go back over it and think it through carefully - it's a very important set of concepts.)      </div> <div>      Let's say you've wandered off to one side of a line and are now reasonably sure which direction to go to relocate it.  A flight path perpendicular to the line is obviously the shortest route back to lift.  However, if the lift has been intermittent (consisting of individual thermals), it could be possible to fly directly across the line between thermals and find no lift.  In that case, returning diagonally (like merging traffic) may improve the chance of recovery before too much altitude is lost. </div> <div>      Confusing?  You bet!  Intriguing?  Definitely!!  </div> <bR> <div>      Whatever the momentary conditions, and whatever your strategic intentions, the following general method will work well most of the time.  Since air is moving from two directions toward some point or line, either wind will drift a sailplane toward the other wind - and into the better lift occurring where the two winds meet.  So, while cruising in search of stronger lift, allow yourself to drift with the wind toward that other wind.  Meanwhile, though, you must not allow lift on either side to roll your craft away from the lift you do encounter.  It is simpler than it sounds; allow lateral drift but resist any tendency of the sailplane to roll away from momentary lift.  Like a hungry dog patrolling an ally full of garbage cans, you'll follow the  'scent'  <U>in</U>directly toward the best lift that lies ahead in your immediate area.</div> <div> <B>     </B></div> <bR> </td> </tr></table></body>