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Wind Shadows
3/6/2001

On a windy day, a thermal bubble can grow bigger if protected from the wind.  Thermals are usually stronger on the sunny side of a spine, but if the both sides are receiving good sun then the strongest thermal lift will usually come off the lee side because it’s better protected from the wind.  If a spine is lined up with the wind, then the thermals can roll up the spine like a snowball, however, if the spine is across the wind, then the stronger thermals will come from a wind protected source.

You have to be careful when looking on the lee side for a thermal.  If you don’t find one you want to be able to get back to the windward side or keep going to the next potential trigger.

How close the points are spaced will affect your search strategy.

• If your running down wind along a ridge with closely spaced spines that extend out across the wind (eg. Castle Ridge),
  • And you don't need more altitude to get to the next trigger,
    • You can cross the spine and hope you hit a thermal on the lee side.  If you don't find one you just keep going to the next trigger.
  • If you want some more altitude to get to the next trigger higher but don't really need it if you're willing to arrive at the next spot low,
    • You can approach a spine with a high intercept and fish out the lee side with a 45 degree angle.  If you hit a core you can crank, but if you don't you can keep going and try the next trigger.
    • If the wind is lighter, you can try turning in on the lee side instead of out
      • If you're low and you turn out you're probably on a better line to intercept the next source.
  • If you can't get to the next trigger at a reasonable altitude without getting up,
    • You may have to come in and figure eight on the windward side using ridge lift technique to gain some spare altitude for a search.  You don't want to be too selective until you get a little cushion.
    • Again, if the wind is light you can search uphill on the lee side.  If its not there you can get back to the windward side.
    • You don't want to get caught low on the lee side and have to fly out through the lee side sink.
    • Obviously, if you can't hang on you need to look downhill.
• When trying to go upwind
  • It's usually better to duck in behind the spines looking for protection from the wind.  You'll often be drawn toward the lee of a spine by a wind shadow thermal as you approach.
    • Everyone has their bump tolerance and you have to put up with potential turb and sheer on the lee side
      • If you're being drawn toward the lee of a spine you should expect a sheer and sharp edges as the thermal uncovers and gets exposed to the wind.
  • If you swing wide and uncover, you run into more wind and your glide will suffer
    • If you're exposed to the wind you may want to get on the speed bar, but as you approach the lee edge it's hazardous to be flying fast.  The edge is particularly hazardous because it's got the most potential for a sheer and your terrain clearance is likely to be low.  Timid pilots like myself are gun shy about speeding up in potentially sharp air.  I'm always amazed watching the comp pilots step on it through the bumps when my intuition tells me I need to slow down.  I guess their reaction time is keener.
    • I usually prefer to duck in and search for a draw to ride.  If I can't fine one and the wind is strong I may elect to fall back rather than challenge an edge.

When there's wind, you want to stay outside on the downwind glides and inside on the upwind glides.

SD

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