Thermal Soaring


  • Shallow turn when thermal is worse (or you're out, and in sink)
  • Steepen turn when thermal is better
  • Uses the Pseudo-adiabatic Chart

Locating Thermals:

  • Thermals can be located by looking for cues in the environment
  • Clouds:

    • Cumulus clouds in the early building stages are indicitive of rising warm air that is being cooled enough to condense to form a cloud
  • Birds:

    • Sometimes birds can be an indication of rising air
  • Other Gliders:

    • Pay attention to what other glider pilots are doing
  • Wind:

    • Probable thermals will be pushed in the direction of the wind

Soaring Forecast:

  • Soaring forecasts provide insight as to the conditions you can expect
  • Published by a local National Oceanic and Atmospheric Administration (NOAA) office
    • The best way to find soaring forecasts is to type into any search engine your airport or region followed by soaring forecast
      • Example: Warner Springs soaring forecast

Thermal Updraft Velocity:

  • Forecasts the upward velocity of air within the thermal and will never be negative
  • The glider descent rate while thermalling must be subtracted to give the expected variometer reading
  • Find more here

Buoyancy/Shear Ratio:

  • A small Buoyancy/Shear (B/S) Radio indicates wind shear due to wind changing with height
    • A B/S of 5 or less means thermals are likely to be broken to unusable
    • A B/S of 10 or higher means shear is unlikely
  • Find more here

Boundary Layer Top:

  • The boundary layer top is the height of the "mixing layer" where surface turbulence mixes with the atmosphere above it
  • The height where the dry adiabatic lapse rate through surface temperature intersects the temperature profile
    • Essentially where TI=0
  • Find more here

Boundary Layer Max Up/Down Motion:

  • Maximum grid-area-averaged extensive upward or downward motion within the boundary layer as created by horizontal wind convergence
    • Positive convergence is associated with local small-scale convergences lines (shear lines)
    • Negative convergences (divergence) produces subsiding vertical motion, creating low-level inversions which limit thermalling heights
  • Find more here

Boundary Layer Windspeed:

  • The speed of the vector-averaged wind in the boundary layer
  • Find more here

Cu Cloudbase:

  • Plotted where cloud bases are theoretically expected
  • Find more here

Flight Hazards Exist Around Exhaust Plumes:

  • Exhaust plumes are defined as visible or invisible emissions from power plants, industrial production facilities, or other industrial systems that release large amounts of vertically directed unstable gases (effluent)
    • High temperature exhaust plumes can cause significant air disturbances such as turbulence and vertical shear
    • Other identified potential hazards include, but are not necessarily limited to:
      • Reduced visibility
      • Oxygen depletion
      • Engine particulate contamination
      • Exposure to gaseous oxides, and/or
      • Icing
    • Results of encountering a plume may include airframe damage, aircraft upset, and/or engine damage/failure
    • These hazards are most critical during low altitude flight in calm and cold air, especially in and around approach and departure corridors or airport traffic areas
    • Whether plumes are visible or invisible, the total extent of their turbulent affect is difficult to predict
    • Some studies do predict that the significant turbulent effects of an exhaust plume can extend to heights of over 1,000 feet above the height of the top of the stack or cooling tower
    • Any effects will be more pronounced in calm stable air where the plume is very hot and the surrounding area is still and cold
    • Fortunately, studies also predict that any amount of crosswind will help to dissipate the effects
    • However, the size of the tower or stack is not a good indicator of the predicted effect the plume may produce
    • The major effects are related to the heat or size of the plume effluent, the ambient air temperature, and the wind speed affecting the plume
    • Smaller aircraft can expect to feel an effect at a higher altitude than heavier aircraft
  • When able, a pilot should steer clear of exhaust plumes by flying on the upwind side of smokestacks or cooling towers
    • When a plume is visible via smoke or a condensation cloud, remain clear and realize a plume may have both visible and invisible characteristics
    • Exhaust stacks without visible plumes may still be in full operation, and airspace in the vicinity should be treated with caution
    • As with mountain wave turbulence or clear air turbulence, an invisible plume may be encountered unexpectedly
    • Cooling towers, power plant stacks, exhaust fans, and other similar structures are depicted in [Figure 1]
  • Pilots are encouraged to exercise caution when flying in the vicinity of exhaust plumes
    • Pilots are also encouraged to reference the Chart Supplement U.S. where amplifying notes may caution pilots and identify the location of structure(s) emitting exhaust plumes
  • The best available information on this phenomenon must come from pilots via the PIREP reporting procedures
    • All pilots encountering hazardous plume conditions are urgently requested to report time, location, and intensity (light, moderate, severe, or extreme) of the element to the FAA facility with which they are maintaining radio contact
    • If time and conditions permit, elements should be reported according to the standards for other PIREPs and position reports


  • Note: Landing in tow: low tow, no spoilers except to take out slack, no brakes
  • Speed to fly:
    • Note: Speed to fly: add 1/2 headwind, subtract 1/4 tailwind
    • Note: Speed to fly for wind: for sink/lift is based off L/D=50
    • McKreedy number
  • Note that turbulence experienced when thermal souring is less the intensity of the thermal and more its interaction with the winds aloft
  • Remember the golden rule of gliding, to never fly over (glide distance) terrain you cannot make a safe landing on
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