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Soaring Forecast Information Page
Back to the Aviation Soaring PageWhat is a soaring forecast?
A soaring forecast attempts to gage the potential for the atmosphere to provide a thermal lifting mechanism for gliders to use as a source of lift. A more
unstable air mass both in the low levels and aloft is preferred to produce desired thermal activity. The forecast product issued by many (but not all) National
Weather Service (NWS) offices uses indices calculated from an upper air balloon sounding. Most soundings are produced by the NWS but private sources may
also be available as well. The data collected at 12 UTC usually serves as the input to the forecast.
Why produce a soaring forecast from an atmospheric model?
As mentioned above, not all NWS offices provide upper air soundings. This limits the specific locations that soaring forecast indices can be produced. NWS
atmospheric models are able to produce sounding data at points away from upper air sites. Programs have been written (at NWS Spokane, WA) to interpret the
model data and calculate the indices just as they are for traditional sounding sites. This has been done on an experimental basis and user feedback would be
appreciated. The model also provides the ability to calculate the next day's, 12 UTC soaring forecast which may help in planning future soaring activities. So,
the main goal of this project is to provide soaring forecast portability while not sacrificing data and forecast quality, and attempting to provide more than the
current day's soaring forecast.
What are the limitations of the model generated soundings?
Atmospheric models are not sophisticated enough to exactly represent the atmosphere due to computation and atmospheric sampling limitations. The model is
however, a very good approximation. Comparisons against the OTX sounding in Spokane, WA have yielded similar soaring forecast results. However, if the model and its
forecasts are unrepresentative of the atmosphere at that given time then the soaring forecasts will also likely be unrepresentative. It will still take a little intuition from the pilot to
discern whether or not the soaring forecast is on track. This interpretation by the pilot has always been a necessity, so nothing has changed there.
Soaring Forecast Content and Indices
Date/Time - The date which the forecast applies (UTC) and the time that the data was produced, also in UTC (a.k.a. Greenwich Mean Time).
Thermal Index - A basic index which subtracts the temperature along the dry-adiabat from the forecast max temperature at the surface, from the temperature at
the SAME level on the sounding.
Height of the -3 index - Where the Thermal Index is equal to -3. Some soaring forecasts may not produce a -3 index if the air mass is too stable or the forecast
max temperature input is too low.
Top of the Lift - Where the Thermal Index becomes equal to 0. This calculated when the dry-adiabat intersects the sounding.
First Usable Lift - The temperature at the surface calculated by taking the temperature at a level on the sounding (3000 feet plus the station elevation) down to
the surface dry-adiabatically.
Great Basin Soaring Index (maximum lift) - A value derived from the difference between the temperatures at the top of the lift and the first usable lift level.
The equation also applies empirically derived constants to try and achieve realistic values for the region.
Max Lift = 2.4*(HTOL/100)+(10*(TFUSBL-TTOL))
HTOL = Height at top of lift where the Thermal Index is 0.
TFUSBL = Temperature at the First Usable Lift height in degrees Celcius.
TTOL = Temperature at the Top of Lift in degrees Celcius.
Temperature of Convection (TOC) - Experimentally added on to the "traditional" NWS soaring forecast. This value is derived the same way as the first
usable lift calculation. At each level indicated on the sounding, that temperature is brought down dry-adiabatically to the surface giving the TOC at that level.
Some soaring references have recommended this technique because it is max temperature independent. It could also help track thermal heights throughout the
day as each convective temperature is reached.
Potential Lift - This is another experimental value. It uses the Great Basin Soaring Index (max lift) algorithm to calculate the lift expected between the first
usable lift level and the current level. Again, this is experimental and any feedback on usefulness is appreciated.
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