Information:
by Larry Burch
NWS Meteorologist
Glider pilot (commercial), Airplane pilot (ATP), Flight Instructor (CFI-A, CFII, MEI)
FAA Aviation Safety Counselor for Salt Lake FSDO
email questions or comments to Larry.Burch@noaa.gov

Traditional soaring forecasts (also known as "Soaring Index" or "Thermal Index") are derived from weather balloon data (also known as RAOB, or Sounding). Weather balloons are launched twice daily at approximately 1115Z and 2315Z. The instrument attached to the weather balloon measures temperature, humidity, and pressure. A transponder within the instrument package is tracked by a radar to obtain wind direction and speed.

One may be surprised to find out that weather balloons are not launched from every weather station or airport. They are launched at only a few places, and the distance between sites can be several hundred miles. They are launched from Salt Lake City UT, Elko NV, Reno NV, Desert Rock NV, Boise ID, Riverton WY, Grand Junction CO, Denver CO, Flagstaff AZ, Tucson AZ. That's it for the area in and around the State of Utah.

So how can a pilot get a soaring forecast for a place which is not near a weather balloon site, for example Parowan Utah? There are several methods, and I'll discuss four.

The first method is to interpolate between nearby soaring forecasts. Parowan is between three weather balloon sites that also produce soaring forecasts, Salt Lake City, Desert Rock NV, and Flagstaff AZ. Just take an average of the 3 soaring forecasts.

A second method is to use the Temp/Winds Aloft table, also known as the FD Winds. Pick a location, such as ELY, and look at the temperatures at 9000 feet, 12000 feet, and 18000 feet. The dry adiabatic lapse rate is 3 degrees per 1000 feet. Thus a perfect soaring forecast (monster lift) would have the temperature at 12,000 feet being 9 degrees cooler than 9,000 feet. And the 18,000 temperature would be 27 degrees cooler than 9,000 feet. Weaker lift would be 2 degrees per 1000 feet. This method can also be used to obtain trends in the stability of the atmosphere, i.e., is it warming or cooling at 12,000 feet and 18,000 feet?

A third method is to use the Temp/Winds Aloft interactive maps from ADDS. Select Temperature and 9000 feet. Note the temperature for the place you want. Repeat for 12000 feet, and 18000 feet. Compare the temperature differences with the dry and standard lapse rates. This page also allows you to select temperature and winds for the next two days.

A fourth method is to use a computer derived sounding. Computer model data is available for any location in the country. Click on "Computer Sounding" to access NOAA's Forecast Systems Lab (FSL) sounding analyses and forecast page. RUC model soundings can be obtained by selecting RUC-2 as the "Input Data Source" at the top, then selecting the time of day you want to see (for example 18 UTC is noon). Enter an airport ID (for example Parowan is 1L9) and select "plot sounding". You'll get a graph of temperature in red, , dew point in blue, and associated wind barbs. From this data you can view how the temperature changes with height, and see where the lift stops, as well as the wind at these altitudes. You will not get a "soaring index", rather you get to see the whole picture. You can also get RUC sounding forecasts for SLC valid for the afternoon and compare it with the actual weather balloon sounding taken that morning to see what changes are forecast during the day, i.e., is the temperature at 10,000 feet warming or cooling. The light blue slanted lines are "dry adiabatic lines". When the temperature line parallels dry adiabatic lines, its monster lift!

Enjoy Utah's soaring weather and be safe!
Larry
10-07-03