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OBJECTIVE FORECAST METHODS TO EVALUATE
THERMAL CHARACTERISTICS FOR SOARING FLIGHT

PART ONE

INTRODUCTION

Soaring pilots of all persuasions and manner of dress, have long sought timely and useful information that would help them determine whether a particular flying day held any special potential. If a pilot could reasonably expect to speck out in one of the day/s best thermals, then extra clothing could be worn. If the lift could be expected to turn on early, then an X-C flight might be considered.  If the lift rate was likely to be strong, then the proper size butt plug could be selected while still safe on the ground.

For the most part, sailplane pilots have lead the way in this effort by developing and refining several derived indices that can give soaring pilots meaningful guidance about likely maximum altitude gains, average lift rates, and approximate thermal trigger temperatures. These indices include: Thermal Index (TI), Top of Lift, Height of -3 TI, Lift Rate, and Trigger Temperature of First Useable Lift. Part One of this article will briefly explain the data sources necessary to derive these indices and how the Thermal Index is calculated. Part Two will describe the important TI values and the other soaring indices.

DATA SOURCES

Calculation of soaring indices requires either 1) observed temperature data from a morning rawinsonde (RAdio WINd SONDE) balloon flight close to the flying site or 2) forecast temperature data from numerical weather prediction model output. For Wind Rider HGC flying sites, the closest upper air station is located at Dulles International Airport (IAD) in northern Virginia. To varying degrees, numerically modeled data are available for any point on the globe with forecast increments from one to 12 hours with projections extending out though 240 hours from the model/s initialization time. ePilots can easily find links to these data sources via the WRHGC Soaring Weather page on the rows labeled 'Soundings' and 'Vertical Profile Temp, Wind.'

DATA ISSUES

Each data source has its strengths and weaknesses. Upper air sites from around the world launch their rawinsondes twice a day at 12z and 00z to observe the atmosphere/s vertical profile of temperature, humidity, and wind velocity. The main shortcoming of using the observed data is its timeliness because it is not routinely available until midmorning during daylight savings time, which can mean a later that desired departure time for the launch site. This shortcoming is somewhat lessened because the data are real, and therefore will be more accurate when compared to the output from weather forecast models. On the other hand, new data from numerical weather prediction models are available every six to 12 hours, depending on the model. This means the soaring indices can be computed a day or two before aviation is committed, which is a very useful commodity for flight planning purposes. The main shortcoming of using model data is that it is sometimes inaccurate, however not so much so that it/s rendered completely useless with regard to the atmosphere/s vertical temperature profile, however there/s often a lot to be desired when it comes to the wind/s velocity. (Note: Velocity is a vector quantity, which means speed *and* direction...which reminds me of a similar story about heat lightning....)

THERMALS

In the general case, thermals are a byproduct of differential heating of the Earth/s heterogeneous surface by incoming short wave radiation from the Sun. The insolation process leads to density discontinuities of air parcels at the surface, which means some of these air parcels are less dense or more buoyant than others and therefore more likely to 'float' or ascend if they can be dislodged. As long as the parcel is lighter/warmer/less dense/more buoyant than its surroundings, the parcel will continue to rise until it reaches an equal temperature, or equilibrium with the environment. For the sake of our discussion, a thermal can be considered an air parcel.

THERMAL INDEX (TI)

The TI is defined as the arithmetic difference between the environmental temperature and the adiabatically cooled air parcel temperature, measured in degrees Celsius. The TI is commonly calculated from the surface to any given height using the forecast daily high temperature as a starting point. The forecast high temperature is then adiabatically cooled as it is lifted to successively greater heights.

For example, if the environmental temperature at altitude is observed to be 15°C (59°F) and the air parcel/s temperature is 20°C (68°F), then the TI is 15°C - 20°C, or -5°C. Since the air parcel at this level is still warmer than the environment, it will continue to rise. As the parcel rises higher and higher, the temperature difference becomes smaller and smaller, which at some point the parcel temperature will reach equilibrium with the environmental temperature, or the top of the lift.

It should be fairly obvious that the usefulness of the TI is strongly dependent on having an accurate temperature forecast, although small errors on the order of a couple degrees won/t generally ruin your day. An objective derived forecast for the day/s maximum temperature can be found on the WRHGC Soaring Weather page on the row labeled 'Model Output Statistics' (MOS).

To compute the TI for the WRHGC Weekend Soaring Forecast, forecast sounding text data from the National Weather Service/s three operational numerical weather prediction models are manually harvested from NOAA/s Air Resource Laboratory/s READY web site. The data set is then imported into a spreadsheet using an original application written in Lotus Script. Once the data are loaded, the TI is automatically calculated for flight levels up to 10,000 feet. Commercially available software, such as RAOB or the interactive, on-line application known as the 'Thermal Index Report Generator' by Kevin Ford are available to compute the TI using real-time rawinsonde data.

TI FORECAST VERIFICATION

Beginning with the Spring 2000 soaring season, the weekly issue of the 'Weekend Soaring Forecast' has reported TI statistics based on the latest numerical weather forecast data, when applicable. Based on preliminary verification reports from pilots, the TI has shown respectable skill, making it a useful tool for flight planning purposes. Pilots are encouraged to relay their post-flight reports of altitude gains, lift rates, and trigger times as part of a continuing effort to improve the soaring forecast.